Brake pedal switch contact sequence (ABS/ETC problem).

I have a long standing problem with an occasional ABS/ETC Traction Failure fault on my 1999 DSE (with 4-wheel ETC).

Sometimes, after a mild "urban" braking (for a junction, pedestrian crossing etc.) the above fault lights and messages will appear. The brake pedal will often pulse - as with an application of the ABS system, I believe. Checking with my Nanocom, there has never been a fault found, although I have been able to cancel the fault messages.

Reading up on the large number of posts on the topic I have checked the following:

Earth/ground connections - OK.
Alternator - OK.
Battery - wrong size & rating - changed.
Battery connector - loose - repaired.
ABS accumulator - original and losing charge - replaced.
ABS ECU - getting desperate for a logical solution, now, so replaced with a used later version.

The fault has continued to appear, with increasing frequency.

Yesterday, I was checking the Nanocom readings carefully, whilst reading the Nanocom notes. I should have done this before, but reading instruction books isn't what real engineers do, is it? Anyway I noticed that the brake switch contacts - there are two sets - were making contact almost at the same point as I pressed the brake pedal. In fact, it was difficult to find a point in the brake pedal travel when Switch 1 was operated and Switch 2 wasn't. Now the Nanocom notes say that Switch 1 should operate as soon as the pedal is touched and Switch 2 should operate towards the bottom of the pedal's travel. Clearly mine isn't doing this and is therefore probably defective.

My quesion is "is this the likely cause of my problem"? I haven't read about this sequential operation of the switches anywhere else. Can anyone tell me if I have probably found the cause of the problem or am I chasing another false lead?

Thanks in hopeful anticipation of a positive reply!!!

John

Yes if switch is faulty it will bring ABS fault up on message centre. As you press pedal switch 1 should go from low to high and switch 2 should go from high to low. If you fit a new switch, plunger should be fully extended and will be self adjusted as the pedal is pressed. That is for Wabco C system yours will be Wabco D system can't see it being any different for switch operation.

Thanks for the quick reply. The Nanocom reports both switches as changing from "CLOSE" to "OPEN" as the pedal is pressed. Looking at the wiring diagram, I think that this is the correct action for the WABCO D system, but the operating point of Switch 2 is clearly wrong. I can imagine that the 2 switches are used by the ECU to determine how hard and/or quickly the brake pedal has been pressed.

I'll order a new switch now and hope that I will have finally cracked it.

Cheers,
John