its the gearbox earth. if its poor, the starter motor tries to earth itself thru the ecu, which blows it up. IIRC its one of the components on the PCB that fails rather than the PCB itself, which means it may not be obvious where the issue is. There isnt really anything else which causes the ECU to fail like this short () of damage to the ecu wiring perhaps but if you can see no obvious signs in the engine loom its probably not the cause. the earth for the ecu is on the passenger side of the head, there should be at least 2 brown wires with a ring terminal on, this is the earth the starter motor tries to fry the ecu through if the box earth is poor. the clocks also earth to the side of the head and I believe a poor gearbox earth can also damage them, I've seen several where one of the tracks on the PCB is melted and I suspect this is why. you could avoid this issue ever happening again simply by earthing the ecu and dash wires direct to the battery, then the starter wont be able to earth itself via the ecu
I was tracing the ECU earths last autumn to look for a possible cause of poor idling. IIRC there are two earths for the ECU, one for the ECU itself and one for the injectors (which are earthed through the ECU). One of these earths connects to the side of the cylinder head, and the other was connected to the battery clamp. Now I did get some odd resistances once voltages were applied which I still need to look into further. My earth straps looked good however, so I just cleaned them and put them back
ah yes you're right i forgot about that one. it runs alongside the earth for the TCI-H unit and is crimped to the same ring terminal
Sweet, I won't be doing all this work until the weekend now so wish me luck! If it doesn't sort the issue I think it's ABF time..
All fixed! I changed all engine earths, cleaned up all the contact points, replaced the ECU and the ECU relay. Runs, idles and starts much much better now. Although 30 seconds into the test drive a got a stone chip/crack.. I need a new windscreen. I mean come on.. When will this **** end!? But thanks for all your advice! Love you x
