Long-Term Pavement 

Performance Program 

Falling Weight 

Deflectometer

Maintenance Manual

Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

Publication No. FHWA-HRT-05-153 December 2006
Foreword 

Many pavement engineers rely on falling weight deflectometer (FWD) technology. FWDs “thump” the 

pavement and record information related to its structural integrity. 

FWDs are often preferred over destructive methods of testing because they are much faster than 

destructive tests and do not entail the removal of pavement materials. In addition, the testing apparatus is 

easily transportable as a trailer or vehicle-mounted. 

The Federal Highway Administration’s (FHWA) Long-Term Pavement Performance (LTPP) program 

operates eight Dynatest® Model 8000 FWDs to collect load and deflection data on in-service pavement 

test sections across North America. LTPP has collected pavement deflection data in daily operations for 

15 years, and in that time, the FWDs have had very little downtime. Continuous preventive maintenance 

is necessary to keep the complex hydraulic-electrical-mechanical FWDs operating under demanding 

conditions to collect high quality data and pass rigorous annual reference calibrations. The owner’s 

manual from the manufacturer provides guidance on most repairs, maintenance and troubleshooting; 

however, eventually FWDs require service beyond routine maintenance—in other words, the time comes 

for a complete overhaul. 

In spring 2003, the LTPP Southern Region support contractor overhauled one of the FWDs operated for 

the LTPP program. During the overhaul, the contractor documented the process photographically and 

described the process of disassembling and reassembling the FWD components and subcomponents. This 

document provides FWD owners, operators, and technicians information as a supplement to the Dynatest 

8000 owner’s manual. Maintenance guidelines are based on continuous operation of FWDs. Less frequent 

operation may require adjusting maintenance frequency; however, the owner’s manual from the 

manufacturer should be followed. 

Gary L. Henderson 

Director, Office of Infrastructure 

 Research and Development 

Notice 

This document is disseminated under the sponsorship of the U.S. Department of Transportation in the 

interest of information exchange. The U.S. Government assumes no liability for the use of the information 

contained in this document. This report does not constitute a standard, specification, or regulation. 

The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers' names 

appear in this report only because they are considered essential to the objective of the document. 

Quality Assurance Statement 

The Federal Highway Administration (FHWA) provides high-quality information to serve Government, 

industry, and the public in a manner that promotes public understanding. Standards and policies are used 

to ensure and maximize the quality, objectivity, utility, and integrity of its information. FHWA 

periodically reviews quality issues and adjusts its programs and processes to ensure continuous quality 

improvement. 
1. Report No 2. Government Accession No. 3. Recipient's Catalog No. 

 FHWA-HRT-05-153 N/A N/A 

4. Title and Subtitle 5. Report Date 

Long-Term Pavement Performance Program December 2006 

Falling Weight Deflectometer Maintenance Manual 

6. Performing Organization Code 

 N/A 

8. Performing Organization Report 

No. 

7. Authors(s) 

Robin Belt, Tyler Morrison, Eric Weaver 

 N/A 

9. Performing Organization Name and 

Address 

 10. Work Unit No. (TRAIS) 

 N/A 

11. Contract or Grant No. 

 DTFH61-01-C-00087 

13. Type of Report and Period 

Covered 

Fugro Consultants, LP 

8613 Cross Park Drive 

Austin, TX 78754 

Final – Spring 2003 

12. Sponsoring Agency Name and Address 14. Sponsoring Agency Code 

Office of Infrastructure R&D 

Federal Highway Administration 

6300 Georgetown Pike 

McLean, VA 22101 2296 

15. Supplementary Notes 

 Contracting Officer’s Technical Representative: Jack Springer, PE, HRDI-13 

16. Abstract 

The Federal Highway Administration’s (FHWA) Long-Term Pavement Performance (LTPP) program operates eight 

Dynatest Model 8000 FWDs to collect deflection data on in-service pavement test sections across North America. 

LTPP has collected pavement deflection data in daily operations for 15 years, and in that time, the FWDs have had 

very little downtime. Continuous preventive maintenance is necessary to keep the complex hydraulic-electricalmechanical FWDs operating under demanding conditions to collect high quality data and pass rigorous annual 

reference calibrations. The owner’s manual from the manufacturer provides guidance on most repairs and 

troubleshooting; however, eventually FWDs require service beyond routine maintenance—in other words, the time 

comes for a complete overhaul. 

In spring 2003, the LTPP Southern Region support contractor overhauled one of the FWDs operated for the LTPP 

program. During the overhaul, the contractor documented the process photographically and described the process of 

disassembling and reassembling the FWD components and subcomponents. This document provides FWD owners, 

operators, and technicians information as a supplement to the Dynatest 8000 owner’s manual. Maintenance guidelines 

are based on continuous operation of FWDs. 

17. Key Words 18. Distribution Statement 

Falling weight deflectometer, FWD, 

pavement deflection, maintenance, Dynatest, 

pavement testing equipment 

No restrictions. This document is available to the 

Public through the National Technical Information Service, 

Springfield, VA 22161 

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price 

 Unclassified Unclassified 88 

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized (art. 5/94)
ii 

SI* (MODERN METRIC) CONVERSION FACTORS 

APPROXIMATE CONVERSIONS TO SI UNITS

Symbol When You Know Multiply By To Find Symbol 

LENGTH 

in inches 25.4 millimeters mm 

ft feet 0.305 meters m 

yd yards 0.914 meters m 

mi miles 1.61 kilometers km 

AREA 

in2 square inches 645.2 square millimeters mm2

ft2 square feet 0.093 square meters m2

yd2 square yard 0.836 square meters m2

ac acres 0.405 hectares ha 

mi2 square miles 2.59 square kilometers km2

VOLUME 

fl oz fluid ounces 29.57 milliliters mL 

gal gallons 3.785 liters L 

ft3 cubic feet 0.028 cubic meters m3 

yd3 cubic yards 0.765 cubic meters m3 

NOTE: volumes greater than 1000 L shall be shown in m3

MASS 

oz ounces 28.35 grams g

lb pounds 0.454 kilograms kg

T short tons (2000 lb) 0.907 megagrams (or "metric ton") Mg (or "t") 

TEMPERATURE (exact degrees) o

F Fahrenheit 5 (F-32)/9 Celsius oC 

or (F-32)/1.8 

ILLUMINATION 

fc foot-candles 10.76 lux lx 

fl foot-Lamberts 3.426 candela/m2 cd/m2

FORCE and PRESSURE or STRESS 

lbf poundforce 4.45 newtons N 

lbf/in2 poundforce per square inch 6.89 kilopascals kPa 

APPROXIMATE CONVERSIONS FROM SI UNITS 

Symbol When You Know Multiply By To Find Symbol 

LENGTH

mm millimeters 0.039 inches in 

m meters 3.28 feet ft 

m meters 1.09 yards yd 

km kilometers 0.621 miles mi 

AREA mm2

 square millimeters 0.0016 square inches in2 

m2 square meters 10.764 square feet ft2 

m2 square meters 1.195 square yards yd2 

ha hectares 2.47 acres ac 

km2 square kilometers 0.386 square miles mi2 

VOLUME 

mL milliliters 0.034 fluid ounces fl oz 

L liters 0.264 gallons gal 

m3 cubic meters 35.314 cubic feet ft3 

m3 cubic meters 1.307 cubic yards yd3 

MASS 

g grams 0.035 ounces oz

kg kilograms 2.202 pounds lb

Mg (or "t") megagrams (or "metric ton") 1.103 short tons (2000 lb) T 

TEMPERATURE (exact degrees) o

C Celsius 1.8C+32 Fahrenheit oF 

ILLUMINATION 

lx lux 0.0929 foot-candles fc 

cd/m2 candela/m2 0.2919 foot-Lamberts fl

FORCE and PRESSURE or STRESS 

N newtons 0.225 poundforce lbf 

kPa kilopascals 0.145 poundforce per square inch lbf/in2

*SI is the symbol for th International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E e 380. 

(Revised March 2003)
iii 

TABLE OF CONTENTS 

CHAPTER 1. INTRODUCTION................................................................................................ 1

Background................................................................................................................................. 1 

Organization................................................................................................................................ 1 

CHAPTER 2. FALLING WEIGHT DEFLECTOMETER TRAILER................................... 3

Trailer Frame .............................................................................................................................. 3 

Accessories ................................................................................................................................. 6 

Torsion Axles.............................................................................................................................. 7 

Hydraulic Brake System ............................................................................................................. 8 

Tires And Wheels ..................................................................................................................... 12 

Trailer Lights ............................................................................................................................ 13 

CHAPTER 3. SUBASSEMBLY ................................................................................................ 15

Subassembly Maintenance........................................................................................................ 16 

Guide Profiles ....................................................................................................................... 19 

Drop Weight Assembly......................................................................................................... 20 

Hydraulic System...................................................................................................................... 23 

Main Cylinder ....................................................................................................................... 29 

Center Cylinder..................................................................................................................... 29 

Main Cylinder Gland ............................................................................................................ 30 

Catch Flange and Plunger Piston .......................................................................................... 32 

Bench Assembly Method...................................................................................................... 33 

Upper Catch Assembly ......................................................................................................... 36 

Side Cylinders........................................................................................................................... 46 

CHAPTER 4. STRIKE PLATE, LOAD CELL, RAISE/LOWER ASSEMBLY ................. 49

Strike Plate................................................................................................................................ 49 

Load Cell................................................................................................................................... 50 

Raise/Lower Bar ....................................................................................................................... 54 

Geophone Holder...................................................................................................................... 59 

CHAPTER 5. ELECTRONICS................................................................................................. 67

Proximity switches.................................................................................................................... 67 

Electrical Connection Box ........................................................................................................ 67 

System Processor ...................................................................................................................... 68 

DMI Proximity Switch.............................................................................................................. 69 

Vehicle Charge Plug Connection.............................................................................................. 70 

Power Control Board ................................................................................................................ 71 

Fuse Blocks............................................................................................................................... 72 

Electronics Battery.................................................................................................................... 72 

CHAPTER 6. CONCLUSION................................................................................................... 73

APPENDIX: SCHEDULED MAINTENANCE LOG ............................................................. 75
iv 

LIST OF FIGURES 

Figure 1. Corrosion caused by battery. ........................................................................................... 3 

Figure 2. Second battery ground connection. ................................................................................. 4 

Figure 3. Missing bolt drilled out, tapped, and replaced. ............................................................... 4 

Figure 4. Stripped and sandblasted trailer ready to go to powder coat facility. ............................. 5 

Figure 5. Rear view of trailer before powder coat was applied...................................................... 5 

Figure 6. Front of newly powdered trailer...................................................................................... 6 

Figure 7. Rear of newly powdered trailer. ...................................................................................... 6 

Figure 8. Lower trailer covers......................................................................................................... 6 

Figure 9. Top trailer cover (“dog house”)....................................................................................... 6 

