1992 Chevy P30

[In-Tech]

Hiya boba,
The next thing I would do is a replacement internal harness for the 4L80E(another notorious fail). You're going to want one for the rebuild anyway(as well as all solenoids during rebuild), so, money well spent. Easily can be causing all your trans and comm issues.

[boba]

I have one, I ordered it when I replaced all the trans solenoids early on, I didn't know it needed an external adapter piece because of the connector style change and the parts supplier didn't tell me, so the old harness is still in there.
I dropped the pan once out here in the dessert, I don't want to do that again - I'll let a shop look at it & if it's just a harness I'll be grateful.

[boba]

Well I'm ready to be done with this adventure.
I've poured all the blood sweat tears and resources into this POS that I care to. It's still f*cked and there doesn't seem to be any way the un-f*ck it.
I got my transmission external wiring harness adapter (for $117) yesterday and very carefully went over every wire before I installed it.
Dropped the pan and replaced the internal harness and got all of that squared away.
There is no damage to the original harness that I can find, and the result is the same, MIL and limp mode.
I'm so mad I can barely type this out.
The tech info I've been able to get is horrid and the build quality is worse.
At this point I either put a haltech on this thing or burn it, but GM can kiss my ass.

[boba]

My apologies for being a salty cracker in the previous post...one little update FWIW.
Stewing in my melancholy I was thinking through the schematics, just rolling the last six weeks over in my to see if something - anything - would come to mind that I could recheck.
5 volt ref's came up, specifically was there a 5v ref for the trans, and if so what else would be on that circuit?
I don't think there is a 5v ref for the trans, but there are two sensors on the engine called out on the transmission control page that *do* share the same 5v source and those are MAP and TPS.
I have used spares for both, and had changed out the MAP when this failure first happened, why I don't know - I was just hunting and pecking to uncover a problem and I didn't know nearly as much about the system then as I have learned now.
So after changing the trans harness and still being in limp mode yesterday I was pretty well stumped on where to go next (except to the junkyard) so I changed out the MAP for the old one.
Problem went away. When this thing runs right it runs really good, and it was back to it's old self.
So my theories about why this has been such a headache, I believe the MAP did go bad, perhaps the one I'd originally changed out was also tired, if it was pulling the 5v ref to ground it would also pull ref power for the TPS, and since the PCU needs both signals to control output's for the trans I can see why I was getting limp mode there.
There may be some additional issues yet that I haven't found, I did find and repair some overheated wiring that routes over the left side exhaust manifold and sends power to the fuel pump & sender and that may have pulled too much current at the PCU to begin with, right now that part of the electrics seems to be working ok.
So I have a new MAP ordered and will pop that in tomorrow and see if it gets me back on the road.
I really did think I'd killed the second PCU and I was pretty upset about that, but maybe it isn't as bad as I had feared.
Anywho, that's the update, I know I'm not the only one who's gone rounds with a 7060 PCU so maybe someday this will be useful to someone else.
Edit - Nevermind. Still bad. Thought I'd recheck fuel pump wire through it's connector. Key on after that & it's back to brain dead.

[boba]

I thought about failure modes a bunch last night, going over all the symptoms I've had in this debacle, the chattering from the fuel pump relay, injectors dumping fuel randomly, loss of idle control and finally limp mode with the trans and finally had to get up and get out my schematic book and look through it to see if there was one component somehow connected to these systems at least in a peripheral way.
And after some consideration I came up with the oil pressure switch.
On the motorhome and perhaps some other rigs with the 7.4 the fuel pump power runs through a switch in the O/P sender and returns a signal to the PCM, so if the sender failed it could possibly pull the F/P circuit low and pull who knows how much current off of other PCU circuits if a contact ring in the sender was making contact with the metal case wall.
Anyway, that was my theory and I was able to get one at the parts store this morning so with high hopes I put it in along with a new MAP sensor.
There was no change it how it ran though. So I hunted & pecked, rechecked power supply to everything I could think of finding no problems, and then put my O-scope on the IAC to see if I could catch any wave form there.
Saw nothing on the scope, checked that I was in the right ranges and all I could see was a little static.
The only other thing I found that was suspect was a spark plug wire with a slightly melted plug boot so I changed that.
Took a sit and a think for awhile and decided I had nothing to lose popping the old PCM back in to check, the one I was pretty sure I'd killed.
So I did. It gave me a double blink with the MIL in KOEO - something I've taken as a sign of life, and it lit right off and ran beautiful with no MIL when I turned the key.
Huh?
If you feel confused imagine my disposition seeing that.
Went for a test drive, it died at the first three stops, but stayed running if I used neutral when stopping. Restarted immediately each time though and wasn't setting a MIL.
I found a place to pull over and relearn the IAC, plus made a small adjustment to the TPS which was reading .6 of a volt instead of the 5.8~5.9 it's supposed to have.
The rest of the test drive was perfect, didn't die at stops, got to full temp and stayed running smooth.
So I'm wondering something on this, is there a PCU failure mode where the only way to clear the gremlins is to power it down and restart from scratch?
If so this is the first time I've run into it, but considering I may have been running leaky voltage all over the harness with a bad oil pressure sender I wouldn't blame the thing for going turtle on me.
Anyway, it seems to be doing Ok right now, I'm still going to be nervous driving it at least for a while - but if it still runs in the morning I'm going to set out and see how far I get.
I've got 1200 miles to go and that seems like a big ask of this old rig, but I'm not finding anything else wrong so here goes...

[boba]

Found a bad fuel injector, it was grounding to the case when up to temp.
Replaced both injectors with new ones.
Driveability got better for a while, then the gremlins came back, still going into limp mode, loses com, MIL on and running poorly. It's random but recurring.
Wired the trans for full manual control to get around limp mode, at least it was driveable for a day.
Second day was bad, MIL would come on and engine would die and not restart. PCU seems very unhappy.
Currently broke down in rural central Nevada awaiting (yet another) replacement PCU.

[In-Tech]

Hiya Bob,
Did you make it home ok?