We were contacted by a rather desperate lady asking if we could fix her car. After an initial conversation we took on the task but could we succeed where others had failed?
We carefully interviewed the customer and got a complete background to the faulty vehicle. The issue started not long after a service at the main dealer including a new battery. The customer described how the vehicle would randomly cut out or fail to start.
Returning to the dealer they ran some diagnostic tests but couldn’t get to the bottom of the issue although in fairness the fault was quite sporadic.
Shortly after collecting her car it cut out and failed to start with various warning lights on the dash.
Not feeling confident in the main dealer, the vehicle was recovered to a local VW specialist to see if they could help locate the fault. After several weeks and some replacement parts (a gateway module and second-hand engine ecu) they were no further forward and suggested maybe we should take a look.
Our initial investigation revealed multiple faults and lack of communication with the engine ecu. After studying manufacturers wiring information, we discovered that all the issues were on the powertrain data bus. Testing the resistance of the data bus revealed an issue with incorrect readings, as the main lack of communication was with the engine ecu we decided to start here and found that the terminating resistor inside the ecu had failed. We substituted the resistor at the ecu wiring plug and we could then communicate with everything on the vehicle apart from the engine ecu. We know that a second-hand ecu had been tried so was there more than one issue? We decided before doing anything else we would prove all feeds and grounds were available at the engine ecu. Again checking with manufacturers data we load tested the circuits and discovered that a main ignition feed was missing. Tracing the wiring back across the bulkhead of the vehicle we found that the wiring harness had water ingress inside and half way across the bulkhead the feed wire had corroded through. After repairing the section of wire, we contacted the customer with our findings and recommended a new engine ecu
On arrival the new ecu was fitted and we were confident that this would when coded and adapted fix the issues. Imagine our surprise when we fitted the new unit to discover that we couldn’t communicate with the new unit! Retracing our diagnostic process we couldn’t see where we had gone wrong so once again we fitted a substitute resistor in the engine ecu harness and restored communication , checking the new ecu the terminating resistor was correct so this only left the possibility of a connection issue in the ecu wiring pins and sure enough with the new ecu connected if you put a sideways strain in the wiring harness we could then communicate with the new ecu, the pins had been damaged by incorrect probing .
The main ecu connector plug was stripped and two new pins installed for the can network constant communication was then restored and after coding and adaption the vehicle was back up and running with no faults.
So our conclusion was that the original fault was the corroded wire in the main wiring harness and that the other issues were all introduced whilst the vehicle was being tested although in truth we will never really know .As for the customer she was just glad to get her car fixed and running correctly and back on the road .