Figure 10. Front trailer cover.......................................................................................................... 7 

Figure 11. Torsion axle and mounting plate. .................................................................................. 7 

Figure 12. Torsion tube housing. .................................................................................................... 8 

Figure 13. Top view of torsion axle and shims............................................................................... 8 

Figure 14. Trailer tires. ................................................................................................................... 8 

Figure 15. Hydraulic hitch assembly and hand brake..................................................................... 9 

Figure 16. Mounted view of complete brake assembly. ................................................................. 9 

Figure 17. Brake drum with adjusting slot.................................................................................... 10 

Figure 18. Flexible brake lines...................................................................................................... 11 

Figure 19. Flexible brake line behind tire..................................................................................... 11 

Figure 20. Rigid brake lines.......................................................................................................... 11 

Figure 21. Brake drum. ................................................................................................................. 12 

Figure 22. New axle wheel bearings and components.................................................................. 12 

Figure 23. Trailer brake and license plate lights and backup alarm. ............................................ 13 

Figure 24. Trailer turn signal and brake lights.............................................................................. 13 

Figure 25. Freshly painted subassembly with joints reinforced. .................................................. 15 

Figure 26. Subassembly mounted on trailer. ................................................................................ 16 

Figure 27. FWD attached to tow vehicle. ..................................................................................... 17 

Figure 28. Disconnect battery terminals. ...................................................................................... 18 

Figure 29. Multisignal connection to control box......................................................................... 18 

Figure 30. Reinforced weld on top of subassembly..................................................................... 19 

Figure 31. Reinforced weld on bottom of subassembly. .............................................................. 19 

Figure 32. Guide locations on a subassembly............................................................................... 19 

Figure 33. Drop weight assembly. ................................................................................................ 20 

Figure 34. Placement of glide ring, bottom view of weight package. .......................................... 21 

Figure 35. Closeup view of glide ring. ......................................................................................... 21 

Figure 36. Roller guides mounted to drop plate. .......................................................................... 21 

Figure 37. Buffer pads and drop plate in position. ....................................................................... 22 

Figure 38. Lift collar..................................................................................................................... 22 

Figure 39. Weights on the drop weight assembly......................................................................... 23 

Figure 40. Hydraulic pump, motor, and fluid tank. ...................................................................... 24 

Figure 41. Hydraulic pump........................................................................................................... 25 

Figure 42. Hydraulic motor........................................................................................................... 25 

Figure 43. Brushes and retainer. ................................................................................................... 25 

Figure 44. Manifold block and A/B valve mounted to hydraulic pump....................................... 26 

Figure 45. Components of the A/B valve and manifold block. .................................................... 26 

Figure 46. PS1 and PS2 switches.................................................................................................. 27 
v 

Figure 47. Threads on the PS1/PS2 switch................................................................................... 27 

Figure 48. Solenoid mounted to hydraulic motor. ........................................................................ 27 

Figure 49. Wiring for Motor and Solenoid. .................................................................................. 28 

Figure 50. Electrical wiring for the hydraulic motor. ................................................................... 28 

Figure 51. Main cylinder components. ......................................................................................... 29 

Figure 52. Center cylinder and components. ................................................................................ 29 

Figure 53. Center cylinder. ........................................................................................................... 30 

Figure 54. Oil hole port................................................................................................................. 30 

Figure 55. Main cylinder gland and components.......................................................................... 31 

Figure 56. Assembled main cylinder gland. ................................................................................. 31 

Figure 57. Catch flange, release piston, and plunger shaft. .......................................................... 32 

Figure 58. Top view of catch flange. ............................................................................................ 32 

Figure 59. Assembled catch flange and release piston (bottom view). ........................................ 33 

Figure 60. Assembled catch flange and release piston (top view)................................................ 33 

Figure 61. Center cylinder and gland............................................................................................ 33 

Figure 62. Inner tube..................................................................................................................... 34 

Figure 63. Assembled main cylinder. ........................................................................................... 34 

Figure 64. O-ring installed in the main cylinder........................................................................... 34 

Figure 65. Main cylinder attached to strike plate. ........................................................................ 35 

Figure 66. Upper catch shell and interior components. ................................................................ 36 

Figure 67. Upper catch shell. ........................................................................................................ 36 

Figure 68. Inner body.................................................................................................................... 37 

Figure 69. Placement of the guide pins......................................................................................... 37 

Figure 70. Ball bearings in place on the inner body. .................................................................... 37 

Figure 71. Inner body set in place................................................................................................. 38 

Figure 72. Inner body and hex bolts. ............................................................................................ 38 

Figure 73. Inner catch top cover installed..................................................................................... 39 

Figure 74. Outer shell, top down, with interior components exposed. ......................................... 39 

Figure 75. Interior of catch assembly with compression springs.................................................. 40 

Figure 76. Inner ring. .................................................................................................................... 40 

Figure 77. Outer shell with inner ring in position......................................................................... 41 

Figure 78. Inner ring in position inside the outer shell................................................................. 41 

Figure 79. Insert outer flange........................................................................................................ 42 

Figure 80. Guide flange and spring............................................................................................... 42 

Figure 81. Upper catch.................................................................................................................. 43 

Figure 82. Attach the upper catch................................................................................................. 43 

Figure 83. Align top cover............................................................................................................ 44 

Figure 84. Tighten all bolts........................................................................................................... 44 

Figure 85. Complete upper catch assembly with cover. ............................................................... 45 

Figure 86. Side cylinder and components..................................................................................... 46 

Figure 87. Mounted side cylinder. ................................................................................................ 46 

Figure 88. Front side cylinder, looking up, from under FWD...................................................... 47 

Figure 89. Rear side cylinder looking up from under FWD......................................................... 47 

Figure 90. Load cell mounted to the strike plate. ......................................................................... 49 

Figure 91. Strike plate................................................................................................................... 50 

Figure 92. Bottom view of load cell swivel and retaining bracket on strike plate. ...................... 50 

Figure 93. Load plate (base down). .............................................................................................. 51 

Figure 94. Load plate with mounting brackets. ............................................................................ 51 
vi 

Figure 95. Load cell with brackets mounted................................................................................. 52 

Figure 96. Mounting brackets and lock flange attached to strike plate. ....................................... 52 

Figure 97. Strike plate with attached load cell and pad (shown inverted).................................... 53 

Figure 98. Adhesive applied to PVC disk..................................................................................... 53 

Figure 99. Raise/lower bar guide location before assembly(rear). ............................................... 54 

Figure 100. Raise/lower bar guide during assembly..................................................................... 54 

Figure 101. Raise/lower bar guide in location.............................................................................. 55 

Figure 102. Raise/lower bar.......................................................................................................... 55 

Figure 103. Center lift nut and attaching bolts. ............................................................................ 56 

Figure 104. Closeup of cable connection...................................................................................... 56 

Figure 105. Raise/lower bar, guide rod, and attaching cable........................................................ 57 

Figure 106. Raise/lower cover. ..................................................................................................... 57 

Figure 107. Attached cable. .......................................................................................................... 57 

Figure 108. Mounted geophone holders to raise/lower bar. ......................................................... 58 

Figure 109. Completed assembly of raise/lower bar. ................................................................... 58 

Figure 110. Rear view of raise/lower assembly............................................................................ 59 

Figure 111. Geophone holder components. .................................................................................. 59 

Figure 112. Assembled geophone holder...................................................................................... 60 

Figure 113. Geophone components. ............................................................................................. 60 

Figure 114. Ground ring and geophone. ....................................................................................... 61 

Figure 115. Mounted ground ring................................................................................................. 61 

Figure 116. New geophone cable.................................................................................................. 61 

Figure 117. Separated wire. .......................................................................................................... 62 

Figure 118. Attached ground lug. ................................................................................................. 62 

Figure 119. Geophone cable soldered to geophone...................................................................... 62 

Figure 120. Silicone-filled housing............................................................................................... 63 

Figure 121. Base cover on housing............................................................................................... 63 

Figure 122. Geophone with base plate attached. .......................................................................... 63 

Figure 123. Attached magnet........................................................................................................ 64 

Figure 124. Completely assembled geophone. ............................................................................. 64 

Figure 125. Raw cable and DIN plug housing.............................................................................. 64 

Figure 126. Internal view of the DIN plug. .................................................................................. 65 

Figure 127. Completed DIN plug. ................................................................................................ 65 

Figure 128. Four types of proximity switches. ............................................................................. 67 

Figure 129. Electronics connection box........................................................................................ 68 

Figure 130. System processor....................................................................................................... 68 

Figure 131. Rear view of vertically mounted system processor................................................... 69 

Figure 132. Vehicle-mounted proximity switches........................................................................ 70 

Figure 133. Vehicle charge plug................................................................................................... 70 

Figure 134. Trailer-to-vehicle charge plug. .................................................................................. 71 

Figure 135. Power control box (board behind front panel). ......................................................... 71 

Figure 136. Trailer fuse block....................................................................................................... 72 

Figure 137. Electronics battery inside the vehicle........................................................................ 72 
vii 

LIST OF TABLES 

Table 1. Truth Table for Hydraulic System Operation................................................................. 24 

1 

CHAPTER 1. INTRODUCTION 

BACKGROUND 

Understanding why some pavements perform better than others is essential to building and 

maintaining a cost-effective highway system. Since 1987, the Long-Term Pavement Performance 

(LTPP) program, a comprehensive 20-year study of in-service pavements, has conducted a series 

of rigorous long-term field experiments monitoring more than 2,400 asphalt and portland cement 

concrete pavement test sections across the United States, Puerto Rico and Canada. 

Established as part of the Strategic Highway Research Program (SHRP) and now managed by 

the Federal Highway Administration (FHWA), LTPP was designed as a partnership with the 

States and Canadian Provinces. LTPP’s goal is to help the States and Provinces make decisions 

based on the performance of existing pavement designs that will lead to better performing and 

more cost-effective pavements in the future. 

One aspect of the LTPP experiments is the long-term and seasonal structural response of 

pavement. Many pavement engineers rely on falling weight deflectometer (FWD) technology to 

determine this information. FWDs “thump” the pavement and record information related to its 

structural integrity. 

Falling weight deflectometers are often preferred over destructive methods of testing because 

they are much faster than destructive tests and do not entail the removal of pavement materials. 

In addition, the testing apparatus is easily transportable as a trailer, or vehicle-mounted. 

The FHWA’s LTPP program operates eight Dynatest® Model 8000 FWDs to collect deflection 

data on in-service pavement test sections across North America. LTPP has collected pavement 

deflection data in daily operations for 15 years, and in that time, the FWDs have had very little 

downtime. Continuous preventive maintenance is necessary to keep the complex hydraulicelectrical-mechanical FWDs operating under demanding conditions to collect high quality data 

and pass rigorous annual reference calibrations. The owner’s manual from the manufacturer 

provides guidance on most repairs and troubleshooting; however, eventually FWDs require 

service beyond routine maintenance—in other words, the time comes for a complete overhaul. 

