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School Bus Wabco ABS Light Staying On

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wabco school bus abs lights staying on

Wabco ABS Dash Warning Light Staying On

This problem has stumped us to the point that Thomas Bus Tech Support was our last resort. The ABS light was illuminating steady but there were no codes found in the Wabco software we have on our shop laptop. Most times it’s the wheel sensor that’s at fault but they all checked out fine. The wiring schematic in the video is not totally accurate because there is no blink code switch on the Thomas C2 school bus.


The Blink Code Switch

The blink code switch is on the Freightliner FS65 conventional models located behind the dash on the right side where the diagnostic link is located. With the blink code switch you count the fault light flashes on the dash and refer to the blink code chart to determine what the problem is. That’s older technology but the wiring from the ABS lamp to the ABS electronic control unit shown in the video is correct.

It’s a ground wire that is switched on and off inside the module depending if an error / fault code is present or if there is nothing wrong at all. In our case the light stays on without faults. So the first steps we took was to test the ABS lamp relay and the wire to the ABS module. It seemed straight forward at first but after running a jumper wire to the relay from the module, confirming ignition power to the relay we stalled out. Grounding the relay turned the ABS light off so either there is another circuit in the ground side we don’t know about or an open circuit.

Thomas Bus Tech Support

The Thomas Bus tech support was very helpful and sent us wiring schematics for the ignition feed to the relay then helped us locate the ground wiring location on the frame. The schematics indicated that there are two connections on the ground wire to the ABS module. We found one connection and noticed a loose terminal. Wiggling that terminal around turned off the ABS light but turned on the retarder indicator lamp. This bus does not have a retarder but at least we can see how it’s related to the ABS system.

Any Kind Of Progress is Good

We have made some progress finding the loose ground wire connection so on Monday we will carry on in that area of the harness. It was another needle in the haystack but at least there is something to sink our teeth in to. The reason we have to fix this ABS fault is the gov’t inspector would hang us out to dry if this bus was in an accident with an ABS fault code. An ABS code disables the system and the stability of the bus on slippery roads is compromised so it can not go back on it’s run until this problem is repaired.