Extracting data

14 October 2013  

By James Dillon

Figure 1

In my experience of this type of failure, it is caused by either a component which has short circuited, causing excessive current flow, or the battery polarity will have been inadvertently reversed. The customer assured me that battery polarity hadn’t been reversed. In fact, he’d never had the battery off the vehicle (or done anything relating to the battery), so, I am probably looking for a component failure. The customer arranged to drop the stricken vehicle in to me. My task was to use a wiring diagram and the ECU internal view to review which pin in the multi-plug connector the damage related to. This way, I’d be able to see which component(s) were connected and begin my search for the root cause. If the customer fitted a replacement ECU to the vehicle without doing this fault finding, the replacement ECU may suffer the same consequence as the first.

The vehicle arrived and I checked out the ECU and engine bay wiring loom. None of the connection wires showed any sign of heat damage. On reviewing the wiring diagram, the larger gauge pin which led on to the ECU’s internal resistor was a main power supply – there wasn’t a component attached. So, my short circuit component theory had to be put to one side. I called the customer and discussed my findings. “It looks like the ECU has had the power and ground reversed,” I said. “Did you connect the battery up the wrong way?” “No,” was the answer. “Definitely not.” “So the vehicle just stopped one day,” I asked. “Yes,” he said. My sixth sense was clanging like Big Ben at this stage. “In my 25 years of experience or electronic system repair…..” I went on to let the customer know that I had serious doubts over the accuracy of his explanation of the run up to the fault occurring (that’s a nice way of putting it, isn’t it?), and confirmed to him that telling the whole story would likely result in a smaller repair bill. The customer persisted with his version of events.

figure 2

The replacement ECU was delivered a couple of days later. I had a visitor from a college at TTHQ with me for the day and it was a great opportunity to test out my new course and coaching skills. David, my guest, had never soldered or de-soldered components before and he commented that his standard wire soldering skills could be best described as ‘rusty’. After 20 minutes tinkering, he’d managed to extract all of the salient information from the original ECU and transferred it to the replacement unit – job done. We fitted the cover back on the new ECU and reconnected it to the vehicle. I turned the key and it fired up (no sign of magic smoke either), an altogether nice repair.

ECU: The faulty ECU was destined to become a doorstop

Figure 1: A resistor had blown and smouldered the ECUs casing

Figure 2: A short circuit caused major damage