In spring 2003, the LTPP Southern Region support contractor overhauled one of the FWDs 

operated for the LTPP program. During the overhaul, the contractor documented the process 

with photographs and captured illustrations of the FWD components and subcomponents. This 

document provides FWD owners, operators, and technicians information as a supplement to the 

Dynatest 8000 owner’s manual. Maintenance guidelines are based on continuous operation of 

FWDs. Less frequent operation may require adjusting maintenance frequency; however, the 

owner’s manual from the manufacturer should be followed. 

Maintenance activity frequencies described as “routine” or “periodical” are roughly equivalent to 

monthly and semiannually, respectfully. Maintenance frequencies described in distance roughly 

equate to operating time as 1 hour equals 80.47 kilometers (km) (1 hour equals 50 miles (mi)). 

ORGANIZATION 

This document is divided into six chapters. Chapter 2 discusses the FWD trailer. Chapter 3 

covers the subassembly. Chapter 4 discusses the strike plate, load cell, and the raise/lower bar. 
2 

Chapter 5 covers the electronics, and chapter 6 is a conclusion. A maintenance log is provided in 

the appendix at the end of the manual.
3 

CHAPTER 2. FALLING WEIGHT DEFLECTOMETER 

TRAILER 

The FWD in this manual is mounted on a towed trailer, which requires some routine 

maintenance. This section discusses and illustrates the following trailer parts: 

• Frame. 

• Accessories. 

• Torsion axles. 

• Hydraulic braking system and tires. 

• Lights. 

• Optional covers. 

TRAILER FRAME 

The trailer frame requires little maintenance. Monthly, a technician should thoroughly check the 

frame for signs of cracks, corrosion, and missing bolts. Figure 1 shows corrosion on the trailer 

frame that occurred under the battery box near the battery ground pole. Figure 2 shows a second 

ground pole, another site of possible corrosion. Figure 3 shows a missing bolt that was drilled out 

and replaced. The missing bolt was from an optional cover. Damage such as broken bolts can be 

expected on units with covers, and bolts that attach the covers should be checked monthly. 

Figure 1. Corrosion caused by battery. 

The 8D battery 

ground pole 

should be 

cleaned monthly 

to prevent 

corrosion and 

checked for 

loose cable lug 

connection. This 

is one of two 

trailer ground 

lug locations. 
4 

Figure 2. Second battery ground connection. 

Figure 3. Missing bolt drilled out, tapped, and replaced. 

NOTE: The missing bolts that have broken off are from the optional covers. Check the covers 

monthly and repair or replace missing bolts. This damage is common on units with covers. 

Second 

ground 

connection 

location

This broken off, missing 

bolt is from the optional 

covers. 
5 

Figures 4 and 5 show the trailer after it was stripped of all paint and parts to prepare it for the 

major overhaul. 

Figure 4. Stripped and sandblasted trailer ready to go to powder coat facility. 

Figure 5. Rear view of trailer before powder coat was applied. 
6 

Figures 6 and 7 show the newly powder-coated trailer. 

Figure 6. Front of newly powdered 

trailer. 

Figure 7. Rear of newly powdered trailer. 

ACCESSORIES 

All eight of the Federal LTPP programs’ FWDs have covers (figures 8, 9, and 10). These covers 

provide some security and help protect sensitive components from outside elements. The covers 

should be washed and inspected monthly for cracks and dents. Covers should be waxed every 6 

months to help prevent scratches caused by dirt and debris. Monthly, all hinges should be 

lubricated and mounting bolts checked to ensure they are tight. 

Figure 8. Lower trailer covers. Figure 9. Top trailer cover 

(“dog house”). 

Check for 

weld cracks 
7 

Figure 10. Front trailer cover. 

TORSION AXLES 

The torsion axles must be checked for the correct toe settings and camber. The trailer axles are 

set at the factory and rarely need to be adjusted; however, it is a good idea to check their 

alignment every 3 months. 

To do this, look at the wear on the tires to check them visually or run your hand across the tire 

surface to feel any abnormal wear such as on the edges or scuffing of the tires. An abnormal 

amount of wear on the interior or exterior edge of the tire indicates the axles may be out of 

alignment. Some wear is normal because of the vertical movement in the torsion axle itself. 

To help reduce unnecessary wear on the tires, rotate them every 8,047 to 16,093 km (5,000 to 

10,000 mi). Excessive wear between rotations may indicate the alignment needs to be corrected. 

To correct this alignment, take the trailer to a reputable trailer repair facility for service; most are 

equipped to correct alignment. 

Figure 11 shows the torsion axle and mounting plate configuration. During tire rotation, check 

the rubber inside the tube housing (figure 12). It should be intact with no pieces missing. In some 

cases, the rubber may separate from the housing. If this occurs, the axle may need to be replaced. 

Consult a repair facility to help determine this. 

Figure 11. Torsion axle and mounting plate. 
8 

Figure 12. Torsion tube housing. 

Figure 13 shows a bird’s eye view of the torsion axle and shims. Check wheels for loose bearings 

and check tires for wear and condition (figure 14). 

Figure 13. Top view of torsion axle and shims. 

Figure 14. Trailer tires. 

HYDRAULIC BRAKE SYSTEM 

Make the following precautionary checks before any trip: 

• Secure all fasteners on the hydraulic hitch assembly. Also check all exposed brake system 

parts (hydraulic lines, park lock, clamps, and brackets). Check trailer hitch brake fluid and 

mounting bolts. Figure 15 illustrates parts of the hydraulic brake system. 

Shim 

Rubber 
9 

Figure 15. Hydraulic hitch assembly and hand brake. 

• Visually inspect brake shoes every 48,280 km (30,000 mi) or at least once a year (figure 16). 

FWD units in the LTPP program should be checked at 40,234 km (25,000 miles) or 6 

months. If brake shoes are worn and need replacing, the drums should be turned or replaced. 

When new shoes are replaced, verify that the hydraulic wheel cylinder is not leaking or 

showing signs of seepage. If it is, replace it, and bleed the brakes. 

Figure 16. Mounted view of complete brake assembly. 

Hand brake 

Brake fluid 

reservoir 

(use DOT 

3-type fluid) 

Hydraulic 

hitch 

assembly 

Hydraulic 

brake lock 

Mounting bolts 

Rear primary shoe 

Backing plate Hydraulic wheel cylinder 

Front brake shoe 

or secondary shoe (short) 

(Always to the front of the 

trailer.) 

Brake shoe adjustment 
10 

• Adjust the brake shoe adjuster (figure 17). This will determine the brake drum to brake shoe 

clearance. It is recommended that shoes be adjusted out until the drum just starts to lightly 

drag from contact. Do not over adjust; doing so may cause the wheels to lock up. If a lockup 

occurs, reduce the shoe to drum clearance. Depending on the brake drum manufacturer, some 

adjusting slots are on the front of the brake drum, while some are located on the back plate. 

Figure 17. Brake drum with adjusting slot. 

• Check the flexible brake lines (figures 18 and 19) for cracks, leaks, or signs of rubbing 

against the frame that could have occurred during travel. 

• Check rigid brake lines for leaks (figure 20). 

• Ensure safety chains are crossed and attached to tow vehicle. 

• Check the audible backup alarm. 

Adjusting slots
11 

Rubber 

grommet 

(protects brake 

line from 

rubbing against 

frame) 

Flexible brake 

line

Brake hose 

brackets

Flexible brake 

line

Rubber 

grommet

Rubber 

grommet 

(protects brake 

line from 

rubbing against 

frame) 

Flexible brake 

line

Brake hose 

brackets

Flexible brake 

line

Rubber 

grommet

Figure 18. Flexible brake lines. 

Figure 19. Flexible brake line 

behind tire. 

Figure 20. Rigid brake lines. 

Tee fitting

Crossover tee 

(check for signs 

of leaks)

Rigid brake 

lines

Tee fitting

Crossover tee 

(check for signs 

of leaks)

Rigid brake 

lines
12 

TIRES AND WHEELS 

• Check tire inflation pressure weekly. Consult the tire manufacturer’s recommendation for the 

correct kilopascals (kPa) (pounds per square inch (psi)) inflation. Use tires designed 

specifically for trailers. 

• Daily check wheel lug nuts visually and torque them monthly to 122.2 to 162.7 Newtonmeter (N-m) (90 to 120 pound-force per foot (lbf)), depending on lug and stud diameter. 

• Check wheel bearings for looseness by feeling for play when the hub is moved up and down, 

or side to side (figure 21). 

• Yearly repack wheel bearings and replace grease seals (figure 22). Remove wheel bearings 

and races to check for bluing, pitting, or excessive wear. Replace wheel bearings as a 

complete set, not individually (including inner and outer wheel bearings and races). 

Figure 21. Brake drum. 

Grease seal

Inner bearing

Locking washer

Thrust washer

Castle nut

Brake drum Races Outer bearing Lug nuts

Grease seal

Inner bearing

Locking washer

Thrust washer

Castle nut

Brake drum Races Outer bearing Lug nuts

Figure 22. New axle wheel bearings and components. 

Wheel bearings 

behind dust cap
13 

TRAILER LIGHTS 

Check trailer lights before traveling. Daily, check brake lights, turn signal, running lights, and 

strobes to make sure they are working properly (figures 23 and 24). 

Figure 23. Trailer brake and license plate lights and backup alarm. 

Figure 24. Trailer turn signal and brake lights. 

Strobe

Brake light

License plate light

Backup alarm

Turn signals

Brake lights

15 

CHAPTER 3. SUBASSEMBLY 

The subassembly is a rigid component that bolts directly to the trailer frame with four bolts and 

nylon lock nuts to secure the bolts. The top of the subassembly, or tower bridge, is bolted into 

place using eight bolts and washers. Use thread locker (medium-strength blue Loctite® or 

equivalent) to prevent loosening. Every 3 to 6 months, examine all eight bolts, tighten, and 

replace any missing bolts).

Figure 25 shows the subassembly and figure 26 shows the subassembly mounted on the trailer. 

Figure 25. Freshly painted subassembly with joints reinforced.

Top of 

subassembly 

Raise/lower guide 

rails (attach to 

strike plate) 

Base of 

subassembly 

Bolt locations 

where 

subassembly 

attaches to trailer 

(two on each side, 

four total) 

Oil pump mounting 

location 

Hydraulic oil 

fill neck 

Hydraulic oil sight 

glass 
16 

Figure 26. Subassembly mounted on trailer. 

SUBASSEMBLY MAINTENANCE 

Conduct a visual examination monthly to ensure no cracks develop. The most common points 

where cracks may develop are the welds between the channel iron and the hydraulic tank (both 

sides) and at the opposite ends of the channel. Cracking can be caused if only one side of the 

channel is welded, rather than both sides. This was a common problem on older units. 

If cracks develop, a reputable welding shop needs to make the repairs, observing the following 

four steps: 

1. Disconnect the charging plug, light plug, safety chain, and breakaway cable from the tow 

vehicle (figure 27). 

