Ground fault on V3 board.

[DaveEFI]

Repairing an MS1 V3 which isn't totally dead, but exhibits some weird faults. So many I concluded there would be a common cause. 5v reads just fine - which confused me. It goes online ok and reads RPM from the Jimstim in Tunerstudio, and most things sort of work on the Stim. But no injector LEDs. Pulses are present at the output of U4.

To cut a long story short, there is no ground continuity between ground at the DB37, and pin 2 of U5, or any of the other grounds on the high current side like R37 and 38. The proto ground is fine to all the ground pins on the DB37. Examination showed the track to pin2 of U5 burnt out - but is this the only ground path to the high current side? Repaired the track and it now seems normal on the Stim - but does that thin track really take all the injector driver current?

[Matt Cramer]

Check under the heat sink - there's a ground path that comes out to the right of Q16 and continues under the DB37 that carries all the high current. I've seen maybe 5 times that's failed, usually from an ignition fault.

[DaveEFI]

Thanks Matt. Holding the board up to a strong light shows the gap between the low current and high current sides, but difficult to see totally on a populated board where the internal track(s) are. And don't have a new board to hand at the minute. It's been built for 4 cylinder wasted spark using the existing coil driver and an additional one where R38 goes. R37 has also been removed and a high current FIdle mod fitted there. So I did wonder if it was the lack of max current limiting on the injector drivers which had caused the damage, after a fault in the wiring.

One injector driver was faulty - Q15 short circuit and red hot - and one ignition output not working fixed by re-loading the firmware. Since the ground to the 5 v reg was non existent, it's lucky the processor wasn't toast too.

[john.p.clegg]

Thanks boys,I'll put my soldering iron back now...

John