11 ~ Recommended Practice 

RP 239A VMRS 017 

COMMERCIAL VEHICLE TIRE INFLATION AND/OR 

MONITORING SYSTEMS GUIDELINES 

PREFACE 

The following Recommended Practice is subject to 

the Disclaimer at the front of TMC's Recommended 

Engineering Practices Manual. Users are urged to 

read the Disclaimer before considering adoption of 

any portion of this Recommended Practice. 

PURPOSE AND SCOPE 

This Recommended Practice (AP) covers current 

capabilities and basic operation of tire inflation and 

pressure monitoring systems. It is designed to help 

fleets select tire inflation and monitoring systems 

for commercial truck tires. This RP applies to tire 

pressure equalizers, electronic and mechanical tire 

pressure monitoring and maintenance devices as 

well as central tire inflation type systems. 

INTRODUCTION 

Maintaining proper tire inflation will maximize tire life 

and casing durability. This results in reduced overall 

tire costs, downtime, tire replacement, irregular wear, 

wheel replacement, road debris and the natural resources required to manufacture tires and retreads. 

Correct inflation will increase benefrts such as fuel 

efficiency, safety, driver retention and uptime, all of 

which have a direct effect on cost per mile. 

The following sections will explore the basic characteristics of the most common types of systems 

currently available. While these systems may reduce 

tire labor, it is still necessary to periodically inspect 

tires to ensure they are serviceable, properly inflated 

and the systems are working correctly. All of these 

systems need to be properly installed and maintained 

to deliver the benefits they provide. 

I. Equalizing SystemsDual Tire Pressure Equalizers 

Basic System Operation 

Tire pressure equalizers are designed to maintain 

the same pressure between two tires of a dual assembly and indicate the state of inflation pressure in 

the tires. By equalizing tire pressure overall individual 

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tire performance and cosVmile are optimized. As a 

result improper load distribution and consequently 

irregular tire wear may be reduced. 

Tire pressure equalizer systems monitor and equalize 

tire pressure by allowing air to transfer from one tire 

to another. This is accomplished by the use of two 

hoses that are plumbed to a check valve and each 

attaches to the valve stem of a dual tire. The check 

valve opens to allow airflow between the tires but 

closes and shuts the air off in case of an instantaneous 

air loss, which prevents both tires from going flat. In a 

slow leak situation, the valve isolates both tires after 

a pressure drop of approximately 1 0 psi. 

Since a central valve is located in these devices, a 

single airing point is provided that eliminates the need 

to remove the hoses to add air to the tires. 

These systems also provide a visual indicator of 

air pressure conditions. They do not provide actual 

inflation pressures but provide different types of 

go/no-go gauges that can indicate whether the tires 

are underinflated, overinflated or at the correct pressure. The operating pressure of equalizers is set at 

the factory. 

Typical Maintenance Requirements 

This equipment should be inspected to ensure hoses 

are intact and attached to valve stems and that the 

valve and display are also intact. Periodically pressures should be checked with a calibrated air gauge 

to ensure the equalizer is still operating accurately. 

Training Requirements 

Drivers and technicians must be trained to read and 

understand the displays on the axle end. 

II. Tire Pressure Monitoring System (TPMS)-

A. Mechanical 

Basic System Operation 

All of these systems attach to the valve stem and 

monitor tire pressure through a mechanical device 

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Issued 312007 

Revised 512014 
that works much like an air gauge. They provide indicators or visual displays on the valve stems. There 

is no display or alarm in the cab since these devices 

do not have the capability oftransmitting information. 

The pressure range the monitor operates in is set 

at the factory. 

The primary difference between mechanical devices 

is the display on the valve end. Some products provide 

a dial gauge that indicates exact pressure, others 

provide a valve cap type of package that indicates 

the tire's pressure or pressure range. 

Maintenance Requirements 

It is critical to periodically check tire pressure with a 

calibrated air gauge to determine if the monitors are 

working correctly. These devices must be replaced 

if found to be out of calibration. 

Training Requirements 

Drivers and technicians must be trained to read and 

understand the displays on the valve stems. 

B. Electronic 

Electronic tire pressure monitoring systems utilize 

electronic sensors that monitor tire temperature 

and/or pressure. These sensors can be mounted on 

the wheel, at the external end of the valve stem, at 

the base of the valve stem, or on the tire innerliner. 

They use radio frequency to transmit tire data to the 

receiver included in the system. Vehicle application 

and the number and types of users of the tire data 

must be considered when selecting a TPMS. The type 

of data transmission also needs to be considered. 

There are three types of electronic TPMS: 

a. Basic TPMS- Basic electronic TPMS provides an in-cab display which delivers audible 

and/or visual alerts to the driver when a tire 

problem occurs while the vehicle is in motion. 

Basic TPMS alerts can be as simple as illuminating a warning light or as sophisticated as 

providing the driver with a schematic of the 

vehicle's tires and pressure and temperature 

values. The driver or technician can see the 

current tire inflation pressure on the display 

when the ignition is turned on, eliminating the 

need to manually check tire pressures before 

the trip. Some systems that monitor tire temperature may provide cold inflation pressure, 

but not all systems do. (See TMC RP235A, 

Guidelines tor Ttre Inflation Pressure Maintenance Guidelines.) The technician can set the 

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target pressure by using the in-cab display in 

conjunction with adjusting tire pressure. 

b. Remote-reading TPM5-This type of TPMS 

is linked to the vehicle databus, telematics 

or infrared data transfer systems for remote 

reading. These types of systems send data 

on all or some of the tires on the vehicle to a 

telematics system that communicates the data 

via satellite or cellular network to a remote 

location for analysis. Alerts can then be sent 

via e-mail and remote access to tire status is 

obtained through the internet. These configurations may also provide an in-cab display for 

the driver that provides audible and/or visual 

alerts. Changes in target pressure may be 

made over the internet or may have to be made 

on the vehicle, depending upon the system. 

c. Off-board Gate, Lane or Handheld Reading 

TPMS-This system type utilizes off-board 

gate, lane, or handheld readers that provide 

interval monitoring of tire pressures and temperatures at static or low vehicle speeds. It initiates data transmission to the various readers 

and/or data storage devices. In-cab displays 

may not be included with these systems. 