Hydraulic tank

Channel iron
17 

Figure 27. FWD attached to tow vehicle. 

Power plug 

Light plug 

Chains 

Breakaway cable 
18 

2. Raise the trailer hitch completely off the tow vehicle. 

3. Disconnect the positive (+) and negative (−) battery terminals (figure 28). 

4. Disconnect the multisignal cable from the power control box (figure 29). 

Figure 28. Disconnect battery terminals. 

Figure 29. Multisignal connection to control box. 

Disconnect all electrical components during the welding to reduce the chance of electrical 

current flowing through the trailer, which might damage sensitive electronics. 

Positive

Negative

Multisignal

cable terminal 

must be 

disconnected
19 

After the repairs are made, the affected area can be cleaned and painted. Figures 30 and 31 show 

the reinforced welds on the subassembly. 

Figure 30. Reinforced weld on top 

of subassembly.

Figure 31. Reinforced weld on 

bottom of subassembly.

Guide Profiles 

Annual maintenance is recommended on the guide profiles including shimming the rollers for 

correct alignment. Rails should be checked every 6 months. They may need to be shimmed 

periodically, depending on the amount of use. Yearly, apply a liberal amount of grease to the 

guide roller post (figure 32). This requires removing the guide profile rails as seen in figure 25. 

Figure 32. Guide locations on a subassembly. 

Subassembly top joint reinforced weld Subassembly bottom joint reinforced weld

Guide roller and shim locations

Mounting bolts (4) attach 

subassembly to trailer
20 

Although the subassembly frame does not require any specific maintenance, it is recommended 

that all associated fasteners be checked for tightness every 3 to 6 months. Other components also 

may require periodic maintenance and repair: 

• Main cylinder. 

• Hydraulic motor. 

• Upper catch assembly. 

• Hydraulic pump. 

• Side cylinders (two). 

• Flow control valves and pressure switches. 

Drop Weight Assembly 

Minimal maintenance is required on the drop weight assembly: 

1. Replace the lift collar semiannually. Unscrew the collar and replace, but do not apply thread 

locker. Use the setscrew to secure the lift collar. Rotate the upper catch daily to ensure equal 

wear on the collar (figure 33). 

Figure 33. Drop weight assembly. 

2. Install the glide ring on the drop weight assembly (a new feature not found on older models). 

This glide ring prevents metal chaffing from occurring between drop weight package and the 

main cylinder. After disassembly and during assembly, apply a small amount of grease to 

the glide ring (figures 34 and 35). Thereafter, lubricate weekly with a spray silicone. 

Lift collar

Dowel pins for FWD 

weights

Buffer stem slots
21 

Figure 34. Placement of glide ring, bottom 

view of weight package. 

Figure 35. Closeup view of glide ring. 

3. Place four adjustable drop height targets (activators) in a vertical rail on the drop weight. 

These targets, which are adjustable to various loads, can be selected to determine weightheight target locations (figure 36). 

Figure 36. Roller guides mounted to drop plate. 

Glide ring 

location

Glide ring

Later models have a 

machined groove for 

glide ring in base bottom

Lubricate weekly 

with spray silicone

Roller bearing 

guide bar

Original lift collar

Set screw

Adjustable drop 

height targets

Dowel pins

Roller bearing 

guide bar

Original lift collar

Set screw

Adjustable drop 

height targets

Dowel pins
22 

4. Check buffer pads for proper seating and any cracks or splitting (figure 37). 

Figure 37. Buffer pads and drop plate in position. 

5. Place the lift collar on the drop plate (figure 38). 

Figure 38. Lift collar. 

Drop plate

Buffer pads

Stop bolt

Transport lock

Center cylinder flange 

Lift collar 

(secure setscrew with 

medium-strength 

thread locker) 
23 

6. Stack the weights over the buffer plates on the drop plate and tighten securely (figure 39). 

Figure 39. Weights on the drop weight assembly. 

HYDRAULIC SYSTEM 

The hydraulic system is one of the most important parts on the FWD. It can also be the most 

complicated to assemble and troubleshoot. The following paragraphs describe and illustrate 

various components of the hydraulic system. 

The hydraulic system incorporates the following main items: 

• A hydraulic cylinder (“main cylinder”) for raising and lowering the weight catch. 

• Two parallel connected cylinders (“side cylinders”) for raising and lowering the falling 

weight subassembly. 

• A hydraulic pump (figures 40 and 41) including an adjustable excess pressure valve operated 

by: 

− A 12 volt (V) direct current motor. 

− A directional control (“DC”) valve (also called the “A/B valve”) with two-12V direct 

current coils, denoted “A” and “B.” 

− A normally open solenoid valve with a 12 V direct current coil, denoted “C.” 

− A normally closed solenoid valve with a 12 V direct current coil denoted “D.” 

− A hydraulic oil reservoir (tank), which is an integral part of the falling weight 

subassembly frame. 

− Two pressure sensitive switches (one normally closed contact and one normally open 

contact) used in the electronic control circuits. 

Weight height (WH) 

proximity switch 

Target 

(lower to increase load, 

raise to decrease load) 

Spacer 

(length varies be weight 

configuration used) 

Guide roller 

(lubricate weekly) 

Tightening handle and 

screw 

(check daily) 

Weights 

Buffer pads 

(check that buffers are 

seated correctly in drop 

weight assembly and 

check for cracks or 

splitting 
24 

The modes of operation of the hydraulic system are to raise and lower the load plate, as well as 

raise and drop the weights. The modes of operation are best explained in a “truth table,” as 

shown in table 1. It is also useful to install a hydraulic pressure gauge to assist in 

troubleshooting. 

Table 1. Truth Table for Hydraulic System Operation. 

Mode A B C D M1

Raise weight (lower plate) OFF ON OFF OFF ON (Notes 1, 2, 3) 

Drop weight (lower plate) OFF ON ON OFF ON (Notes 1, 3) 

Lower catch ON OFF ON OFF ON (Note 4) 

Raise plate ON OFF ON ON ON – 1

M is the coil of the motor starting relay. 

Note 1: “Plate” means falling weight subassembly. 

Note 2: If the weight has dropped, then this will be a “raise catch” mode. 

Note 3: This mode will also cause lowering of the plate and will not be fully active until both shafts of the two “raise 

plate” cylinders (side cylinders) have been pressed out completely to their bottom position. 

Note 4: This mode also lowers the catch. 

The hydraulic pump is mounted to the top of the hydraulic fluid tank (figures 40 and 41). A 

single bolt, which runs through the bottom of the tank, holds the pump in place. An O-ring is 

used to seal the pump to the tank. The pump has proven very reliable. Yearly, remove the 

hydraulic motor and lubricate the drive coupler with high quality moly-type grease. The only 

other routine monthly maintenance needed are changing oil and filter, tightening motor to pump 

bolts, and checking for excess pressure and adjusting if necessary (as explained in the Dynatest 

Owner’s Manual). A drain plug for replacing or adjusting hydraulic fluid is located on the 

bottom of the hydraulic tank. 

Figure 40. Hydraulic pump, motor, and fluid tank. 

Hydraulic pressure 

gauge

Filler cap

Hydraulic tank

Hydraulic line

A/B valve

Hydraulic motor

Hydraulic hoses and 

connections

Oil filter

Manifold block

Hydraulic pressure 

gauge

Filler cap

Hydraulic tank

Hydraulic line

A/B valve

Hydraulic motor

Hydraulic hoses and 

connections

Oil filter

Manifold block
25 

Figure 41. Hydraulic pump. 

Semiannually check all steel line clamps. These clamps are used to keep the steel lines from 

rubbing against each other and from contacting areas where a hole could result. Check the 

clamps every 6 months and apply a medium-strength thread locker to fasteners. 

The hydraulic motor mounts directly to the pump (figure 42). The 12 V-motor is a brand that is 

equipped with inspection covers, and it has replaceable brushes and brush retainers. Brushes and 

retainers (figure 43) should be checked every 3 months. 

Figure 42. Hydraulic motor. 

Figure 43. Brushes and retainer. 

Cover plate for hydraulic 

motor brushes

Hydraulic motor

Hydraulic pump 

Hydraulic manifold block

(bolts here)

Oil ports

Cover plate for hydraulic 

motor brushes

Hydraulic motor

Hydraulic pump 

Hydraulic manifold block

(bolts here)

Oil ports

Retaining clip

(brushes behind clip)

Hydraulic fill 

Hydraulic drive coupler 

(lubricate yearly) 

Hydraulic pump and excess 

pressure bypass valve 

location (not installed in this 

picture) 

Rubber O-ring location under 

pump housing 

Steel hydraulic line clamp 
26 

The manifold block is attached to the pump housing (figure 44). A gauge helps in 

troubleshooting pressure-related problems. If one is not installed, it is recommended to install 

one. 

Figure 44. Manifold block and A/B valve mounted to hydraulic pump. 

NOTE: Rubber O-rings are used to seal the manifold block to the pump and the A/B valve to the 

manifold (figure 45). 

Figure 45. Components of the A/B valve and manifold block. 

Hydraulic motor

Manifold block

Oil filter

(replace yearly)

“A” valve

Hydraulic pressure gauge

(optional but recommended)

“D” valve

“C” valve

“B” valve

Hydraulic motor

Manifold block

Oil filter

(replace yearly)

“A” valve

Hydraulic pressure gauge

(optional but recommended)

“D” valve

“C” valve

“B” valve

“C” valve

“D” valve

Manifold block

Hydraulic line fitting

Pressure switch (PS2)

(yellow or green dot on end)

Pressure switch (PS1)

(red dot on end)

A/B valve

Hydraulic line fitting

“C” valve

“D” valve

Manifold block

Hydraulic line fitting

Pressure switch (PS2)

(yellow or green dot on end)

Pressure switch (PS1)

(red dot on end)

A/B valve

Hydraulic line fitting
27 

PS1 and PS2 are pressure sensitive switches (figures 46 and 47). PS1 is a pressure sensitive 

switch with a normally closed contact that opens when the pump outlet pressure exceeds 

approximately 40 bar (34.47 to 4,136.85 kPa (5–600 psi)). PS1 is used for excess pressure 

detection; thus, the (red) PS1 light emitting diodes (LED )must turn off at excess pressure. 

PS2 is a pressure sensitive switch with a normally open contact, which closes from the pressure 

in the return oil during catch lowering (setting approximate 2 bar (25–30 psi)). PS2 must stay on

while the catch is moving downward, and it must come off when the catch stops at its bottom 

position. If any of these operations fail, check to see if oil is leaking from the end of the switch. 

A leak is a good indication the switch has failed. 

NOTE: If PS1 or PS2 must be replaced, use Teflon® tape on the threads to ensure a secure, leakfree seal. Take care to not over tighten to the point of breaking the fitting or causing thread 

damage. Over tightening may also damage the manifold block. 

