Traction control system for use with four wheel drive vehicles having on-demand transfer cases

A traction control system for a four wheel drive vehicle including generating a first value corresponding to an average rotational speed of front wheels, generating a second value corresponding to an average of a rotational speed of rear wheels, comparing the first and second values, and generating a first signal corresponding to a difference between the first and second values, are provided. The first signal is used for transferring torque to the secondary axle if the rear wheels are rotating faster than the front wheels. The system also generates a second signal corresponding to a difference in speed between a first axle shaft and a second axle shaft of the front axle. The second signal is used for transferring torque to the other of the front axle shafts if the one of the secondary axle shafts is rotating faster than the other of the front axle shafts.

FIELD OF THE INVENTION 
The present invention relates to traction control systems in four wheel 
drive vehicles, and more particularly, to traction control systems for use 
with on-demand four wheel drive systems. 
BACKGROUND OF THE INVENTION 
Traction control systems are commonly integrated into anti-lock braking 
systems. Anti-lock braking systems typically modulate the pressure of 
hydraulic fluid delivered to a vehicle wheel brake to prevent the vehicle 
wheel from locking up in the braking condition. Conversely, a traction 
control system, when integrated into the anti-lock braking system, 
actuates the brakes to prevent spinning of a vehicle wheel, thereby 
maximizing the traction which can be exerted by that wheel. 
On-demand four wheel drive systems enable an automatic switching from two 
wheel drive used during normal operating conditions to four wheel drive 
responsive to slippage of the primary axle used for two wheel drive. 
On-demand four wheel drive systems can also be placed in the four wheel 
drive mode by the driver actively selecting that mode with a switch or 
button. 
Known traction control systems for four wheel drive vehicles operate 
independently of engagement of the secondary axle through a transfer case. 
The benefit of traction control in preventing slipping, and maintaining 
the stability of vehicle handling under slippery conditions, for a four 
wheel drive vehicle is greatly reduced when the four wheel drive system is 
operating in a two wheel drive mode, particularly if the rear wheels are 
being driven in the two wheel drive mode. 
Four sensor anti-lock systems employ a wheel rotation speed sensor at each 
of the four vehicle wheels, enabling individual braking control of each 
wheel. Three sensor anti-lock braking systems are a less expensive 
alternative to the four sensor systems. With three sensor systems, each of 
the two front wheels in the vehicle has a wheel speed sensor, and a third 
sensor is used to monitor the speed of the rear drive shaft. Given that 
the rolling radius of the vehicle wheels and the final drive axle ratio 
for the front and rear axles are known, the speeds of the wheels and the 
axle shaft can be compared to determine if there is any incipient wheel 
lock up occurring in either of the front wheels, or of either of the rear 
wheels during a brake application. If such incipient lock up is detected, 
the front two wheels can be controlled individually, and the two rear 
wheels can be controlled simultaneously with each other by selectively 
relieving brake pressure. 
It is desired to provide a traction control system compatible with 
on-demand four wheel drive systems and further compatible with both three 
sensor and four sensor anti-lock brake systems. 
SUMMARY OF THE INVENTION 
A traction control system for a four wheel drive vehicle includes a rear 
axle, a front axle, a transfer case connecting the rear axle and the front 
axle, a hydraulic circuit, and an electronic controller. The rear axle has 
rotatable left rear and right rear axle shafts and a rotatable rear pinion 
member. A left rear wheel is rotatably fixed to the left rear axle shaft. 
