Need an EFI tech!

[Jeff Chambers]

Originally Posted by Capri306

Sounds like bad grounds or something not pinned-out right in the harness. I'd double check the wiring second, the ground connections first, all with a schematic and test light...or something like that. Hopefully it's something relatively simple and easy to fix. Good luck!

Grounds are the first thing I'd check as well. I've seen multiple sensor codes like this before and found that the EEC ground (located up in the right kick panel) had broken. Fixed the ground, fixed the problem.

[ripperdog]

Well, I found the problem. There is a burnt trace on the ECU circuit board leading to pin #46 (sig rtn). This explains why all the sensors on the sig rtn circuit were reading out of range and why the engine was running so badly. The cause of the problem, however, was a bit more difficult to find.

I didn't know at the time but, when I bought the harnesses I was incorrectly supplied with an O2 sensor harness for an automatic transmission car instead of manual transmission. Automatic cars supply 12volts through the neutral switch circuit where as manual transmission cars work with the 5volt vref. The cross-over is done in the O2 harness. The first time I stepped on the clutch it dumped 12 volts down the sig rtn wire and cooked the computer.

The fix was simple...just move one wire in the O2 sensor harness plug. Unfortunately, now I need a new computer. Anyone have one for sale?

Anyone planning an automatic to 5spd tranny-swop should make sure that they get a new O2 sensor harness if they are planning to change to a 5spd computer.

Thanks again for all the help! With any luck I should have this thing back-up and running sometime soon.

[Capri306]

Originally Posted by ripperdog

The fix was simple...just move one wire in the O2 sensor harness plug.

HA! I'm not surprised in the least amount by this news, but still, ain't it a hoot? Electronics and electrical problems in general are usually like this. Guess that's why I'm studying to be an electrical engineer: if you know what to look for, you're 90% there.

I'm very happy that you found out what it was so quickly, though. Props to you, but sorry I can't help you with a new EEC-IV PCM. I'd probably look for one in a boneyard out of a REALLY wrecked Mustang, because unless it was absolutely FUBAR in the passenger side, the computer should be fine.

tmoss: I think he'd be best off finding a whole new PCM because if there was a voltage surge up to the PCB, not to mention through the signal circuits, it's cooked and may present problems later down the road, even if the gap is bridged. There is a gap there for a reason, and bridging it would be like sticking a piece of aluminum foil into a fuse socket " 'cuz the damn thing keeps blowing." hehehe, i have to laugh a little because I've done that before to get home...but, I digress.