Figure 46. PS1 and PS2 switches. Figure 47. Threads on the PS1/PS2 switch. 

The hydraulic motor solenoid switch requires no routine maintenance (figure 48). Periodically 

check to make sure the connections are tight. 

Figure 48. Solenoid mounted to hydraulic motor. 

Periodically check the wiring carefully for loose connections (figures 49 and 50). Be careful not 

to over tighten connections, as damage can occur to terminal lugs. As part of the monthly 

maintenance, remove A/B wiring plugs and clean with contact cleaner. Also, remove and clean 

coils on each end of the A/B valve. Remove and clean the stems on C/D valve assembly. 

Teflon tape

Hydraulic motor solenoid
28 

Figure 49. Wiring for Motor and Solenoid. 

Figure 50. Electrical wiring for the hydraulic motor. 

Negative

Positive

“C” valve

“D” valve

A/B wiring

(remove yearly)

Negative

Positive

“C” valve

“D” valve

A/B wiring

(remove yearly)

Positive from hydraulic motor 

Positive to manual control box 

Negative ground for motor 

Ground from manual control box 

(check periodically that it is tight) 

Excessive pressure adjustment 

Pump 
29 

Main Cylinder 

The main cylinder is made up of four major components: 

• Center cylinder. 

• Main cylinder gland. 

• Catch flange and release piston. 

• Outer tube. 

Figure 51shows the main cylinder partially disassembled. Damaged seals could result in leaking 

hydraulic fluid or loss of hydraulic pressure. 

Figure 51. Main cylinder components. 

Center Cylinder 

The center cylinder shaft is used to raise and lower the weight package catch assembly. The shaft 

is composed of a piston, two O-rings, seal, guide and wiper seals, and three attaching bolts. The 

main cylinder gland guides the shaft. The piston moves up and down inside the inner tube 

(figure 52). 

Figure 52. Center cylinder and components. 

The shaft should be straight and free of nicks and pitting. The piston, shaft, seals, and O-rings 

should be examined if there is a problem with piston operation. Figure 53 shows a completed 

center cylinder after all seals and O-rings have been replaced. Figure 54 shows the oil hole port 

through which the hydraulic fluid flows to into the cylinder. 

O-ring (inside base of tube)

Main cylinder nut

(high-strength thread locker)

Beveled edge to bottom

Piston with seals installed

Main cylinder outer tube

Center cylinder tube

Inner tube

(holes to top)

Main cylinder gland with outer O-ring 

installed

Center cylinder shaft

O-ring (inside base of tube)

Main cylinder nut

(high-strength thread locker)

Beveled edge to bottom

Piston with seals installed

Main cylinder outer tube

Center cylinder tube

Inner tube

(holes to top)

Main cylinder gland with outer O-ring 

installed

Center cylinder shaft

Guide 

seal

Seal

Attaching bolts

(use high-strength 

thread locker)

Center cylinder

O-rings

Guide 

seal

Seal

Attaching bolts

(use high-strength 

thread locker)

Center cylinder

O-rings
30 

Figure 53. Center cylinder. 

Figure 54. Oil hole port. 

Main Cylinder Gland 

The main cylinder gland is screwed into the main cylinder tube to guide the center cylinder. It is 

also equipped with a bleed screw to bleed the hydraulics of excess air. Numerous O-rings and 

seals may need replacement 

Figure 55 shows the main cylinder gland and all necessary seals used in this component. 

Replacement of these seals is relatively simple. First, remove the old seals. Second, install the 

O -ring (A), then the internal seals (B) at the same location. Third, install the glide ring seal. (It 

may be necessary to form this seal first.) Fourth, put the outer O-ring into place. Fifth, install the 

dust seal. Finally, install the bleed screw and sealing ball (figure 56). 

Center cylinder

(activates catch 

flange plunger)

Shaft 

(check for nicks, pitting, 

excessive wear, chrome 

peeling or flaking and 

replace if necessary) 

Piston 

Center cylinder

Guide seal 
31 

Figure 55. Main cylinder gland and components. 

Figure 56. Assembled main cylinder gland. 

Internal O-rings (A)

Glide ring seal

Internal seal wiper (B)

(located same as each 

O-ring)

Main cylinder gland

Outer O-ring

Sealing ball

Bleed screw

Dust seal

Internal O-rings (A)

Glide ring seal

Internal seal wiper (B)

(located same as each 

O-ring)

Main cylinder gland

Outer O-ring

Sealing ball

Bleed screw

Dust seal

Glide ring

O-ring installed, 

then internal seal

(set one)

Wiper seal

O-ring under 

internal seal

(set two)

Bleeder set screw

(do not overtighten

when bleeding 

hydraulics)

Glide ring

O-ring installed, 

then internal seal

(set one)

Wiper seal

O-ring under 

internal seal

(set two)

Bleeder set screw

(do not overtighten

when bleeding 

hydraulics)
32 

Catch Flange and Plunger Piston 

The catch flange is screwed into the center cylinder and the release plunger freely floats until 

hydraulic pressure builds up to the release pressure. The upper catch assembly attaches to the 

catch flange. After the weight package is raised to the target height, the release plunger is forced 

up and releases the inner catch collar. Figure 57 shows the O-rings and seals required for this 

component. Figures 58 through 60 show various views of the catch flange. 

Figure 57. Catch flange, release piston, and plunger shaft. 

Figure 58. Top view of catch flange. 

Release plunger shaft

Seal ball 

Bleed screw

Release piston

Outer O-ring

Inner glide seal

Bleed screw 

location for catch 

flange

Catch flange

Locking set screws

(medium-strength 

thread locker)

Inner O-ring

Inner seal

Release plunger shaft

Seal ball 

Bleed screw

Release piston

Outer O-ring

Inner glide seal

Bleed screw 

location for catch 

flange

Catch flange

Locking set screws

(medium-strength 

thread locker)

Inner O-ring

Inner seal

Set screw location

Set screws

(medium-strength thread locker)
33 

Figure 59. Assembled catch flange and 

release piston (bottom view). Figure 60. Assembled catch flange and 

release piston (top view). 

When installing this component, do not use a thread locker. Setscrews are used to secure this 

flange. A pipe wrench can be used to tighten flange to the center cylinder; however, great care 

should be used to avoid marring or damaging the outer surface. This component does not require 

excess force to tighten. 

NOTE: Always install a new outer O-ring if this component has been removed. Also, before 

reinstalling this component, a fine metal file should be used to remove any dimples caused by the 

setscrews on the cylinder. This will help ensure a flush surface and reduce the chance of 

hydraulic leaks. 

Bench Assembly Method 

To bench-assemble the main cylinder components, follow these steps: 

1. Slide the gland over the center cylinder shaft (figure 61). 

Figure 61. Center cylinder and gland. 

Set screws

(medium-strength 

thread locker)

Bottom

A

Top
34 

2. Carefully slide the shaft piston into the center tube. Lower both components into the outer 

tube housing (see figure 51 above and figure 62). 

Figure 62. Inner tube. 

3. Insert the center cylinder and gland into the inner tube, piston first. After this has been done, 

these two components can be inserted into the main cylinder (figure 63). 

Figure 63. Assembled main cylinder. 

4. Insert the inner tube with attached center cylinder and gland into the main cylinder tube. 

Make sure the O-ring is installed around the hub inside the main cylinder (figure 64). This 

O-ring is not installed on replacement tubes from the manufacturer. 

Figure 64. O-ring installed in the main cylinder. 

Holes to 

top

B A

Main cylinder nut

Pressure or inlet 

port

Return or 

discharge port

Main cylinder

Main cylinder gland

Catch flange

Catch release 

piston

Center cylinder

Main cylinder nut

Pressure or inlet 

port

Return or 

discharge port

Main cylinder

Main cylinder gland

Catch flange

Catch release 

piston

Center cylinder

O-ring installed on hub inside main cylinder

(Replacement tubes from manufacturer do 

not have this O-ring installed.)
35 

To assemble the main cylinder on the unit, follow these steps, which are similar to those when 

assembled on a bench: 

1. Install inner tube (figure 62) on to the center cylinder tube, beveled to the bottom and holes 

to the top. 

2. Slide the center cylinder shaft piston into the inner tube (figure 63). Slide the shaft down onto 

the tube until the gland threads are in contact with the outer tube threads. 

3. Use a spanner wrench to tighten the gland. 

4. Secure the main cylinder to the strike plate by using the main cylinder nut (figure 65). Check 

the nut annually and torque to 190-230 N-m (140-170 lbf-ft). Apply a high-strength thread 

locker. 

Catch flange

Center cylinder

Main cylinder

Hydraulic lines

Strike plate

Weight height (WH) 

proximity switch

Subassembly

Trigger (TRG) 

proximity switch

Catch flange

Center cylinder

Main cylinder

Hydraulic lines

Strike plate

Weight height (WH) 

proximity switch

Subassembly

Trigger (TRG) 

proximity switch

Notice the wear on the outer tube. Newer units use a glide ring to prevent such wear. 

Figure 65. Main cylinder attached to strike plate. 
36 

Upper Catch Assembly 

The upper catch assembly grabs the weight package lift collar so that the weight package can be 

raised and dropped. The upper catch is equipped with numerous moving components that will 

wear with use. (figures 66 and 67). It is important to keep all parts lubricated and free of debris, 

which could cause binding or excessive wear. Clean and inspect the moving components 

annually. Replace worn components. 

NOTE: This repair can be made on the FWD without removing the cylinder shaft. 

Figure 66. Upper catch shell and interior components. 

Figure 67. Upper catch shell. 

Top

Inner body shell

Outer shell

Inner catch top cover

Catch shell top cover

Inner ring

(designed to wear 

out)

Catch top cover

M5 ×12 mm hex 

bolts

Ball bearings (6)

Compression 

springs

Heavy-duty spring

Inner plunger 

guide for heavy 

spring flange

Spring retainer 

for heavy spring

Catch shell top cover

M5 × 35 mm hex bolts

M6 × 20 mm hex bolts 

Outer shell

(designed to wear out)

Guide pin

Inner body

M6 × 60 mm hex bolts

Inner ring

(designed to wear 

out)

Catch top cover

M5 ×12 mm hex 

bolts

Ball bearings (6)

Compression 

springs

Heavy-duty spring

Inner plunger 

guide for heavy 

spring flange

Spring retainer 

for heavy spring

Catch shell top cover

M5 × 35 mm hex bolts

M6 × 20 mm hex bolts 

Outer shell

(designed to wear out)

Guide pin

Inner body

M6 × 60 mm hex bolts
37 

After all components are clean, begin reassembly. Following are the steps to assemble the upper 

catch: 

1. Place the inner shell top down on a workbench (figure 68). Make sure the guide pins are 

tight. If they are loose, clean pin threads and use a high-strength thread locker (figure 69). 

Figure 68. Inner body. 

Figure 69. Placement of the guide pins. 