A right rear wheel is rotatably fixed to the right rear axle shaft. A left 
rear wheel brake is functionally disposed between a non-rotatable 
structure of the vehicle and the left rear wheel wherein the left rear 
wheel brake operably resists rotation between the left rear wheel and the 
vehicle structure. Similarly, a right rear wheel brake is functionally 
disposed between the same or a different non-rotatable structure of the 
vehicle and the right rear wheel wherein the rear right wheel brake 
operably resists rotation between the right rear wheel and the vehicle 
structure. The front axle has rotatable left front and right front axle 
shafts and a rotatable front pinion member. A left front wheel is 
rotatably fixed to the left front axle shaft. A front right wheel is 
rotatably fixed to the front right axle shaft. A left front wheel brake is 
functionally disposed between a non-rotatable structure of the vehicle and 
the left front wheel wherein the left front wheel brake operably resists 
rotation between the front left wheel and the vehicle structure. 
Similarly, a right front wheel brake is functionally disposed between a 
non-rotatable structure of the vehicle and the right front wheel wherein 
the right front wheel brake operably resists rotation between the right 
front wheel and the vehicle structure. The transfer case has an input 
shaft receiving input torque from a drive unit and also has a primary 
output shaft rotatably connected to the rear pinion member and to the 
input shaft. The transfer case also has a secondary output shaft rotatably 
connected to the front pinion member. The transfer case further includes a 
selectively engageable clutch rotatably connecting the primary output 
shaft with the secondary output shaft. The clutch is responsive to an 
electrical signal and is electrically connected to the electronic 
controller. A source of pressurized fluid is electrically connected to the 
electronic controller and is responsive to electrical signals therefrom. A 
hydraulic circuit connects the source of pressurized fluid with the wheel 
brakes. A first solenoid operated valve is disposed in the hydraulic 
circuit between the source of pressurized fluid and the left front brake, 
selectively blocking a flow of hydraulic fluid from the source of 
pressurized fluid to the left front wheel brake. The first solenoid 
operated valve is responsive to an electrical signal and is electrically 
connected to the electronic controller. A second solenoid operated valve 
is disposed in the hydraulic circuit between the source of pressurized 
fluid and the right front wheel brake, selectively blocking a flow of 
hydraulic fluid from the source of pressurized fluid to the right front 
wheel brake. The second solenoid operated valve is responsive to an 
electrical signal and is electrically connected to the electronic 
controller. A third solenoid operated valve is disposed in the hydraulic 
circuit between the source of pressurized fluid and at least one of the 
rear wheel brakes and selectively blocks a flow of hydraulic fluid from 
the source of pressurized fluid to the at least one of the wheel brakes. 
The third solenoid operated valve is responsive to an electrical signal 
and is electrically connected to the electronic controller. A first rotary 
speed sensor is located at one of the left front wheel and a part that 
rotates substantially in unison with the left front wheel. The first 
rotary speed sensor provides an electrical signal corresponding to the 
speed of the left front wheel and is electrically connected to the 
electronic controller. A second rotary speed sensor is located at one of 
the right front wheel and a part that rotates substantially in unison with 
the right front wheel. The second rotary speed sensor provides an 
electrical signal corresponding to the speed of the right front wheel and 
is electrically connected to the electronic controller. A third rotary 
speed sensor is located proximate to a rear ring gear or any element 
rotating at a fixed speed ratio with respect to the rotary speed of the 
rear ring gear including a rear pinion enabling an approximate measurement 
of the rotary speed of the rear ring gear. The third rotary speed sensor 
provides an electrical signal relating to the speed of the rear ring gear 
and is electrically connected to the electronic controller. The electronic 
controller includes means for determining values relating to the rotative 
speeds of the front and rear ring gears using signals from the sensors and 
also including means for comparing the values relating to the front and 
rear rotative speed signals and means for generating an electrical signal 
used to control the clutch within the transfer case based on a difference 
between the values relating to the front axle and rear axle ring gear 
speeds. The electronic controller also includes means for comparing the 
electrical signals from the first and second rotary speed sensors after 
engagement of the transfer case clutch and generating a signal to the 
source of pressurized fluid and a signal to at least one of the first and 
second valves responsive to a difference in signals from the front rotary 
speed sensors indicative of a slipping front wheel wherein a brake torque 
applied to the slipping front wheel transfer torque to the non-slipping 
front wheel. 
