How to make diagnostics pay

James Dillon on the importance of knowing what to measure

Published:  21 April, 2014

By James Dillon

The next step was to confirm with the scope and Diagnostic Assistance software that we have the correct voltages and signals at the sensors. I measured the voltage at the sensors first because they are relatively easy to access compared to the ABS ECU. Figure 1 below shows the scope being attached to the ABS sensor via the wheel arch liner access plate to the ECU side of the sensor wiring loom. With the engine started and a gear engaged to allow the wheels to spin, I took the measurement (Figure 3). It shows a flat line from the sensor supply. So it is likely that either the voltage supply which comes from the ECU is faulty (or its wire) or the sensor is shorted internally. On this system, a quick disconnection of the sensor will prove if the sensor is at fault. If it is faulty, the supply voltage will return when the sensor is disconnected. With the sensor disconnected, the signal remained at zero volts. This indicated a fault in the wiring or within the ECU. I had no choice but to remove the engine ECU and relay plate to gain access to the ABS ECU and back probe the wires directly at the control unit. The fault code for this WSS suggested a short which was confirmed by the voltage I had measured.

The measurement showed zero volts at the ABS ECU. This narrows it down to an ABS ECU fault or a short circuit. How do I quickly prove which one it is? If I isolate the wiring between the ABS ECU and the sensor, and the voltage returns, I have a wiring fault, if not, I have an ECU fault. This next step required that I cut the wire in the loom. As long as the 'after test' repair is done correctly (solder or crimp and weatherproof heatshrink etc) this should not be a problem. I connected the scope, channel A to the sensor end and channel B to the ECU end of the wire. I was expecting the voltage on channel B to rise to the supply voltage level if the fault is in the wiring. If it didn't increase, I had an issue with ABS ECU. A quick snip later and the voltage had increased. Bingo! The vehicle had a short in the wiring loom (exactly as the fault code suggested).

As this was a trade job they may well want to locate the actual fault site and repair the wiring themselves, as there may be much labour involved in exposing the wire along its length within the loom. Therefore, to complete the diagnosis, and prove that a good repair would cure the symptom, I had to make a final step and jump the faulty wire from the ABS ECU to the sensor. It was important that I isolated the signal wire at the sensor end to prevent the short pulling my jumper wire voltage down. This would confirm that the system function would be restored if the wire is repaired correctly. The wire is jumped and the sensor function is restored. Nice!

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