I know the ECU gets its earth from the plenum. How is the plenum grounded though? I thought there is an earth wire connection to the throttle body which also grounds the plenum. There also seems to be an indication of a wire going there but it's been ripped off in my case.
The plenum has it's own earth strap which runs from the main loom behind the engine right where the two connectors to the ECU split out. But failing that the plenum is bolted to the main block which will no doubt be earthed too.
The differences between a 156 and a 147 appear to extend beyond the (to quote Gus) "internal strategies" within the ECU. For instance, it would appear that the throttle pedal connection shown in this pic;
Is 147/GT only, the 156 lacks it. So you may wish to consider these wiring diagrams as being for the 147/GT only;
Chris, thanks for that.
I'm studying these diagrams now and I'm guessing the C40 and C10 earths join on the plenum since on one diagram for the ECU there is only a C10 earth and on another there is only a C40 earth.
I'm also attaching a photo of my mystery connector which I'm guessing is a C40 earth. So if I'm reading this correctly there should be another wire from it also going to the ECU and joining the C10 earth since according to the diagrams C10 is definitely on the ECU?
I guess I really like to complicate things :biglaugh:
That's definitely a broken earth there! I would get a replacement cable on that asap, you might find there is just enough copper remaining to keep the ecu happy. If that breaks completely while you're driving though everything will go down!
Edit: What's that cable that (in the second picture) appears to have a light purple line running down it. From the camera angle it appears to bolt onto the same place of the rocker cover as the "broken" earth, and then loop back up to the plenum / ecu ? It looks like a non-factory cable to me and it could be this that is earthing your ecu and throttle body?