A traction control system for a four wheel drive vehicle includes means for 
generating signals, means for comparing signals, and means for 
transferring torque. Means for generating a first value corresponding to 
an average of a rotational speed of front wheels, means for generating a 
second value corresponding to an average of a rotational speed of rear 
wheels, means for comparing the first and second values, and means for 
generating a first signal corresponding to a difference between the first 
and second values, are provided. The signal signal corresponding to the 
difference in average wheel speeds is used by means for transferring 
torque to the secondary axle if the rear wheels are rotating faster than 
the front wheels. Also provided is means for generating a second signal 
corresponding to a difference in speed between a left axle shaft and a 
right axle shaft of the front axle. The second signal is used by means for 
transferring torque to the other of the front axle shafts if the one of 
the front axle shafts is rotating faster than the other of the front axle 
shafts. 
A method for developing driving traction in a four wheel drive vehicle 
includes the steps of generating a first value corresponding to an average 
of a rotational speed of front wheels, generating a second value 
corresponding to an average of a rotational speed of rear wheels, 
comparing the first and second values, and generating a first signal 
corresponding to a difference between the first and second values, and 
transferring torque to the secondary axle if the rear wheels are rotating 
faster than the front wheels. The method also includes the steps of 
generating a second signal corresponding to a difference in speed between 
a left axle shaft and a right axle shaft of the front axle, and 
transferring torque to the other of the front axle shafts if one of the 
front axle shafts is rotating faster than the other of the front axle 
shafts. 
A traction control system is provided which is compatible with on-demand 
four wheel drive systems, and anti-lock brake systems employing either 
three sensors or four sensors. 
Other features of the invention will become apparent by reference to the 
following specification and to the drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
An integrated traction control/anti-lock brake system 10 employing three 
speed sensors is shown schematically in FIGS. 1 and 3, which for the 
purpose of brevity, will be referred to as the traction control system 10. 
Traction control system 10 is shown integrated into a four wheel drive 
vehicle, with the configuration shown being typical of that used for light 
trucks. 
An engine 12 is mounting at the front of the vehicle in a fore-aft 
direction. A transmission 14 is mounted to a rear of the engine 12 and 
receives torque therefrom. A transfer case 16 is mounted to the rear of 
transmission 14 and receives torque therefrom. A primary or rear drive 
shaft 18 connects transfer case 16 with primary or rear axle 20. A 
secondary or front drive shaft 22 extends from a forward side of transfer 
case 16 to secondary or front axle 24. 
An electronic controller or control unit (ECU) 26 is electrically connected 
to various signal sources and to a plurality of controlled elements, 
including transfer case 16 as well as valves in traction control system 
10. 
A hydraulic control circuit 28, while shown in part in FIG. 1, is shown 
substantially in its entirety in FIG. 3. Valves controlled by ECU 26 are 
disposed in hydraulic control circuit 28. 
Rear axle 20 as best seen in FIG. 1 includes a rear axle ring gear 29 fixed 
to a differential 30 driven by a rear pinion 32. A left rear axle shaft 34 
and a right rear axle shaft 36 are both connected to differential 30 and 
are disposed within rear axle housing 37. Rear axle housing 37 typically 
consists of a cast iron differential housing with steel tubes extending 
therefrom enclosing right and left axle shafts 36 and 34. Rear axle 
housing 37 comprises a significant part of the vehicle structure and is 
attached to a vehicle frame (not shown) by suspension springs and shock 
absorbers. Other examples of vehicle structural elements include the 
frame, steering knuckles, suspension struts, and the vehicle body. Axle 
housing 37 may also be engaged by struts (not shown) pivotably connecting 
it with the vehicle frame (not shown). It should be appreciated that 
alternative rear axle structures can be employed without any effect on 
this invention. An axle suited for use with independent suspension, 
employing light weight alloy differential housings, and no axle tube, is 
but one alternative example. 