2. Put ball bearings in position by placing the outer shell top down on the workbench and turn 

the inner body top up. Place the six ball bearings in the correct locations, making sure to 

grease each bearing with a liberal amount of light all-purpose grease. Lower the inner shell 

into the outer catch shell (figures 70 and 71). 

Figure 70. Ball bearings in place on the inner body. 

Inner body

Guide pins (3)

Ball bearings 

in place (6)
38 

Figure 71. Inner body set in place. 

3. Place four M6 × 60 mm hex bolts in the four holes on the top of the inner body (figure 72). 

These secure the upper catch assembly to the top of the main cylinder. 

Figure 72. Inner body and hex bolts. 

M6 × 60 mm

hex bolt holes

Inner body set 

in position
39 

4. Place the inner catch top cover on the outer shell. Use a medium-strength thread locker on all 

six M5 × 12 mm hex bolts (figure 73). 

Figure 73. Inner catch top cover installed. 

5. After the cover is installed, turn the assembly over so the top rests on the workbench 

(figure 74). 

Figure 74. Outer shell, top down, with interior components exposed. 

Ball bearings (6)

M6 ×60 mm hex 

bolts

M5 × 35 mm hex 

bolt (2) holes for 

securing upper 

catch assembly 

to the guide 

flange

Guide pins

Ball bearings (6)

M6 ×60 mm hex 

bolts

M5 × 35 mm hex 

bolt (2) holes for 

securing upper 

catch assembly 

to the guide 

flange

Guide pins
40 

6. Put the three compression springs over the guide pins (figure 75). 

Figure 75. Interior of catch assembly with compression springs. 

7. Place the inner ring (figure 76) on the three springs. The inner ring has two sets of holes used 

to rotate the inner ring after it shows signs of wear. Rather than replace the inner ring, it can 

be rotated to the previously unused set of holes for extended life. 

Figure 76. Inner ring. 

One set of holes

Signs of wear
41 

8. Slide the inner ring in position over the springs by picking up the assembly and turning it 

upside down. This allows the ball bearings to release the inner rings and slide over the 

compression springs. The collar then can move to the bottom of the outer shell, allowing the 

ball bearings to retract enough for the inner ring to reach its lowest possible position (figures 

77 and 78). 

Figure 77. Outer shell with inner ring in position. 

Figure 78. Inner ring in position inside the outer shell. 

Inner collar facing up 

toward outer catch 

shell top. This is 

correct orientation.
42 

9. Insert the outer spring retainer over the four M6 × 60 mm hex bolts (figure 79). 

Figure 79. Insert outer flange. 

10. Attach the upper catch to the main cylinder; it takes time and practice to accomplish this. 

With the plunger shaft extended, place the spring guide on top of the plunger. Next, put the 

plunger spring on top of the spring guide (figure 80). In this picture, the main cylinder is not 

attached to the FWD to show how the upper catch is attached. 

Figure 80. Guide flange and spring. 

Flange

Heavy spring

Spring guide

Release piston

Holes for attaching bolts below 

(4)

In this photo the main cylinder is 

not attached to the FWD to better 

show how the upper catch is 

attached.

Heavy spring

Spring guide

Release piston

Holes for attaching bolts below 

(4)

In this photo the main cylinder is 

not attached to the FWD to better 

show how the upper catch is 

attached.
43 

11. Hold the upper catch in your hands and carefully place your fingers at the base of the outer 

flange and inner ring (figure 81). Turn the assembly so the top faces up and the outer flange 

and inner ring slide down. Hold in place until the inner ring is resting on the guide flange 

with the outer catch shell. 

Figure 81. Upper catch. 

12. Place the upper catch over the guide flange and spring. The inner collar should help guide the 

catch over the spring and onto the catch flange. 

13. Tighten the four M6 × 60 mm hex bolts on the catch flange and rotate the upper catch until 

the bolts line up with the threads on the catch flange (figure 82). Do not tighten the bolts 

completely. Next, reach under the catch and collapse it by pushing upward on the collar, 

which contains the ball bearings. 

Figure 82. Attach the upper catch. 

Inner collar

4 bolts attach to 

4 holes above

(medium-strength 

thread locker)

Hex wrench
44 

14. Using an O-ring pick or similar tool, align the two center holes on the inner catch cover with 

the holes on the spring guide (figure 83). Use two M5 × 35 mm hex bolts and mediumstrength thread locker to secure the top (figure 84). After tightening these two bolts, go back 

and tighten the four M6 x 60 mm hex bolts. 

Figure 83. Align top cover. 

Figure 84. Tighten all bolts. 

Align top cover

It may be necessary to rotate 

the catch assembly for proper 

alignment.

Lubricate weekly
45 

15. Place the top cover on the upper catch assembly and secure with two M6 × 20 mm hex bolts, 

then use medium-strength thread locker (figure 85). 

Figure 85. Complete upper catch assembly with cover. 
46 

SIDE CYLINDERS 

Two side cylinders (front and rear) raise and lower the load/strike plate (figure 86). They are 

assembled similar to the main cylinder; therefore, a detailed description of assembly is not 

necessary. 

Figure 86. Side cylinder and components. 

To install the side cylinders after they have been assembled, follow these steps: 

1. Attach hydraulic lines to the fluid ports. The longer line goes to the top fluid port (lower 

plate) and the shorter line attaches to the bottom fluid port (raise plate) (figure 87). 

Figure 87. Mounted side cylinder. 

Flange

Lock collar

Shaft

Fluid port (raise)

Fluid direction 

Outer housing

Flange, O-rings, 

and seats

Piston

Fluid port (lower)

Side cylinder pin 

location

Oil pushes the 

piston downward

Flange

Lock collar

Shaft

Fluid port (raise)

Fluid direction 

Outer housing

Flange, O-rings, 

and seats

Piston

Fluid port (lower)

Side cylinder pin 

location

Oil pushes the 

piston downward

Guide roller shafts

Hydraulic line 

supplies lower 

mode oil flow

Hydraulic line 

supplies upper 

mode oil flow

Side cylinder pin location

(lubricate monthly, check 

bolts monthly for 

tightness)

Side cylinder

Side cylinder shaft 

(extended)

Guide roller shafts

Hydraulic line 

supplies lower 

mode oil flow

Hydraulic line 

supplies upper 

mode oil flow

Side cylinder pin location

(lubricate monthly, check 

bolts monthly for 

tightness)

Side cylinder

Side cylinder shaft 

(extended)
47 

2. Put the cylinder into position on the subassembly frame. The top of the cylinder has a hole 

for the pin to anchor it to the subassembly. Install anchor bolts and apply medium-strength 

thread locker. 

3. Attach the flexible hydraulic lines to the rigid mounted hydraulic lines already in place on the 

subassembly frame. 

4. Put the side cylinder shaft through the strike plate and attach it with a washer and bolt that 

secures it to the strike plate (figures 88 and 89). Use medium-strength thread locker. 

Figure 88. Front side cylinder, looking up, from under FWD. 

Figure 89. Rear side cylinder looking up from under FWD. 

NOTE: The rear-stepped washer is designed to have no contact with the plate low-proximity 

switch when the cylinder is fully retracted. 

5. Attach hydraulic lines. 

Raise/lower pulley 

and cable

(lubricate monthly)

Hydraulic 

lines

Bolt

Washer for 

front side 

cylinder

(mediumstrength 

thread locker)

Raise/lower pulley 

and cable

(lubricate monthly)

Hydraulic 

lines

Bolt

Washer for 

front side 

cylinder

(mediumstrength 

thread locker)

Large washer

Bolt threads into 

side cylinder shaft

(medium-strength 

thread locker)

Plate low proximity 

switch

Large washer

Bolt threads into 

side cylinder shaft

(medium-strength 

thread locker)

Plate low proximity 

switch

49 

CHAPTER 4. STRIKE PLATE, LOAD CELL, 

RAISE/LOWER ASSEMBLY 

Figure 90 shows the strike plate, load cell, and the raise/lower assembly. 

Figure 90. Load cell mounted to the strike plate. 

STRIKE PLATE 

The strike plate (figure 91) requires routine cleaning and semiannual visual inspection of the 

welds. The following components should also be checked annually or as indicated: 

• Load cell and mounting bracket. 

• Center cylinder mounting nut. 

• Hydraulic line mounting clamps. 

• Rear raise/lower bar shaft and mounting bracket. 

• Guide profile rails. Check cap screws every 3 to 6 months. 

Guide rails

Strike plate

Load cell

Manual 

transport locks 

Hydraulic line 

mounting clamp

Guide rails

Strike plate

Load cell

Manual 

transport locks 

Hydraulic line 

mounting clamp
50 

Figure 91. Strike plate. 

The load cell swivel and retaining plate are located on the bottom of the strike plate (figure 92). 

Figure 88 shows a different view of the raise/lower car mounting bracket. Check bolts annually 

and apply medium-strength thread locker. Annually remove the load cell swivel and clean. 

Figure 92. Bottom view of load cell swivel and retaining bracket on strike plate. 

LOAD CELL 

The load cell attaches to the strike plate, and the load cell swivel is pressed in to fit in the strike 

plate, making a snug fit. The following illustrations show the steps taken during a major overhaul 

of a load cell: 

Center cylinder 

mounts here

Hole for center 

cylinder

Rear raise/lower bar 

mounting bracket

Holes for 

raise/lower guide 

rails

Hole for side 

cylinder

Holes for load 

cell attachment 

brackets

Center cylinder 

mounts here

Hole for center 

cylinder

Rear raise/lower bar 

mounting bracket

Holes for 

raise/lower guide 

rails

Hole for side 

cylinder

Holes for load 

cell attachment 

brackets

Strike plate

Holes for bolts to 

secure retaining 

plate

(mediumstrength thread 

locker)

Load cell swivel

(remove and clean 

yearly)

Retaining plate

Strike plate

Holes for bolts to 

secure retaining 

plate

(mediumstrength thread 

locker)

Load cell swivel

(remove and clean 

yearly)

Retaining plate
51 

1. Place the load cell base down (figure 93). 

Figure 93. Load plate (base down). 

2. Attach the mounting brackets (figures 94 and 95). The brackets should be checked annually. 

Make sure all bolts are in place and the rubber is not torn. 

Figure 94. Load plate with mounting brackets. 

Holes (4) for attaching 

brackets

Holes (3) for PVC plate 

under load cell

Collar for attaching strike 

plate

Load DIN plug connector 

location

Holes (4) for attaching 

brackets

Holes (3) for PVC plate 

under load cell

Collar for attaching strike 

plate

Load DIN plug connector 

location

Load cell

Attaching 

bolts

(mediumstrength 

thread 

locker)

Brackets (4)

Load cell

Attaching 

bolts

(mediumstrength 

thread 

locker)

Brackets (4)
52 

Figure 95. Load cell with brackets mounted. 

