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ABS / Brake Lights ON - ABS Failure

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ABS Failure - Diagnostics Tips

4 Wheel Anti-Lock Brake System

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2 Wheel Anti-Lock Brake System (RWAL - Rear Wheel Anti-Lock)

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I find its really common to hear people complain about ABS and BRAKE lights being lit. There is a few simple things you can do to diagnose your problem.


Check your ABS fuses under the hood fuse #11 and check the #3 fuse inside the cab (driver side door jamb). Replace any that are blown.


Step on you brake pedal and see if the tail lights light up and turn off when released. If not repair the brake light switch.


Shift the transfer case into 4WD and check if the 4WD light comes on. (4WD equipped vehicles). If not repair the 4WD switch.


Does the speedometer work? If so the rear speed sensor in the differential is functional.

So now that you done that much that means that it one of the front speed sensors that has either become disconnected or the sensor has failed.

If the lights remain the only way I know to diagnose this correctly is at a Dodge dealer. They have the tools to hook up to the ABS computer and pull the error codes and tell you what has failed.

Once you've repaired the problem you must drive the vehicle to reset the ABS and BRAKE lights. Also take the second and check the error codes on the ECM/PCM and reset any error codes.

Description - CAB (Controller Antilock Brakes)

The Controller Antilock Brakes (CAB) is a microprocessor which handles testing, monitoring and controllingthe ABS brake system operation (Fig. 10). The CAB functions are:


Perform self-test diagnostics.


Monitor the RWAL brake system for proper operation.


Control the RWAL valve solenoids.

NOTE: If the CAB needs to be replaced, the rear axle type and tire revolutions per mile must be programed

into the new CAB. For axle type refer to Group 3 Differential and Driveline. For tire revolutions per mile,(Refer to 22 - TIRES/WHEELS/TIRES - SPECIFICATIONS) . To program the CAB refer to the Chassis Diagnostic Manual.

Operation - System Self-Test

When the ignition switch is turned-on the microprocessor RAM and ROM are tested. If an error occurs during the test, a DTC will be set into the RAM memory. However it is possible the DTC will not be stored in memory if the error has occurred in the RAM module were the DTC’s are stored. Also it is possible a DTC may not be stored if the error has occurred in the ROM which signals the RAM to store the DTC.

CAB Inputs

The CAB continuously monitors the speed of the differential ring gear by monitoring signals generated by the rear wheel speed sensor. The CAB determines a wheel locking tendency when it recognizes the ring gear is decelerating too rapidly. The CAB monitors the following inputs to determine when a wheel locking tendency may exists:


Rear Wheel Speed Sensor


Brake Lamp Switch


Brake Warning Lamp Switch


Reset Switch


4WD Switch (If equipped)

CAB Outputs

The CAB controls the following outputs for antilock

braking and brake warning information:


RWAL Valve


ABS Warning Lamp


Brake Warning Lamp

P0500 Error Code - Explained

Since 4 wheel antilock brakes rely on 2 front axle sensors and 1 rear wheel sensor. The CAB is watching the speed output from all 3 sensors at one time. So when on a slick surface like ice, gravel, wet pavement, etc. when you accelerate rapidly and cause the rear tires to spin the rear speed sensor jumps up in speed rapidly and the front 2 sensor could be at zero speed yet. So the CAB can't understand how the rear half of the truck is doing say 35 MPH and the front half is doing 0 MPH. POOF! P0500 code is thrown.

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