A left rear wheel 38 is rotatably fixed to left rear axle shaft 34 for 
rotation therewith. A right rear wheel 40 is rotatably fixed to right rear 
axle shaft 36 for rotation therewith. Other elements, such as brake rotors 
or drums may rotate in unison with the wheels as well. 
A left rear brake 42 and a right rear brake 44 are shown schematically in 
FIG. 1 as disposed outside left rear wheel 38 and right rear wheel 40. It 
should be appreciated that the actual brakes would be disposed radially 
within wheels 38 and 40 as is normally found in motor vehicles. The term 
wheel as used in the present application is intended to refer to the 
combination of a tire and wheel. 
A rear ring gear speed sensor 46 is shown disposed at the rear differential 
30 proximate to ring gear 29. Alternatively, sensor 46 may be located 
within transfer case 16, at rear pinion 32, or proximate to any element 
that rotates at a fixed ratio of speed with respect to ring gear 29. Where 
ever sensor 46 is located, it must provide a rotation signal at a fixed 
ratio to rotation of ring gear 29. 
Front axle 22 includes a front differential 48 with a ring gear 47 driven 
by a front pinion 49. A front left axle shaft and a front right axle shaft 
50 and 52 respectively extend from front differential 48. 
A front axle housing 53 is shown enclosing front ring gear 47, differential 
48, front left axle shaft 50 and front right axle shaft 52. Ring gear 47 
is fixed to differential 48 for unitary rotation therewith. Front axle 
housing 53 comprises part of the vehicle structure, similar to rear axle 
housing 37. It should be appreciated that as with rear axle 20, there are 
alternative configurations to the front axle illustrated here. One 
alternative type of construction is to provide an axle housing which 
encloses substantially only the differential. The axle shafts are joined 
to the differential by flanges or universal joints. 
A left front wheel 54 and a right front wheel 56 are rotatably connected to 
the front left axle shaft 50 and the front right axle shaft 52 
respectively. Left front brake 58 and right front brake 60 are shown 
schematically in FIG. 1. 
A common feature in four wheel drive vehicles, particularly in light 
trucks, is a mechanism enabling the disconnection of front wheels 54 and 
56 from differential 48 so that when the vehicle is operating in the two 
wheel drive mode, the wheels 54 and 56 are not backdriving differential 48 
and drive shaft 22. This is done primarily to conserve fuel, and 
secondarily to reduce noise. One method of achieving this is to provide 
hub locks (not shown) between each of wheels 54 and 56 and their 
respective axle shafts 50 and 52. Such hub locks are well known in the 
field of four wheel drive vehicles. Alternatively, a center disconnect can 
be provided proximate to differential 48, disconnecting one of axle shafts 
50 and 52 from differential 48. With the axle shaft disconnected, wheels 
54 and 56 cannot back drive differential 48. It is assumed for the 
purposes of this disclosure that if the wheels 54 and 56 are not 
constantly drivingly engaged with differential 48, then the chosen axle 
connect/disconnect means will provide the desired connection to enable the 
transmission of torque from transfer case 16 to wheels 54 and 56. 
A left front wheel speed sensor 62 is located proximate to wheel 54. 
Similarly, right front wheel speed sensor 64 is located proximate to wheel 
56. It should be appreciated that wheel speed sensors 62 and 64 can be 
positioned anywhere convenient to any object having a fixed rotational 
relationship with the wheels 54 and 56, such as their respective brake 
rotors. It is anticipated that sensors 46, 62, and 64 will be magnetic 
pickups although other known rotational speed sensors, such as magneto 
resistive sensors, or Hall effect sensors may be used instead. 
Hydraulic circuit 28, best shown in FIG. 3, is a split system, having a 
front portion 65 and a rear portion 66. Both the front portion 65 and rear 
portion 66 are pressurized by a master cylinder 67 acted upon by a brake 
pedal 68. Commonly, a power booster (not shown) is disposed between brake 
pedal 68 and master cylinder 67 to provide an assist to the driver in 
applying braking force to the master cylinder. 
