ABS Troubleshooting


ABS Componant Functions:

Speed sensors and tone rings on each wheel monitor wheel rotation.
Each speed sensor communicates wheel rotation pulses to the Electronic Control Unit. ECU receives the speed sensor inputs, interprets the signal pulses and calculates the speed and acceleration rates of each wheel.

Based on the speed sensor input, the ECU detects impending wheel lock and operates the ABS Modulator Valves required for proper control. The Modulator Valves can be operated in the air, release or hold modes to regulate air pressure to the brake chambers.

The braking force is applied at a level that minimizes the stopping distances while maintaining as much lateral stability as possible.

The ABS will perform a diagnostic indicator lamp check and self- diagnostic test each time the ignition is switched to the on position. The ABS dash indicator light momentarily (2.2 seconds) illuminates verifying the self-check test. If the ABS indicator light remains on, or illuminates while the motorhome is being operated, there is a fault within the anti-lock brake system. This fault will not affect normal service braking. The motorhome will need to go to a service center to repair the problem.

First, listen for the clicks when you turn the key (but don't start the engine).  There should be one for each wheel.  On our 2003 tag axle coach, that's 8 clicks.  Missing any clicks?  Something's not communicating.

Try re-making all the connections on the Wabco box.  We've had an ABS light twice that was a connection issue.  Our box is silver and in the bay with the HWH pump and inverter.  You WILL need to drive a short distance to reset the ABS light if that was the problem.


To Retrieve Blink Code(s):

Turn ignition key to the ON position.

Wait for three seconds prior to proceeding.

Apply and release the service brake pedal once.

Press and hold the momentary test button under dash panel for one to two seconds.

Engine Diagnostic Switch Engine Diagnostic Switch

2003 U320

ABS switches 2000 U320

2000 U320

The indicator light will illuminate while test button is pressed.

After releasing test button, observe the ABS warning indicator lamp.

The lamp will turn off and the blink code will be displayed: 1-1 indicates no system faults.

ABS Troubleshooting Codes


To Reset ABS system:

“Turn Key On”;

Hit ABS Momently – (Right Button under Dash)

Under Dash Momentary Switches – Left = Engine & Right = ABS Diagnostics.


ECU Rev C : The ABS warning lamp comes on when the ignition switch is activated. The lamp goes out when the speed of the vehicle reaches approximately 4 mph (6 km/h).  If the warning lamp remains lit or comes on at any other time during vehicle operation, it signals a malfunction in the ABS

ECU Rev D (started around 2000-2001): ABS lamp comes on at ignition momentarily for a bulb check, then goes out.  If there is an Existing Fault the ABS lamp does not go out at ignition. If Lamp does not go out at speeds above 4 mph (6 km/h) — a fault still exists in the ABS system.

ECU Rev E (somewhere around 2010?) Same start-up as Rev D

© Barry Brideau 2003-2018