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ABS Troubleshooting for Trucks, Trailers, and Buses

ABS Warning Lamp

A question that we are commonly asked is, " How do you know what the warning lamp in the cab or on the trailer should be doing?"

The answer is, thankfully, more straightforward than might first appear and starts with a simple fact, namely, that there are only three main warning lamp sequences currently in use, which are:

1. ON/OFF at standstill
2. ON and remains ON until the vehicle is driven above 7kph
3. ON/OFF/ON, and remains ON until the vehicle speed exceeds 7kph

How do I know which sequence is fitted to my vehicle?

These three warning lamp sequences are used by both Wabco, Haldex and Knorr-Bremse, who between them account for the majority of ABS systems fitted to commercial vehicles in the U.K. To find out which sequence applies to your vehicle follow these simple steps.

FOR TRAILERS

If you have an ISO 7638 connector (ABS socket) on both your truck and trailer, connect it and turn the ignition ON. The trailer warning lamp in the cab should come ON.

• If the lamp goes ON/OFF then that is your sequence (number 1. above )
• If the lamp stays ON, drive the truck and trailer above 7kph. If the warning lamp in the cab then goes OFF, your sequence is number 2. above
• If the warning lamp stays on after the vehicle has exceeded 7kph, it means there is a fault in the ABS system (number 3. above), which should be fixed before driving further

FOR TRUCKS

Turn the ignition ON:

• If the warning lamp goes ON/OFF then that is your sequence (number 1. above )
• If the lamp stays ON, drive the truck above 7kph. If the warning lamp in the cab then goes OFF, your sequence is number 2. above
• If the warning lamp stays on after the vehicle has exceeded 7kph, it means there is a fault in the ABS system (number 3. above), which should be fixed before driving further

How the warning lamp works

The warning lamp is controlled by the electronic control unit (ECU) fitted to the vehicle. When the ignition is turned ON, the ECU checks the external components to make sure they are working correctly. After this initial test has been completed, some ABS systems will turn the warning lamp OFF, and it will then only reappear if a fault is detected once the vehicle is moving (faster than 7kph). If the warning lamp stays ON when the vehicle is stationary it may indicate a fault, alternatively it could be that the ECU is waiting to receive the correct wheel speed signals from the ABS sensors. Once this happens from all the ABS sensors fitted to the vehicle, the warning lamp will go OFF.

One thing all ABS systems have in common, is that the warning lamp should go OFF when the vehicle is driven avove 7kph. If the vehicle lamp stays ON above this speed, it means there is a fault in the system. NB - some electronic dashboards do not show a warning lamp until a fault appears.