A left front pressure cut-off valve 70 is disposed along front circuit 
portion 65 between master cylinder 67 and brake 58. A left front pressure 
relief solenoid operated valve 72 is connected to pressure line 71 at a 
point between brake 58 and valve 70 and is connected on its other side to 
a relief line 73. 
A right front solenoid operated pressure cut off valve 74 is connected to 
pressure line 71 in parallel with left front solenoid operated pressure 
cut off valve 70, between master cylinder 67 and brake 60. A right front 
solenoid operated pressure relief valve 76 is connected on one side to 
pressure line 71 between valve 74 and brake 60, and on a second side to 
relief line 73. A front low pressure accumulator 78 is also connected to 
relief line 73. 
Rear circuit 66 has only one cut-off valve 80 and only one relief valve 82 
which control both rear brakes 42 and 44. Rear axle solenoid operated 
pressure cut-off valve 80 is disposed in rear pressure line 81 between 
master cylinder 67 on one side and both rear brakes 42 and 44 on the other 
side. Rear axle solenoid operated pressure relief valve 82 is connected on 
one side to rear pressure line 81 between valve 80 and brakes 42 and 44, 
and is connected on the other side to rear relief line 83. A rear low 
pressure accumulator 84 is also connected to relief line 83. A pump motor 
85 is drivingly connected to a front circuit pump 87 and a rear circuit 
pump 88. Check valves 89 are disposed along relief lines 73 and 83 and 
prevent flow from pumps 87 and 88 backward through relief lines 73 and 83 
respectively. A rear return line 90 connects an output side of pump 88 
with pressure line 81 between master cylinder 67 and valve 80. A front 
return line 91 is connected to an output side of pump 87. Check valves 89 
are also disposed on an output side of both pumps 87 and 88, preventing 
fluid from being forced back into the output side of pumps 87 and 88. 
A pressure biased valve 92 is disposed between pressure line 71 at a point 
between valves 70 and 74, and master cylinder 67, and relief line 73 
between check valve 89 and an intake side of pump 87. Another check valve 
89 is disposed between the intake port of pump 87 and the connection 
between valve 92 and relief line 73. Valve 92 is biased such that if 
pressure in master cylinder 67 is greater than that in accumulator 78, 
then valve 92 closes. Valve 92 is typically closed when the brake pedal 68 
is depressed and open when it is not. 
A solenoid operated traction control valve 93 is disposed along pressure 
line 71 between valves 70 and 74, and where valve 92 connects to pressure 
line 71. A pressure relief valve 94 is parallel with valve 93 along 
pressure line 71. Front return line 91 connects to pressure line 71 
between valve 93 and valves 70 and 74. Valves 70, 72, 74, 76, 80, 82, and 
93 are electrically connected to 26 and are responsive to electrical 
signals therefrom. 
An alternative form of traction control system 10 is shown in FIGS. 4 and 
5, differing principally in that it employs four wheel speed sensors 
instead of three. Since four wheel speed sensors are employed, there is a 
set of controlling valves for each wheel of the rear axle, instead of a 
single set for the entire axle. A left rear wheel speed sensor 95 and a 
right wheel speed sensor 96 are located proximate to left and right rear 
wheels 38 and 40. 
As shown in FIG. 5, front circuit portion 65 remains unchanged. However, an 
alternative rear circuit portion 98 is employed. A left rear solenoid 
operated pressure cut-off valve 100 is disposed in rear pressure line 81 
between master cylinder 67 and left rear brake 42. A left rear solenoid 
operated pressure relief valve 102 is disposed between pressure line 81 at 
a point between valve 100 and brake 42, and relief line 83. 
