We have recently completed a challenging diagnostic job that perhaps highlights just how difficult even the apparently simplest of diagnostic jobs can be . A Volkswagen Golf was booked in for a simple engine management light coming on , the car had a history of a brake light switch fault that had already been replaced which seemed to be a short term fix . Interrogation the engine ecu revealed the same fault for brake light switch if the fault was cleared and the brake pedal operated several times the fault would reoccur. Live data in engine seemed to show the brake light working normally even when the fault was present . Consulting manufacturers wiring data showed that the brake light switch was not just a single switch but a rather more elaborate affair with two sides to the switch which also went to the ABS and body control modules as well as the engine ecu . Checking live data in the ABS and body modules showed no change in the brake light circuit . This was then confirmed at the brake light switch wiring using an oscilloscope. Checking the feeds and grounds to the switch proved there was no fault there but when the circuits were tested it was quickly established that one side of the switch was going to ground .
The next stage was to find out where this fault was occurring, we started by disconnecting the control units from the circuit ABS ,Body module and the engine ecu , the engine ecu on this vehicle is inside the scuttle and is in a security cage which requires quite a bit of work to remove . After disconnecting these we still had a short to ground , as the wiring goes into the vehicle from the switch we tested as far as we could get access to but quickly realised this was going to be a difficult job as the main wiring loom is the first part installed into the car on manufacture before the crash bars and dash panel are fitted .
VEHICLE INTERIOR STRIPPED OUT..
MAIN WIRING HARNESS..
We now began the strip down removing the center consul , radio , upper and lower dash panels ,steering column and the vehicle crash structure so we could get access to the main wiring loom. Careful inspection revealed that the corresponding wire in the main loom had chaffed through on the steering column support bracket causing the short circuit .Once repaired we tested the circuit and carried out the task of reassembling the interior . The brake light switch and the body module had both failed as a result of the short circuit and had to be replaced , the body module requiring on line coding with Volkswagen .
SHORTING WIRE..
WIRE NEXT TO METAL BRACKET..
After assembly the circuits were rechecked and proved to be working correctly in all control units and the fault was cleared and extensive road testing carried out to ensure a permanent repair .
CRASH STRUCTURE BACK IN PLACE..
TOP DASH GOING BACK IN..
Who said diagnostics is simple we are often asked how much will diagnostics cost and although this is an extreme example it does highlight what lengths we have to go to in a modern car to trace faults in modern cars.