3. Attach the load cell to the strike plate by using a guide tool to center the load cell on the 

swivel collar, which is essential for correct alignment (figure 96). Grease the load cell swivel 

monthly. It does not require a large amount of grease; one or two pumps from a hand-held 

grease gun with high-grade grease are sufficient. Failure to grease the load cell swivel can 

result in load cell calibration errors. 

Figure 96. Mounting brackets and lock flange attached to strike plate. 

4. Assemble the rubber pad and PVC disk before installing them on the load cell (figure 97). 

Use a strong adhesive to secure the rubber pad to the PVC disk before screwing the PVC disk 

to the load cell. Secure the PVC disk and rubber pad to the bottom of the load cell (figure 

98). 

NOTE: Photos here are part of a major overhaul. It is recommended that if a load cell 

replacement is necessary that the rubber pad and PVC disk be put together before installing on 

the load cell. After replacing a load cell, conduct a full, absolute calibration. 

Lock collar applied 

after load cell is 

pressed into swivel 

and mounted to 

strike plate

Lock collar

Load cell swivel grease fitting 

using a high quality grease 

once a month (only one or 

two pumps with a grease 

gun)

Screws (6)

(mediumstrength 

thread locker)

Brackets (4)

(mediumstrength 

thread locker)

Lock collar

Load cell swivel grease fitting 

using a high quality grease 

once a month (only one or 

two pumps with a grease 

gun)

Screws (6)

(mediumstrength 

thread locker)

Brackets (4)

(mediumstrength 

thread locker)
53 

5. Attach the rear raise/lower bar guide to the front of the strike plate. The bottom screw tends 

to work loose; check it annually. Use a high-strength thread locker to secure the screw. The 

bottom screw can be difficult to remove. It may be necessary to apply heat to the bolt to 

remove it. 

Figure 97. Strike plate with attached load cell and pad (shown inverted). 

Figure 98. Adhesive applied to PVC disk. 

Adhesive

Rubber pad

PVC disk

Bolts that attach PVC disk

Load cell

Attaching brackets

Strike plate

Rubber pad

PVC disk

Bolts that attach PVC disk

Load cell

Attaching brackets

Strike plate
54 

RAISE/LOWER BAR 

The raise/lower bar guide raises and lowers the geophone rail to the ground. The raise/lower box 

sleeve in the front of the trailer guides the front end (figure 99). 

Figure 99. Raise/lower bar guide location before assembly(rear). 

The rear end of the raise/lower bar attaches to the rear lift nut (figure 100). 

Figure 100. Raise/lower bar guide during assembly. 

Front raise/lower 

guide rail

Hole for 

raise/lower bar 

guide assembly

Center cylinder

Check 4 bolts 

annually

Hydraulic lines

Hydraulic lines 

and clamp

(check every 3 to 

6 months)

Front raise/lower 

guide rail

Hole for 

raise/lower bar 

guide assembly

Center cylinder

Check 4 bolts 

annually

Hydraulic lines

Hydraulic lines 

and clamp

(check every 3 to 

6 months)

Top attaching bolt and 

washer 

(medium-strength thread 

locker)

Guide shaft

(lubricate weekly)

Raise/lower bar 

attaching nut

Bottom screw

(high-strength thread 

locker)

(check fastener yearly, 

remove and clean 

threads on bolt, reapply 

high-strength thread 

locker)

Top attaching bolt and 

washer 

(medium-strength thread 

locker)

Guide shaft

(lubricate weekly)

Raise/lower bar 

attaching nut

Bottom screw

(high-strength thread 

locker)

(check fastener yearly, 

remove and clean 

threads on bolt, reapply 

high-strength thread 

locker)
55 

A steel cable and pulleys raise and lower the bar (figure 101). Lubricate the front raise/lower 

pulleys monthly and check the cable every 3 to 6 months for damage. Replace it if damaged. 

Note: The bottom screw tends to work loose so it is a good idea to check annually. Use high 

strength lock tight to secure the screw. 

Figure 101. Raise/lower bar guide in location. 

Figure 102 shows the raise/lower bar removed from the FWD, and figure 103 shows the center 

lift nut and attaching bolts. Little maintenance is required. Yearly, remove the attaching bolts on 

the rear, clean, and reapply with medium-strength thread locker. 

Figure 102. Raise/lower bar. 

Center cylinder 

hydraulic lines

Center lift nut

Tab and bottom 

screw

(Check every 6 

months for 

looseness; if 

loose, remove, 

clean, replace, 

apply highstrength thread 

locker.)

Strike plate

Load cell

PVC disk

Ribbed rubber sheet

Center cylinder 

hydraulic lines

Center lift nut

Tab and bottom 

screw

(Check every 6 

months for 

looseness; if 

loose, remove, 

clean, replace, 

apply highstrength thread 

locker.)

Strike plate

Load cell

PVC disk

Ribbed rubber sheet

Guide rod

(lubricate weekly)

Raise/lower bar

Center nut

Attaching bolts

(yearly apply 

thread locker)

Guide rod

(lubricate weekly)

Raise/lower bar

Center nut

Attaching bolts

(yearly apply 

thread locker)
56 

Figure 103. Center lift nut and attaching bolts. 

Following are the steps to assemble the raise/lower bar: 

1. Install the center lift nut over the guide shaft. Place the guide shaft between the strike plate 

and the lower tab as seen in figure 101. After installing this component, assemble the 

raise/lower bar. Align the hole in the rear bar with the hole in the rear lift nut. Apply 

medium-strength thread locker to the bolt. Repeat on the opposite side. 

2. Attach the cable-pulley configuration to the trailer frame (figures 104). Attach a steel cable 

and run it through two pulleys (figure 105). Lubricate the pulleys monthly with silicone 

spray. Periodically remove the cover and inspect components for damage or excess wear, and 

repair or replace as necessary. 

Figure 104. Closeup of cable connection. 

Hole in bar

(lubricate weekly)

Guide rod goes 

through steel sleeve 

on raise/lower box

Pulley

(Lubricate rollers 

monthly; every 6 

months check 

pulleys for damage 

or excessive wear.)

Raise/lower cable

Rails

Set screw

Red arrows indicate

cable routing

Guide rod goes 

through steel sleeve 

on raise/lower box

Pulley

(Lubricate rollers 

monthly; every 6 

months check 

pulleys for damage 

or excessive wear.)

Raise/lower cable

Rails

Set screw

Red arrows indicate

cable routing
57 

Figure 105. Raise/lower bar, guide rod, and attaching cable. 

3. Place stainless steel cover over guide rod (figure 106). Make sure the pin is in place during 

transport. 

Figure 106. Raise/lower cover. 

4. Attach the cable to the top of the rails with straps (figure 107). 

Figure 107. Attached cable. 

Stainless steel 

cover

Guide rod

Raise/lower rails

Pulley

Transport pin
58 

5. Attach the tension spring (figure 108). The tension spring helps take up cable slack on the top 

of the raise/lower bar when the bar is fully raised. 

Figure 108. Mounted geophone holders to raise/lower bar. 

6. The geophone holders are screwed into specific locations along the rails, although the 

locations can be adjusted. Mount the geophone holders to the raise/lower bar and adjust 

position (figures 109 and 110). 

Figure 109. Completed assembly of raise/lower bar. 

Rails

Tension spring

Geophone holder

Mounting bracket
59 

Figure 110. Rear view of raise/lower assembly. 

GEOPHONE HOLDER 

The geophone holder attaches to the raise/lower bar on the FWD. The purpose of the geophone 

holder is to provide a means of adjusting the geophones to various distances from the load plate. 

Figure 112 shows geophone holder components. Upon inspection, if holder springs are weak or 

damaged replace only as a set, not individually, to maintain proper seating pressure. 

Figure 111. Geophone holder components. 

Foam guide

(lubricate weekly)

Geophone holder top bracket

Geophone holding springs

Geophone clamping disk

(check monthly and clean with a 

wire brush if necessary)

Bolts to hold 

geophone holder to 

rail

Clamping bolts

Geophone housing

Foam guide

(lubricate weekly)

Geophone holder top bracket

Geophone holding springs

Geophone clamping disk

(check monthly and clean with a 

wire brush if necessary)

Bolts to hold 

geophone holder to 

rail

Clamping bolts

Geophone housing

Grease fitting

(grease monthly)

Note position of 

geophone. It is for 

LTPP testing. Geo #3 

is located 305 mm (12 

inches) from the center 

of the load cell to the 

center of the holder.

Grease fitting

(grease monthly)

Note position of 

geophone. It is for 

LTPP testing. Geo #3 

is located 305 mm (12 

inches) from the center 

of the load cell to the 

center of the holder.
60 

Geophones are preassembled. Figure 112 shows an assembled geophone holder for an 80 mil 

(2,032 microns) standard geophone. 

Figure 112. Assembled geophone holder. 

Figure 113 shows the components for the geophone. 

Geophone 

Electric cable

Geophone housing

Exterior base 

plate

Interior base

Clamping ring and 

spacer ring

Magnet

Strap/nut Screws

Magnet screw

Cable grommet

Ground lug with screw 

and nut

DIN plug

DIN plug lock

Geophone 

Electric cable

Geophone housing

Exterior base 

plate

Interior base

Clamping ring and 

spacer ring

Magnet

Strap/nut Screws

Magnet screw

Cable grommet

Ground lug with screw 

and nut

DIN plug

DIN plug lock

Figure 113. Geophone components. 

Lubricate monthly with 

low-viscosity lubricant. Check springs 

monthly; weak or 

damaged springs 

can cause 

geophone to have 

inconsistent set 

pressure, which in 

turn causes 

variability in 

geophone 

readings.

Lubricate monthly with 

low-viscosity lubricant. Check springs 

monthly; weak or 

damaged springs 

can cause 

geophone to have 

inconsistent set 

pressure, which in 

turn causes 

variability in 

geophone 

readings.
61 

Use the following steps to assemble your own geophone rather than using a preassembled one: 

1. Mount the ground ring to the geophone (figures 114 and 115). 

Figure 114. Ground ring and geophone. 

Opening for 

geophone cable

Figure 115. Mounted ground ring. 

2. Prepare the geophone cable for assembly. Use a sharp art knife to split the two-strand wire 

and shield. Separate the wires to solder them onto the geophone (figures 116 and 117). 

Figure 116. New geophone cable. 

Geophone

Ground ring

Grommet

Geophone 

cable
62 

 

Figure 117. Separated wire. 

3. Twist the wire shield and attach to the ground lug (figure 118). 

Figure 118. Attached ground lug. 

4. Solder the positive and negative wires to the appropriate prongs on the geophone 

(figure 119). Screw the ground lug into the side to the ground ring. 

Figure 119. Geophone cable soldered to geophone. 

Negative wire

Shield

Positive wire

Prongs for wire

Ground lug 

attached with 

screw

Prongs (2)

Positive/negative

Ground ring

Geophone

Cable and 

grommet

Ground lug 

attached with 

screw

Prongs (2)

Positive/negative

Ground ring

Geophone

Cable and 

grommet

Ground lug
63 

5. Place the geophone in the geophone housing and fill the internal area with electronics-grade 

silicone (figure 120). Install the strap. 

