Chrysler Full-Size Trucks 1989-1996 Repair Guide

Description and Operation

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See Figures 1, 2 and 3

Found on 1993-96 Dakotas and Pick-ups, the all-wheel Anti-lock Brake System (ABS) is a unique combined system. It consists of the Rear Wheel Anti-Lock (RWAL) system components, plus the additional parts needed to create an all-wheel ABS system. For the 1996 model year, the names for some of the components were changed; however, the system remains essentially the same as in the 1993-95 model years.



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Fig. Fig. 1: ABS hydraulic system schematic-1993-96 models

The ABS system will prevent wheel locking under heavy braking. By preventing wheel lock-up, maximum braking effort is maintained while preventing loss of directional control. Additionally, some steering capability is maintained during the stop. The ABS system will operate regardless of road surface conditions.



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Fig. Fig. 2: Major ABS components-Dakota models shown; Pick-ups are similar

There are conditions for which the ABS system provides no benefit. Hydroplaning is possible when the tires ride on a film of water, losing contact with the paved surface. This renders the vehicle totally uncontrollable until road contact is regained. Extreme steering maneuvers at high speed or cornering beyond the limits of tire adhesion can result in skidding which is independent of vehicle braking. For this reason, the system is named anti-lock rather than anti-skid.



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Fig. Fig. 3: Specialized components convert the Dodge RWAL brake system to an all-wheel ABS system

Under normal braking conditions, the ABS system functions in the same manner as a standard brake system. The primary difference is that power assist is gained from hydraulic pressure rather than a conventional vacuum booster.

The ABS system is a three-channel design. The front brake anti-lock valve provides two-channel pressure control of the front brakes. Each front wheel brake unit is controlled separately. Two solenoid valves are used in each control channel.

The rear anti-lock valve controls the rear wheel brakes as a pair. The rear brake valve contains two solenoid valves.

The front and rear anti-lock valves contain electrically operated solenoid valves. The solenoid valves modulate brake fluid to apply pressure during anti-lock braking. The valves are operated by the anti-lock electronic module.

The electrical system for the ABS system is separate and independent from the other electrical circuits in the vehicle. A specially programmed electronic control module is used to operate the system components.

The pump/motor assembly on the front anti-lock valve provides the fluid volume needed during anti-lock braking. The pump is operated by an integral electric motor. The DC type motor is controlled by the ABS module.

The ABS Module (called Controller Anti-lock Brake/CAB in 1996) is the electronic brain of the system, receiving and interpreting speed signals from the four sensors at the wheels. The ABS module will enter anti-lock mode when it senses impending wheel lock at any wheel and immediately controls the line pressure(s) to the affected wheel(s). The hydraulic assembly serves as both an integral master cylinder and the hydraulic booster assembly for the brake system. It contains the wheel circuit valves used to control the brake fluid pressure to each wheel circuit.

During anti-lock braking, line pressures are controlled or modulated by the rapid cycling of electronic valves within the hydraulic assembly. These valves can allow pressures within the system to increase, remain constant or decrease, depending on the needs of the moment as registered by the ABS module. The front wheels are controlled individually, while the rear wheels receive the same electrical signal, based on the wheel with the greatest locking tendency. Anti-lock function is available above 3-5 mph (4.5-8 km/h).

The operator may hear a popping or clicking sound as the pump and/or control valves cycle on and off during normal operation. The sounds are due to normal operation and are not indicative of a system problem; under most conditions, the sounds are only faintly audible. If ABS is engaged, the operator may notice some pulsation in the pedal. If additional force is applied to the pedal during an ABS-engaged stop, the operator will notice extremely hard pedal feel. This is due to isolation of the master cylinder during ABS operation. Some pulsing may also be felt in the body of the vehicle due to suspension movement as brake pressures apply and release at the individual wheels.

The ABS system is equipped with built-in diagnostic capability. At every start-up, the ABS module illuminates the dashboard warning lights and turns them off after checking the circuitry. When the vehicle reaches a certain preset speed, the ABS module conducts a system check, briefly activating all the control valves to confirm their operation. This system check may be noticed by the operator as a series of rapid clicks during initial drive-off; the sound is normal and not indicative of a problem. Some fault conditions will cause the ABS module to set and retain a trouble code which may be retrieved for diagnostic purposes. Stored fault codes will remain stored until cleared by the DRB II, or equivalent scan tool.

The ABS module will illuminate the appropriate dashboard warning lamp according to the fault detected. It is possible to have a fault affecting only the ABS function; in this case, the ABS system will be disabled, but the vehicle will retain normal braking capability.

SERVICE PRECAUTIONS





Certain components within the ABS system are not intended to be serviced or repaired individually. Only those components with removal and installation procedures should be serviced.
 
Do not use rubber hoses or other parts not specifically specified for the ABS system. When using repair kits, replace all parts included in the kit. Partial or incorrect repair may lead to functional problems and require the replacement of components.
 
Lubricate rubber parts with clean, fresh brake fluid to ease assembly. Do not use lubricated shop air to clean parts; damage to rubber components may result.
 
Use only DOT 3 brake fluid from an unopened container.
 
If any hydraulic component or line is removed or replaced, it may be necessary to bleed the entire system.
 