Maintenance Requirements 

Sensors should always be installed so that they 

are positioned with the valve stem. Care must be 

taken when mounting and demounting tires so that 

sensors attached to the wheel or at the base of the 

valve stem are not damaged. As a general rule, 

liquid tire additives (sealants, balancers, coolants) 

should not be used with TPMS sensors used inside 

the tire chamber. Check with the manufacturers of 

other additive products to ensure these products 

will not interfere with TPMS sensor operation. Most 

electronic systems self-calibrate and diagnose system problems. Follow manufacturer instructions for 

maintenance of your respective TPMS. 

Training Requirements 

Drivers must be instructed in the company's policy 

when tire alerts are triggered. This can include 

continuing travel to the company's yard, stopping 

at the nearest tire service location, or stopping immediately and acquiring emergency road service. 

Maintenance personnel should also be instructed 

on how to respond to alerts and any reports that are 

generated by the TPMS. lire technicians must be 

trained to properly install sensors, mount and demount 

tires, and move sensors so that the TPMS receivers 

RP239A-2 
recognize the wheel positions on which the sensors 

are installed. If the tire pressure monitoring solution 

includes software applications that must be used in 

conjunction with the TPMS, then technicians must be 

trained to use them as well. Detailed maintenance 

instructions and training should be provided by the 

system manufacturer. 

Ill. Tire Inflation Systems 

(Monitors, inflates, and /or deflates) 

Basic System Operation 

Tire inflation systems take the air that is stored in the 

air tanks on a vehicle and use it to supply air to the 

tires. This can be done on demand or automatically 

triggered through sensors that monitor tire pressure. 

These systems can be broken down into two types: 

automatic and variable. 

Automatic tire inflation systems maintain tire pressure at a single preset level and are pneumatically 

or electronically activated. Electronically activated 

systems use an ECU to measure tire pressures at 

regular intervals and inflate the tires as required. 

Pneumatically operated systems keep tires inflated 

by maintaining a predetermined air pressure continuously applied to the tires. A warning lamp indicates 

to the driver when tire or system service is required. 

These systems eliminate the need to manually 

inflate tires. Automatic tire inflation systems have 

no operational involvement from the driver. They 

automatically sense the pressure in the tires and 

inflate as necessary. 

Variable- or driver-actuated tire inflation systems 

can raise or lower tire pressures during vehicle 

operation to compensate for varying load and road 

conditions in addition to maintaining tire pressure. 

These systems allow the driver to interact with the 

system and change tire pressure on demand. They 

are electronically activated and must have battery 

power to the ECU for them to function and to retrieve 

fault codes. These systems are usually used for onoff road operations. 

Tire inflation systems use compressed air from the 

vehicle's air system. They transfer this air to a frame 

mounted control unit. Air is then delivered to a wheel 

end rotary union that distributes the air to the tire 

valve stems. 

Different air delivery methods exist in these systems. 

Some systems use the axle tube as a pressure conC 2014- TMC/ATA 

duit, some have a dedicated airline in the axle, and 

others have outboard plumbing. 

All tire inflation systems must be vented to prevent 

hub pressurization in the event of a component failure. Some systems vent air at the wheel ends while 

others vent remotely. 

One tire never supplies air to another tire, leaving 

the systems to maintain correct air pressure settings throughout. Current tire inflation systems have 

safety devices built into them. A pressure protection 

valve (PPV) is required in these systems to ensure 

a rapid loss of tire pressure will not drain the vehicle 

air system below a safe pressure required for proper 

braking. If a significant loss of air should occur in a 

tire or the inflation system itself, the remaining tires 

are protected from air loss by check valves in the 

air hoses. 

Some tire inflation systems have the capability to 

communicate to an off vehicle data collection station via communication devices already in service 

as specified by the fleet. These off-vehicle signals 

can be via satellite, cellular or other remote-based 

communication systems. It is the responsibility of 

the data collection provider to have the decoding 

software for integrating the systems. 

Maintenance Requirements 

Recommended inspection procedures need to be 

performed on tire inflation system equipped vehicles 

in conjunction with a fleet's scheduled preventive 

maintenance inspections. It is still necessary to 

periodically inspect tires to ensure they are serviceable, properly inflated and the systems are working 

correctly. These checks will allow the technicians to 

determine whether the system is functioning correctly, 

if tires have leaks or other out-of-service conditions, 

and measure tread depth. 

Training Requirements 

Drivers need to be trained regarding the meaning/operation of the tire inflation system indicator/warning 

light and their expected response. Technicians must 

be trained to set and verify the proper tire pressure 

target and correctly maintain the system. 

Appropriate personnel should be instructed on how 

to recognize four basic items about the system: 

1. How to inspect the system for basic component 

integrity and leaks throughout. 

2. How to properly dismantle and reassemble 

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