1992 Chevy P30

[boba]

I did mostly use a VOM for resistance when checking circuits Jeff, I scoped output on the trans input speed sensor and the distributor but that's as far as I went with actually looking at running voltages.
There is pretty limited access on much of the wiring, not to make an excuse - but it's a painful rig to work on. I do the best I can.
The ground you refer to is called out as G102 on the schematic, I have loosened and re-tightened the nut but I'll go further there, that one is easy enough to do.
I will refresh my memory on how to do voltage drop testing and see how much of that I can get done today.
I agree that it looks like a power or ground issue, I just haven't found the smoking gun...yet.
The replacement ECU was a Cardone unit that I ordered from a shop after providing all the relevant info, but I don't know who the actual supplier was or if it was flashed properly before it was sent to me. It ran worse than the original and I sent it right back.
There seems to be some disagreement on programming the bare metal ECU's, (from auto parts stores anyway) some people say they need a flash and some just look at me like I'm a dummy. I don't know what to believe on that.
I did try a replacement ECU once years ago when I was fighting a bad distributor pickup coil, the engine ran great, but the trans wouldn't shift, so my best guess is that they do need to be flashed.
I'm sure someone here knows the particulars on that, It would be nice to have that confirmed one way or the other.
I did talk to a trans shop and they told me about the failsafe mode, originally I thought that was going to be the issue...but as you say - the symptoms are to random and I'll need to have it running right before I go further with that. I don't want to buy a trans rebuild if I don't need it. That day will come soon enough anyway.
Thanks for your reply, I'll get done what I can and post back.

[boba]

Ok a little more data - the G102 ground was a little bit lazy on the voltage drop test compared to a ground wire to the battery, cleaned it up and it was good after that.
Engine did run better as long as I didn't touch anything, when I shake the ECU pigtails it went bad though.
Substituted a backprobe ground from ECU pins C2 and C3 to ground and things improved, now the only symptom is idle control when I move the wires with engine running, I'll continue chasing that down...BUT in looking through the ground distribution from the schematic I see it lists a G103 generator bracket ground.
My harness doesn't have that, or any hint that it ever had one there. So, quando quando?
Schematic says that's where the ECU grounds go.
I'm going to make an executive decision here and relocate my ECU grounds so I know where they are at least. Maybe that will sort out the IAC issue too, but I'll treat it as it's own issue if not.

[boba]

I put a splice to ground for both ECU ground wires and it seems to have the engine running as it should, I can move the ECU around a little bit now and it doesn't affect idle, it does highlight the next symptom now which looks like the twisted pair wiring for the front speed sensor. Moving them around with the engine running does cause speed fluctuation.
I wasn't expecting that, does it need to routed well away from other high current wires, or perhaps there's a problem with it too?
Or maybe it'll be ok when I tape the wires back up.
Hmmmm...

[MO LS Noobie]

the next symptom now which looks like the twisted pair wiring for the front speed sensor. Moving them around with the engine running does cause speed fluctuation.
I wasn't expecting that, does it need to routed well away from other high current wires, or perhaps there's a problem with it too?
Or maybe it'll be ok when I tape the wires back up.
Hmmmm...

That would be a new one on me, it shouldn't have anything to do with the idle speed fluctuation, but circuit grounds cause all kinds of weird symptoms when voltage gets back through the circuits, instead of being routed ground. Yes, definitely keep everything away from the high current battery cables, alternator output, and all secondary ignition wires.

[boba]

I'll pull the harness out one more time and look at splices over by the DRAC module and steering column, resistance check on the speed sensor pair look ok, not melted together, broken or going to ground as far as I can tell.
It seems like splicing in grounds to the ECU might have done the trick, I'll know more when I get it back together and see if I can actually drive it anywhere. :-)
[circuit grounds cause all kinds of weird symptoms when voltage gets back through the circuits] < that's the part that worries me, I don't know why or where it failed exactly, but it seems like that's what I'm up against on this one.
Thanks for fielding my question here Jeff, I'll post back when I know more...

[MO LS Noobie]

Stray voltage can happen when you have high resistance at ground connections. Instead of going to ground to complete the circuit, it backfeeds into other circuits causing havoc.

[boba]

I put everything back together this morning thinking I'd repaired the bad ground and I was ok. It started and ran fine last night.
It didn't get warmed up to go down to the tank dump this morning before it was acting up again. This has become the consistent pattern.
Got back in and checked the ground on the thermo-housing, cut the ring terminal off and saw that the wire was discolored back under the insulation, got aggressive and cut 4" back off the wire till it was well and clean and crimped on a new extension tail and new ring. Fastened it to the alternator bracket. Seemed ok for a hot minute, but it didn't maintain it's manners for the 1/4 mile to the dump station. And it flipped the speedometer needle all the way over also which I've never seen before.
I ordered a replacement 7060 ECU off of Ebay, I gotta get this thing driveable soon and I can't think of anything I've left un-checked.
Re-checked the TPS voltage (it was a little high) Set the throttle screw to adjust, and then re-did the IAC set procedure too just for grins. I didn't need to adjust for base idle - it was good, but it won't hold target RPM (should be 750, idles at 800).
Did those settings with the scanner so that's what the ECU is seeing.
Anyway, that's where it is for right now. When it went bad it was all of a sudden, and I haven't found a bad sensor anywhere in the system...so with crossed fingers and low hopes...it gets another ECU.
Past that I got nothing.
If I can't get it going it'll be out in the California desert with a signed title in the tray over the drivers seat.