Right rear solenoid operated pressure cut-off valve 104 is disposed in 
parallel with valve 100 along pressure line 81 between master cylinder 67 
and right rear brake 44. A right rear solenoid operated pressure relief 
valve 106 is disposed between pressure line 81 at a point between valve 
104 and brake 44, and pressure relief line 83. Accumulator 84 is also 
connected to pressure relief line 83. A check valve 89 is disposed between 
an inlet side of pump 88 and accumulator 84 blocking a flow of fluid from 
pump 88. A pressure biased valve 107 is disposed between pressure line 81 
and relief line 83. The connection to relief line 83 is made between check 
valve 89 and yet another check valve 89 disposed proximate to the inlet 
port of pump 88. Return line 90 connects an outlet side of pump 88 with 
rear pressure line 81 at a point between valve 107 and master cylinder 67. 
Both the three sensor and four sensor versions of the traction control 
system 10 employ transfer case 16 which has an electromagnetic clutch 108 
electrically connected to electronic control unit 26 and responsive 
thereto. As shown in FIG. 2, an input shaft 110 is rotatably supported in 
transfer case housing 111. Input shaft 110 receives torque from 
transmission 14. A planetary gear set 12 connects input shaft 110 with 
primary output shaft 114. Primary output shaft 114 is also rotatably 
supported in housing 111, and is rotatably fixed to rear drive shaft 18. 
Also rotatably mounted in housing 111 is a secondary output shaft 116 
which is rotatably fixed to front drive shaft 22. A chain link belt 118 
wraps around sprockets on shafts 114 and 116. The sprocket on secondary 
output shaft 116 is fixed thereto for unitary rotation therewith. The 
sprocket on primary output shaft 114 rotates freely thereon unless a 
clutch pack 120 provides a driving connection between shaft 116 and the 
sprocket located thereon. Clutch pack 120 can be selectively engaged by 
energizing a coil within electromagnetic clutch 108 causing cam plates 122 
to index relative to each other. Cam grooves in which cam plate rollers 
124 are disposed cause plates 122 to separate and axially load clutch pack 
120 responsive to an application of current to the coil. 
The invention operates in the following manner. Engine 12 provides torque 
to transmission 14, the torque varying in part as a function of engine 
throttle position. Transmission 14 multiplies the torque from engine 12 by 
a transmission ratio selected to provide an optimal level of output torque 
based in part on engine throttle position and vehicle velocity. 
The output torque from transmission 14 is communicated to transfer case 16 
through input shaft 110. Planetary drive 112 transmits the torque from 
input shaft 110 to primary output shaft 114 at a one-to-one ratio, unless 
the low-gear ratio is elected by the driver, in which case planetary gear 
set 112 multiples the torque at input shaft 110 by a fixed ratio. 
Planetary gear set 112 can also be placed in a neutral mode in which no 
torque is transmitted from input shaft 110 to primary output shaft 114. In 
a two wheel drive mode, essentially 100% of the torque in primary output 
shaft 114 is transmitted to rear drive shaft 18. 
Torque from output shaft 18 is passed through pinion 32 to axle shafts 34 
and 36 via ring gear 29 and differential 30. The ratio between pinion 32 
and the ring gear 29 provides a further multiplication of the torque. 
Under full traction conditions, the torque in both rear axle shafts 34 and 
36 is approximately equal. However, if either of rear wheels 38 and 40 
slip, then the torque in both rear axle shafts 34 and 36 drops to 
approximately zero. As the torque drops to near zero, the forward driving 
force is similarly reduced. This assumes that differential 30 is an open 
differential, and not a locking or limited slip differential which would 
tend to force both axle shafts 34 and 36 to rotate in unison so as too 
apply torque to the non-slipping wheel even if one of wheels 38 and 40 is 
developing little or no reaction torque. 
