Drakefield’s Barry Craven shares his experience of diagnosing hard-to-detect EBS faults on Volvo and Scania models.
Just lately, I’ve had a number of Volvos and Scanias with very similar faults appearing, all of which have proved to be quite hard to diagnose correctly. What I’ve found is that the same fault has appeared on several different trucks, always saying that the rear axle twin EBS modulator is faulty. But here’s the problem: it says unknown fault!
So, the first thing you do is check the power to the valve. You will probably find there is no power, which is normal as the ECU will shut the power down to the module when a fault is detected and only give it power again when the fault is gone. Next, you check the power cables from the front to the back of the vehicle, along with the can wires, but you can’t find anything wrong.
Next, you try a brake reset and a test drive and nothing changes, so now you have to assume the valve is actually faulty, so off you go and replace the valve, but the fault is still there. You subsequently try a brake reset again and clear the fault memory in the ECU, followed by another road test and, guess what? The same fault is still there: Unknown fault!
Here is what I have found: the original fault is nothing to do with the module itself, it’s actually one of the EBS wheel speed sensors that is faulty. From what I can gather, the module sees the sensor as either open or short circuit and decides that, because the sensor is faulty, it can’t correctly control the braking on that wheel. Instead of telling you there is a faulty sensor, it shuts the module down, but because the module has been shut down it can’t even check the sensors to tell you if they are faulty or not. Very frustrating.
As a general rule, I suggest that you always check the sensors first, even if the fault says something different. I can save a whole lot of time and effort. I use the Drakefield SensorMaster to check the sensors – it’s quick and easy, and can pinpoint a sensor fault in seconds without the need for any other diagnostic equipment.
Hopefully this will help you out if you ever come across the same fault.