Power is nothing without control

Tracing a fault on a Golf with a list of DTCs

Published:  09 December, 2013

By Frank Massey

We have seen a few VW Golfs in our time and reckon ourselves to be both familiar as well as confident in their repair. However, the Golf that found its way into our workshop this month had been seen by no less than nine other garages and had trekked all the way from Newcastle upon Tyne to our workshop across the Pennines.

The Golf in question was a 130 Pd 'iJ' chassis with engine code ASJ. The vehicle displayed an intermittent loss of power due to several DTCs, the result of which set the n75 boost control solenoid to default value and thereby prevented any boost. The vehicle had been owned by our client for several years. It had been tuned up but was still beautifully smooth and displayed a remarkable torque throughout the range. For most of the time, a real pleasure to drive given its 86k mileage covered.

We also found that the vehicle came in with a variety of spares, ranging from several relays, in most cases more than one of the same part no, n75 solenoid... as well as other bits and bobs, including a cuddly toy and a dog cage!

The DTCs listed below suggested multiple component or circuit faults. It is our experience and opinion that this is most likely a wiring or common environment issue. The codes specifically focused on circuit or event failure and a short to power or short to ground problems:

? 46ao radiator fan circuit 1 open or short to ground

? 46oc relay [j17 low pressure priming pump] open circuit or short to ground

? 4691 air con interface short to battery +

? 4625 boost valve open circuit short to ground

? 4c03 relay low heating output short to battery +

The most obvious and curious observation confirms no event failure or voltage drop of any significance. The DTCs however direct us to a common relationship, don't they? With the aid of Autodata it is clear that all the circuits do share a common power path through relay k46. I guess this is where caution takes over experience but the other interesting fact was the error often occurred during crank start events. With this in mind (and given all the error circuits share a common power supply chain) the next step focused on the battery and ground references.

The fitted battery passed a conductance and health test but the rating of 550 amps was well below the correct 740 amp rating. Could it be this simple? No! With the following improvements to all ground points and a new battery, the voltage drop during cranking improved by over 1 volt, it did not however cure the problem. At this point, I started to think of RF interference, a voltage spike - after all, one of the affected components, the cooling fan, can present an inrush of 50\60 amps. Detailed examination of the waveform did not however support this theory. We have experienced similar problems as a result of water ingress in looms and/or PCMs.Given the age of the vehicle and evidence of damp, we exposed several sections of the loom without success.

Still suspecting a wiring issue and I guess not accepting what was becoming obvious (namely a powertrain control module problem), we monitored the power supplies at several key locations. The result of which finally convinced us that there was no multiple error at all. So, no errors, yet lots of trouble codes were present. It has to be a faulty PCM... But with a lack of tangible evidence how can we justify or sell it to the customer?

Pic 1:?Tuned 130pd Golf displayed good torque

pic 3:?Taping the loom back was a task in its self

pic 4:?The car arrived with the back seat full of spare parts

pic 5:?The battery was below the recommended amp rating

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