When slip occurs on just one side of rear axle 20, the axle shaft on the 
side which is slipping will rotate at a higher speed than the axle which 
is not slipping. Since the rotational speed of ring gear 29 equals an 
average of the two axle shaft speeds, and the average rear wheel speed, 
the speed of ring gear 29 will vary linearly with the speed of a slipping 
wheel when the speed of the non-slipping wheel remains constant. The 
rotational speed of rear ring gear 29 is monitored by the electronic 
control unit 26 using rear ring gear speed sensor 46. The rotational speed 
of rear ring gear 29 must be compared with the rotational speeds indicated 
by signals from front wheel speed sensors 62 and 64. Vehicle speed is 
estimated by averaging signals from front wheel speed sensors 62 and 64 
and multiplying the average wheel speed in units of RPM by two times .pi. 
times the rolling radius of the tires. If the rolling radius of the tires 
is provided in inches, this yields a velocity in units of inches per 
minute. The speed, however, is directly proportional to front ring gear 47 
and front differential 48. The theoretical speed of front differential 48, 
independent of whether front differential 48 is actually rotating or not, 
can be calculated by averaging the speed of axle shafts 50 and 52. When 
the rotary speed of rear ring gear 29 is greater than the calculated speed 
of front differential 48, it is indicative that one of wheels 38 and 40 is 
slipping. Of course, it should be appreciated that instead of using the 
ring gear and calculated differential speeds, the rear ring gear speed 
could be converted into an equivalent average rear drive shaft speed for 
comparison to a calculated front drive shaft speed. It should further be 
appreciated that the values could be converted to some arbitrary standard 
corresponding to neither wheel speed nor drive shaft speed. When the 
rotational speed of rear ring gear 29 exceeds that of front differential 
48, or exceeds it by some predetermined value, electronic control unit 26 
assumes that one of wheels 38 and 40 is slipping, and initiates engagement 
of electromagnetic clutch 108 within transfer case 16. A value other than 
zero may be used to prevent undesired clutch engagements from occurring. 
The clutch engagement transfers a portion of the torque in primary output 
shaft 114 to secondary output shaft 116. Assuming that neither wheel 54 
nor wheel 56 slips, torque builds in secondary drive shaft 22 and in axle 
shafts 50 and 52, developing vehicle thrust force at the contact patches 
of wheels 54 and 56. However, if one of wheels 54 and 56 slip, then the 
torque in drive shaft 22 is significantly reduced. 
The electronic control unit 26 is able to determine that one of front 
wheels 54 and 56 is slipping by comparing the rotational speeds of the two 
wheels, 54 and 56. If one exceeds the other, or exceeds the other by a 
predetermined value, then the electronic control unit will act to reduce 
and eliminate the relative slipping. The benefit of using a predetermined 
value of shaft speed difference greater than zero for the value needed 
before slip control begins is that it avoids false engagements of slip 
control due to slight variations in wheel speed due to turning and other 
maneuvers creating differential speeds for which it is undesired to 
trigger an unwanted engagement of the traction control. 
The traction control system disclosed relies on applying brake torque to 
the slipping wheel to develop torque at the wheel on the other side of the 
axle where tractive effort is hopefully possible. For traction control of 
the front axle 24, front circuit 65 must therefore be pressurized. When 
ECU 26 determines that one of the front wheels 54 and 56 is spinning too 
fast relative to the other, indicating a slip condition of that wheel, it 
directs a signal to motor 86 driving pumps 87 and 88. To prevent undesired 
braking pressure from building up in rear circuit 66, both valves 80 and 
82 are maintained in an open position by electronic control unit 26 to 
allow fluid to circulate without an increase in line pressure. To prevent 
pressurized fluid in front circuit 65 from merely being diverted through 
master cylinder 67 to a brake fluid reservoir (not shown), valve 93 is 
moved to a closed position by electronic control unit 26. The pressure cut 
off valve of the non-slipping wheel is closed to prevent a brake 
application of that wheel, and its corresponding pressure relief valve is 
opened. The brake pressure cut off valve for the slipping wheel is pulsed 
between an open and closed position as is the corresponding pressure 
relief valve which is pulsed in synchronization therewith. This pulsing 
brake application gradually slows the slipping wheel until its speed 
approximately equals that of the non-slipping wheel and there is 
approximately equal torque distribution between the two. When the speeds 
of wheels 54 and 56 stabilize in an equilibrium condition where the speeds 
are approximately equal to each other, electronic control unit 26 ceases 
the operation of pump 86 and the pulsation of the slipping brake valves, 
and returns the brake control valves back to their normally open and 
normally closed positions, thereby enabling a regular brake apply. 