A clean repair area is essential. Always clean the reservoir and cap thoroughly before removing the cap. The slightest amount of dirt in the fluid may plug an orifice and impair the system function. Perform repairs after components have been thoroughly cleaned; use only denatured alcohol to clean components. Do not allow ABS components to come into contact with any substance containing mineral oil; this includes used shop rags.
 
The ABS module (called Controller Anti-lock Brakes/CAB in 1996) is a microprocessor similar to other computer units in the vehicle. Insure that the ignition switch is OFF before removing or installing controller harnesses. Avoid static electricity discharge at or near the controller.
 
Always wear safety goggles when disconnecting lines and fittings.
 

DIAGNOSTIC PROCEDURE



See Figure 4

ABS system diagnosis involves checking only those components that form the anti-lock system. Diagnosis of the ABS system consists of 3 general steps, performed in order. The first indicator that something is amiss in the ABS system is the illumination of the warning light display. Second, a visual or preliminary inspection is always required before any other steps are taken. Third, the problem should be identified.



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Fig. Fig. 4: Location of the ABS diagnostic connector

In most cases, identification of a problem can ONLY be performed using the DRB II scan tool, or equivalent. The scan tool must be hooked up to the ABS diagnostic connector which is clipped to the underside of the knee bolster, just below the steering column.

The ABS diagnostic connector is a six-way type and is either light blue or black in color. The connector is the ABS access point for the DRB (or equivalent) scan tool. System components can be tested after the tool is attached.

Visual Inspection

Although specific diagnosis requires special diagnostic tools, a careful visual inspection may turn up a simple problem that can be remedied without the need to spend time and effort in having the vehicle looked at by a professional shop.

Before diagnosing an apparent ABS problem, make absolutely certain that the normal braking system is in correct working order. Many common brake problems (dragging lining, seepage, etc.) will affect the ABS system. A visual check of specific system components may reveal problems creating an apparent ABS malfunction.

  1. Inspect the brake fluid level in the reservoir.
  2.  
  3. Inspect brake lines, hoses, master cylinder assembly, brake calipers and cylinders for leakage.
  4.  
  5. Visually check brake lines and hoses for excessive wear, heat damage, punctures, contact with other parts, missing clips or holders, blockage or crimping.
  6.  
  7. Check the calipers and wheel cylinders for rust or corrosion. Check for proper sliding action if applicable.
  8.  
  9. Check the caliper and wheel cylinder pistons for freedom of motion during application and release.
  10.  
  11. Inspect the wheel speed sensors for proper mounting and connections.
  12.  
  13. Inspect the tone wheels for broken teeth or poor mounting.
  14.  
  15. Inspect the wheels and tires on the vehicle. They must be of the same size and type to generate accurate speed signals. Check also for approximately equal tire pressures.
  16.  
  17. Confirm the fault occurrence with the operator. Certain driver induced faults, such as not releasing the parking brake fully, will set a fault code and trigger the dash warning light(s). Excessive wheel spin on low-traction surfaces, high speed acceleration or riding the brake pedal may also set fault codes and trigger a warning lamp. These induced faults are not system failures but examples of vehicle performance outside the parameters of the ABS module.
  18.  
  19. The most common cause of intermittent faults is not a failed sensor but a loose, corroded or dirty connector. Incorrect installation of the wheel speed sensor will cause a loss of wheel speed signal. Check harness and component connectors carefully.
  20.  

Diagnostic Mode

Connect the DRB II scan tool, or equivalent tester, according to instructions furnished with the tool. The system will enter diagnostic mode and prompt the operator through the assorted system checks and tests.

Dashboard Warning Lamp
BRAKE WARNING LAMP

The red BRAKE warning lamp will be illuminated to warn the operator of conditions which may result in reduced braking ability. These conditions include:



Parking brake not fully released.
 
Low brake fluid.
 
Hydraulic assembly or ABS module
 

The lamp will also illuminate whenever the ignition switch is put in the START position or the ignition switch is turned to ON . Under these bulb test conditions, the lamp should stay illuminated for about 2 seconds.


CAUTION
Illumination of the BRAKE lamp indicates a condition affecting the braking ability of the vehicle. The vehicle should not be driven until the seriousness of the problem is determined. In most cases, conditions illuminating the BRAKE lamp will also illuminate the ANTI-LOCK warning lamp, disabling that system as well.

ANTI-LOCK WARNING LAMP

The amber anti-lock light illuminates at start-up as part of the system self-check feature. The light will remain lit two or three seconds, then go off as part of its normal routine.

The amber ANTI-LOCK warning lamp is controlled by the ABS module. If the controller detects a condition resulting in the shut-down of the ABS function, the ANTI-LOCK lamp will be lit. The ANTI-LOCK lamp is normally lit until the ABS module completes its self-tests; if no faults are found, the lamp is turned off.

Display of the ANTI-LOCK warning lamp by itself indicates only that the ABS function has been disabled. Power-assisted normal braking is still available and the vehicle may be driven with reasonable care.

A system fault, such as the loss of speed sensor signal, or solenoid failure, will cause the amber warning light to come on. The most effective procedure here is to check for obvious damage first. Beyond this, the system will have to be checked with a DRB II scan tool, or equivalent tester.

 
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