Elementary fault diagnosis

An Audi A6 is brought back to life by David Massey of ADS

Published:  29 April, 2014

By David Massey

Last week in the workshop we were lucky enough to have the company of Adam from Autologic who wanted to progress within the company into a role which requires in-depth diagnostic skills, in order to carry out on-site equipment demonstrations, so we designed a special training course to sharpen his abilities.

The first job we had in the workshop certainly wasn't for the faint-hearted and presented us with some intriguing problems, which even the most skilled technicians would find challenging.

The said vehicle was an Audi A6 2006 2.0 DSG auto diesel (BRE) 140bhp which had stood for several years and had numerous failed attempts to successfully diagnose and repair it. The vehicle had been presented to us by a local car sales pitch who had taken ownership and responsibility to repair and eventually sell.

The car was in a sorry state and had more plant life in and around the bonnet area than the Amazon rainforest, the windscreen was badly smashed and evidence suggested that it had been used as a taxi judging by the witness marks both on the doors and inside the car.

Even the most experienced diagnostic technicians would admit it's difficult to know where to start on such a vehicle and some wise ones out there would simply avoid it. However, with a belligerent 'never say die' attitude and with Adam being as keen as mustard to roll up his sleeves and get hands-on tackling diagnostics, I took the job on.

It's tempting on such a vehicle to get distracted and misled, due to the very condition the car had been left in. With pen and paper in hand, I started to walk round the vehicle and explain to Adam the true art of diagnostics, which ultimately relies on discipline and a religiously strict testing procedure. I walked Adam around the vehicle and, having spent ten minutes looking inside the car and under the bonnet, I quickly started to draw a picture in my head of what the problem could be.

I asked Adam to imagine this was a murder scene and in order to solve the case we must carefully look at all the evidence. He smiled enthusiastically and I'm sure he thought I was a little mad - he's probably right but it is usually a hereditary condition. Upon my initial visual examination, I found the following evidence.

Vehicle had been used as a taxi

Despite the contaminated crime scene, I felt there was enough evidence here to form a diagnostic plan and know where to start. The vehicle battery was confirmed as dead on arrival so we initially had to replace it, which allowed us to unlock the vehicle and carry out some initial serial diagnostic tests.

I explained to Adam why it was important to gather such an important and accurate picture of what occurred at the moment of death. He was even more intrigued and said, "It's like Sherlock Holmes or CSI!" Elementary my dear Watson... I went on to explain further about the key points we had initially discovered and the importance of assessing the evidence very closely.

It was a taxi so there could have been a possible disturbance to the CAN wiring network when the taxi meter was removed

It wasn't possible to communicate to the engine, transmission or ABS module and according to the central electrics 2 module, DTC evidence suggested there was no communication between these modules.

We pushed the vehicle into the workshop, I wheeled the scope over and decided to take a look at the CAN signal at the ABS control module as it was easily accessible. The CAN network was down completely, showing only a flat line voltage. It's worth noting that this vehicle uses three networks in total - a high speed, high priority for drive train systems (engine, transmission, ABS), a low speed for convenience systems and a LIN network for auxiliary components, wipers, windows etc. The evidence suggested a potential wiring fault on the high speed CAN network.

Figure 2

I soldered and heat shrunk the wiring back together and I was now able to communicate with all the modules. After a quick crank the car sprung into life and stayed running. Admittedly, I did spend the next four hours gloating and decided to completely reassemble the vehicle and professionally finish the job off.

Since we had removed the cowling under the windscreen, I decided to remove the rubber bung drain plugs and steam clean all the plantation in and around the bonnet area. I took the time to carefully dry and lubricate all the electrical sockets and replace all the missing hoses and trim. I then went through all the remaining DTCs and replaced several blown fuses.

I applied a BG 109 engine flush and replaced the engine oil and all the filters. It actually started to look like a presentable and respectable car again. I even took the time to remove the glue left on by the taxi company logo on the side of the doors. Despite the cracked windscreen it looked mint. Case solved.

Test: There was no communication between the modules

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