By way of example, assume that both the right front and the right rear 
wheels 56 and 40 are on glare ice and the left front and left rear wheels 
54 and 38 are on dry pavement. With the vehicle in two wheel drive, 
starting from a complete stop, right rear wheel 40 spins and left rear 
wheel 38, left front wheel 54, and right front wheel 56 all remain 
stationary. While the signals from the two front wheel speed indicators 62 
and 64 indicate to the electronic control unit that the vehicle is not 
moving, rear ring gear speed sensor 46 indicates that at least one of the 
rear wheels 38 and 40 is slipping. Electronic control unit 26 sends a 
signal to transfer case 16, specifically to electromagnetic clutch 108, 
transferring torque from primary output shaft 114 to secondary output 
shaft 116. Torque transmitted to secondary output shaft 116 is 
communicated through drive shaft 22 and through differential 48 to axle 
shafts 50 and 52. However, no significant amount of torque develops 
because right front wheel 56 which is also on ice begins to slip and left 
front wheel 54 remains stationary. 
Electronic control unit 26 compares the speed of wheel 54 with the speed of 
right front wheel 56 and determines that the standard required to 
establish a slipping condition has been exceeded. Electronic control unit 
26 sends out electrical signals opening valve 82 and 80 as required, 
closing valves 93 and 70 and energizing motor 86. Pump 87 supplies 
pressurized fluid to pressure line 71 via return line 91. If, for any 
reason, the pressure in line 91 becomes excessive, then pressure relief 
valve 94 will open to allow pressurized fluid to be returned to a fluid 
reservoir via master cylinder 67. This occurs at approximately 1,500 psi. 
Valve 74 is pulsed between an open and closed position and valve 76 is 
intermittently cycled between closed and open positions in sync with 74, 
providing a pulsing brake application to brake 60. This intermittent 
application of brake 60 transfers torque to left front wheel 54, enabling 
it to move the vehicle forward. As the speed and torque distribution 
between wheels 54 and 56 becomes equalized, and both wheels 54 and 56 move 
to dry pavement, energization of motor 86 by electronic control unit 26 is 
terminated, and the traction control/anti-lock brake system valves are 
returned to the normal conditions shown in FIG. 3. 
The embodiment of FIGS. 4 and 5 works in essentially the same manner, 
except that instead of using a single ring gear speed sensor 46 to 
determine whether there is slippage in the rear axle, the individual wheel 
speeds are compared with each other to determine if there is slippage. 
Alternatively, with a four sensor system, the speeds of wheels 38 and 40 
could be averaged to derive an equivalent axle shaft speed which would be 
used by the electronic control unit in the same fashion as the directly 
measured axle shaft speed from sensors 46 in the first embodiment. Another 
difference between the first embodiment and the embodiment of FIGS. 4 and 
5 would be that upon energization of pump motor 86, both valves 102 and 
106 would need to be moved to their open positions to prevent 
unintentional lockup of the rear wheels by the brakes. 
It is evident that many alternatives, modifications and variations of the 
traction control system of the present invention will be apparent to those 
skilled in the art in light of the disclosure herein. It is intended that 
the metes and bounds of the present invention be determined by the 
appended claims rather than by the language of the above specification, 
and that all such alternatives, modifications, and variations which form a 
conjointly cooperative equivalent are intended to be included within the 
spirit and scope of these claims.