Figure 120. Silicone-filled housing. 

6. Place the plastic base cover on the geophone (figure 121). 

Figure 121. Base cover on housing. 

7. Attach the steel base plate, which will secure the magnet to the geophone housing 

(figure 122). 

Figure 122. Geophone with base plate attached. 

Strap

Silicone

Housing

Cable

Base cover

Center nut 

to secure magnet

Spacing ring

Screws

(high-strength 

thread locker)

Housing

Steel base plate

Center nut 

to secure magnet

Spacing ring

Screws

(high-strength 

thread locker)

Housing

Center nut 

to secure magnet

Spacing ring

Screws

(high-strength 

thread locker)

Housing

Steel base plate
64 

8. Attach the magnet with a screw and medium-strength thread locker (figures 123 and 124). 

Figure 123. Attached magnet. 

Figure 124. Completely assembled geophone. 

9. Pay attention to the DIN (Deutsches Institut fur Normung eV) plug connector at the other end 

of the geophone cable (figure 125). Attach the DIN plug housing to the end of the cable. 

Figure 125. Raw cable and DIN plug housing. 

Screw with 

thread locker

Magnet

Housing

Outer housing 

(first piece)

Raw cable
65 

10. Solder wires to the internal component of the DIN plug (figure 126). 

Figure 126. Internal view of the DIN plug. 

11. Attach the second half of the DIN plug outer housing to the plug (figure 127). 

Figure 127. Completed DIN plug. 

After the geophone is assembled, plug the geophone into the correct DIN connector and run a 

drift screen check to ensure they are functioning properly). Follow by conducting a reference 

calibration. 

Outer housing

(second piece)

Soldered 

wire

67 

CHAPTER 5. ELECTRONICS 

PROXIMITY SWITCHES 

There are four proximity switches (figure 128): 

• Weight height (WH) proximity switch sensor is activated by close presence of any of the 

falling weight stops (located in a rail on the drop weight). The (red) WH LED must turn on at 

each passing of a falling height stop. 

• Trigger (TRG) proximity switch sensor is activated when the drop weight is less than 6 to 

12 mm (0.25 to 0.50 inch) from its lower, resting position. The (green) TRG LED must be on

when the weight is low and it must turn off when the weight has been raised 6 to 12 mm 

(0.25 to 0.50 inch). 

• Plate low (PL) proximity switch sensor is deactivated when the falling weight subassembly is 

no longer suspended from the tow shafts of the side cylinders. This is normally when the 

plate has come to rest on the ground, but it may occur if the subassembly is prevented from 

being lowered, such as if the transport locks have are still locked. The (yellow) PL LED must 

turn OFF when the plate is low. 

• Plate high (PH) proximity switch sensor is activated only when the plate (i.e., the falling 

weight assembly) is close to its highest position. The (green) PH LED must be on when the 

plate is high. It also must be on when the subassembly is resting on the transport locks in 

either or both sides. 

Figure 128. Four types of proximity switches. 

ELECTRICAL CONNECTION BOX 

Check electrical connections and clean annually (figure 129). Refer to the manufacturers wiring 

schematics and owner’s manual for further information on the wiring diagram. 

WH proximity switch

TRG proximity switch

PH proximity 

switch

PL proximity 

switch

(underside of 

strike plate)

WH proximity switch

TRG proximity switch

PH proximity 

switch

PL proximity 

switch

(underside of 

strike plate)
68 

Figure 129. Electronics connection box. 

SYSTEM PROCESSOR 

Annually remove the system processor (figure 130) and clean all internal components. 

Periodically check connections in the rear of the processor (figure 131) to make sure they are 

secure and no strain is put on the cables. Also, check that the processor is securely mounted to 

the vehicle. Refer to the manufacturer’s owner’s manual for repairs and maintenance beyond the 

routine. 

Figure 130. System processor. 
69 

Figure 131. Rear view of vertically mounted system processor. 

DMI PROXIMITY SWITCH 

The vehicle-mounted proximity switch is a distance-measuring instrument (DMI) (figure 132). It 

should be checked monthly to make sure the correct air gap is used. Stainless switches can have 

a sensor gap of up to 8 mm (0.315 inch). The yellow plastic switches can have a gap of up to 

5 mm (0.197 inch). 

Monthly check the wiring and switches to make sure there is no strain on the cable. Visually 

check the switch for damage and replace if necessary. Recalibrate after repairs are made. 

Check connections and make sure 

cables are routed correctly so there 

is no strain on them.
70 

Figure 132. Vehicle-mounted proximity switches. 

VEHICLE CHARGE PLUG CONNECTION 

Monthly clean the power supply connection between the vehicle and trailer (figure 133). Also, 

check that the connection is snug to make good contact between the two plugs (figure 134). 

Figure 133. Vehicle charge plug. 
71 

Figure 134. Trailer-to-vehicle charge plug. 

POWER CONTROL BOARD 

The power control board is located behind the panel shown in figure 135. Remove the front 

panel and inspect all connections, making sure they are tight. Clean all connections annually. 

Figure 135. Power control box (board behind front panel). 
72 

FUSE BLOCKS 

Figure 136 shows the trailer fuse block. Two more fuse blocks are located inside the vehicle. All 

three fuse blocks should be inspected and cleaned monthly. 

Figure 136. Trailer fuse block. 

ELECTRONICS BATTERY 

The electronics battery is located inside the vehicle (figure 137). Check that it is securely 

mounted to the vehicle floorboard, and check and clean the connections monthly. Also, check the 

water level inside the battery. If water needs to be added, use distilled water. 

Figure 137. Electronics battery inside the vehicle. 
73 

CHAPTER 6. CONCLUSION 

Maintaining and regularly servicing FWDs is crucial to equipment performance, longevity, and 

quality of acquired data. Routine maintenance provides insurance against expensive repairs and 

parts replacement, which can also result in expensive downtime. The information in this 

document is to help people who are responsible for ownership, management, operation, or 

service of FWDs. This document is particularly useful to people who use FWDs in highway 

infrastructure construction, rehabilitation, maintenance, and management. 

While the contents of this document directly apply only to the Dynatest Model 8000 in use by 

the LTPP program, the instructions can also help on some earlier or later models, and even other 

brands of FWDs. Troubleshooting and maintenance advice for the tow vehicle were excluded 

from this document. A variety of tow vehicles perform the same function, each having its own 

variables. 

75 

APPENDIX: SCHEDULED MAINTENANCE LOG 
76

SCHEDULED MAINTENANCE LOG FOR FWD TRAILER

 (Revised: 12 July 2005) 

DateOwnerModel No.Serial No.Hour meter/Drop counter - Incoming, Outgoing Date Insp. 

Initls.

 Maintenance Required Maint. 

Performed

Page No./ Figure No.

Hydraulic System and Related Components: Change hydraulic oil/filter Annually P-26/F-44  Inspect rubber pad on load cell Daily P-53/F-97 Grease load cell Monthly P-52/F-96 Lubricate guide rollers Weekly P-23/F-39 Visual inspection of excess pressure Daily P-26/F-44 Inspect load cell swivel operation Monthly P-50/F-92 Inspect load cell attaching brackets Monthly P-51/F-94 Lubricate upper catch assembly Weekly P-44/F-84 Rotate catch assembly Daily P-45/F-85 Check hydraulic oil level Daily P-15/F-25 Check guide profiles and shims Annually P-19/F-32 Check side cylinder bolts Every 6 

months 

P-46/F-87 

 Check center cylinder for leaks Monthly P-35/F-65 Check A/B valve for leaks Monthly P-26/F-44  Check hydraulic lines and connections for leaks Monthly P-24/F-40 
 SCHEDULED MAINTENANCE LOG FOR FWD TRAILER 

 (Revised: 12 July 2005) 

Date

OwnerModel No.Serial No.Hour meter/Drop counter - Incoming, Outgoing

77

 Date Insp. 

Initls.

 Maintenance Required Maint. 

Performed

Page No./ Figure No.

Hydraulic System and Related Components (Continued): Inspect side cylinder for leaks Monthly P-46/F-86 Inspect strike plate hardware Monthly P-49/F-90 Check center cylinder to strike plate nut Annually P-29/F-51 Check condition of buffers Monthly P-23/F-39
 SCHEDULED MAINTENANCE LOG FOR FWD TRAILER 

 (Revised: 12 July 2005) 

Date

OwnerModel No.Serial No.Hour meter/Drop counter - Incoming, Outgoing

78

Electronics/Electrical Components: Inspect trailer battery - Clean terminals/check water level Monthly P-18/F-28  Inspect electronics battery (in vehicle) Monthly P-72/F-137 Clean ground terminals (two in trailer) Monthly P-3/F-1 and 

P-4/F-2

 Inspect four proximity switches Monthly P-67/F-129 Inspect PS1/PS2 switches Monthly P-27/F-46  Clean fuse block terminals (trailer) Every 3 

months 

P-72/F-137 

 Inspect hydraulic motor brushes Every 6 

months 

P-25/F-43 

 Check charging plug from vehicle to trailer connection Every 3 

months 

P-70/F-134 an135 

 Check power control board connections Every 6 

months 

P-18/F-29 

 Check seismic sensor connections Monthly P-71/F-136 
 SCHEDULED MAINTENANCE LOG FOR FWD TRAILER 

 (Revised: 12 July 2005) 

Date

OwnerModel No.Serial No.Hour meter/Drop counter - Incoming, Outgoing

79

Electronics/Electrical Components (Continued): Check air and infra-red sensor connections (optional 

equipment) 

Monthly —

 Clean and inspect multisignal cable Every 3 

months 

—

 Clean inside of 9000 processor Annually P-68-69/ 

F-131 and 132 

 Check air-gap vehicle-mounted DMI Annually P-70/F-133  Clean connectors in electrical control box Annually P-68/F-130Raise/Lower Bar Assembly: Lubricate front pulley wheels Monthly P-57/F-105 Lubricate rear pulley wheels Monthly P-47/F-88 Check rut bar depth - adjust Annually P-59/F-109 Check geophone holders Monthly P-59/F-110 Check geophone holder springs Monthly P-61/F-113  Check geophone clamping disk Monthly P-60/F-112
 SCHEDULED MAINTENANCE LOG FOR FWD TRAILER 

 (Revised: 12 July 2005) 

Date

OwnerModel No.Serial No.Hour meter/Drop counter - Incoming, Outgoing

80

 Date Insp. 

Initls.

 Maintenance Required Maint. 

Performed

Page No./ Figure No.

Raise/Lower Bar Assembly (Continued): Lubricate foam guide Monthly P-60/F-112 Raise/lower bar cable (inspect) Monthly P-57/F-105 Raise/lower bar rear guide and bolt Annually P-55/F-102 
Recycled

Recyclable HRDI-13/12-06(1M)E