30 AFTERMARKET MAY 2024 TECHNICAL www.aftermarketonline.net Fig. 5 Fig. 6 one and splits into two somewhere before reaching the fuse box. We then have a blue/red wire to pin 86 which the ECM grounds to close to the relay which then allows 12v to flow out pin 87 which goes through the fuse mentioned above and back to the engine ECU. This also supplies another fuse box and the glow plug control module as well as the mass air flow sensor. Please refer to Fig.5. Clearly wrong Now we know how the relay operates we can spot one big problem straight away. One of the supply wires to the relay has been cut and bolted with an eyelet to the frame of the vehicle, making it a ground. The yellow arrow shows the rest of the wire and where it should be connected. My first question was why, but then looking closer and testing the relay, I found it to be working. However, the blue/red wire from the engine ECU was providing power instead of a ground, so clearly someone had picked up on this and supplied the other side with a ground thus allowing the relay to close and operate. This was very creative of them, but unfortunately this isn’t how the circuit is supposed to work, so something is clearly wrong. So, we have power on a wire that should be ground with the ignition on or engine running which I also verified with my test lamp. Please refer to Fig.6. The yellow arrow here shows the external fuse I have mentioned a few times which should be taped to the loom but clearly isn’t as someone had been there before me. Thinking to myself, I postulated that the only possible causes for this fault would be either that the wire is shorted to power somewhere, perhaps rubbed against another wire inside the loom that has 12v on it, or if the ECU is outputting 12v instead of a ground. The easiest way to prove this would be to remove the engine ECU connectors and test the wiring and if then ok, test with the unit connected and disconnected. Output I then disconnected the relevant connector at the control unit and with the key in the on position tested for a short to positive with my multimeter. The meter showed that no short was present so next I reconnected my test lamp bulb to the wire at the relay and connecting and disconnecting the plug showed the bulb to light and then go out proving the fault was inside the control unit. Testing the other power supplies and grounds while there showed everything to be ok so we now had an answer and the customer could be contacted to authorise a new engine control module and a new turbo actuator motor. The customer gave approval and the parts were ordered. While I was waiting for them to arrive, I rewired the harness back to how it should be so when the new ECU was fitted the circuit would be how the manufacturer intended it to be. I can only presume as the ECU wasn’t controlling the main relay correctly it didn’t allow the vehicle to start due to a logic conflict internally as it will measure the output to the relay for circuit and fault monitoring. Solid process After a few days the parts arrived and a repair could be carried out. After fitting and programming the new ECU and fitting a new actuator, I turned the key and the engine fired into life for the first time in months confirming we had a fix, as there was no exhaust or coolant lines connected I could not run it for very long but the fact it started was enough indication that I had fixed the problem. A final fault code checked revealed a few open circuit fault codes. Checking at the engine showed them to be left disconnected so these would clear once the vehicle was rebuilt back up. What caused the ECU to supply a power instead of a ground I hear you ask? I’m not sure. With the bonnet being off for so long maybe the ECU got wet internally and corrosion shorted the ground to a power supply. Alternatively, perhaps damage was done during the engine change. I just don’t know; I can only assess the vehicle as it is in front of me. Once again though, like all of my articles, this shows the importance of understanding what you are testing, having good technical information in the form of wiring and technical bulletins and a solid process.
RkJQdWJsaXNoZXIy MjQ0NzM=