1992 Chevy P30

Hi folks, I'm asking for guidance here on driveability issues with my motorhome.
The chassis is a '92 Chev P30, drivetrain is /454 engine /4L80E trans/ 7060 ECU with the AWUM calpack. (I also have a BAMM that I can use)
This rig has been mostly ok in the five years I've had it, but about a month ago driving on the highway (minding my own business!) it jumped into neutral for a few milliseconds, engine revved, set an MIL and then it came back to normal, did it a few more times in the next five miles till I got it home.
Put my scanner on it, no codes.
Since then I've put all new solenoids in the trans, swapped out the IAC MAP EVRV and TPS with spares, tried a replacement ECU, and haven't found a real solution.
It'll run fine for a little while, but always misbehaves in some way if I drive it very long. I don't trust it on the road now for obvious reasons. It weighs six tons and doesn't have a drivers door. No fun when it dies in traffic.
When it runs badly the symptoms are poor idle control, both high and low, no starts - sometimes there's no fuel being sprayed, other times it goes full dump even before I've cranked it over, a lot of times the scanner won't connect and when it does there's usually no codes, though I once got a whole page of sensor codes. Early on I would pull the ECU and let it cool as it seemed to be getting warm. I finally had to re-crimp the pins I'd had it out so many times.
I have replaced the fuel pump in the past and set pressure, the rig is also completely stock.
It seems that I can get it to act up by wiggling the ECU pigtail wires and I've been trying to narrow it down to which one, or at least what group. Haven't had success with that yet.
The ECU does have power and ground where it should, and I've done pinout checks to all the sensors as much as I could without finding any obvious problems. I've checked the distributor pickup coil - @ 800 ohms, I have had problems with that before, but it seems good now.
I've pulled the underdash harness out through the firewall and gone over it looking for bad splices, checked my grounds, (I also got the GM wiring book - P models diagnosis and wiring diagrams so I'd at least have some information).
But I'm a little stuck at getting this narrowed down.
Any help with troubleshooting techniques and/or common failure knowledge on this would be greatly appreciated.
Thanks
Bob.

I know you said that you checked powers and grounds, but how did you do that. With an ohmmeter, or the voltage drop method? An ohmmeter will just tell you whether there is continuity, voltage drop will show you whether or not the conductor can pass current. If you need instructions on voltage drop method you can Google it or we can walk you through it. It really seems like a bad ground or a bad connection at the PCM. If both PCMs work the same, that probably rules out a bad PCM (maybe). I am not sure about the P series vans but the C and K series pickups had a major ground at the thermostat housing that was really bad for corrosion. I usually solder a fresh terminal on the wires and move them to an intake manifold bolt.

The transmission going into "neutral" is something else. The electronic shift transmissions will go into failsafe mode, which I believe is second gear. This would seem like neutral because the engine would rev quite high. That would be an indication of a power or a ground issue. If it was truly going into neutral, if you didn't accidentally manually shift it into neutral, it would most likely be hard parts inside of the transmission. Hard part failure is usually consistent though, not intermittent.

Hope this helps, Jeff

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.

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.

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...

Quote:

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Originally Posted by boba

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...

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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.

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...

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.

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.

Tested the DRAC module, first test was to plug into a breadboard and measure current draw on 12V power.
Failed. Module draws no current. Complete open circuit.
I'm using the pinout from the factory service manual, pretty sure I'm on the right pins.
I have some discrete electronics with me, can this thing be repaired?
Anyone have s schematic?

Quote:

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Originally Posted by boba

I have some discrete electronics with me, can this thing be repaired?
Anyone have s schematic?

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I have no experience with DRAC modules, but a quick Google search turned up:

https://www.udmodule.com/

https://atracom.blob.core.windows.net › gears › 2005 › 2005-04 › 2005_04_04.pdf

https://harristuning.com/Tbi/vssb-drac-calibration/

I hope they help.

Jeff

I was pretty much just chasing down hunches and trying to verify the condition of things that may have been causing a problem, I don't know that the DRAC should draw any current with power connected or not, the output speed sensor will generate a AC voltage as it runs anyway - so that may be what really powers it up.
I'd found the UD and Harris tuning links the other day when looking, can't get the atracom link to work though. It would be nice to get an explanation of how the DRAC does it's job so I'd know what to look for.
I did use the Harris link to set jumpers on the DRAC I do have, the speedo has always read a little fast so it was worth doing and I'm curious to see if it makes a difference.
I honestly think this is going to end up being a 7060 with bad output drivers.
Nothing else makes sense to me after all I've looked over, the replacement ECU will be here early next week...and if it's the correct one I'll know more then.

Replacement ECU showed up today, engine started right up & ran fine. Now for a road test & see if the trans will shift.
Edit - mixed success. Didn't work so well once I drove it, trans did shift which was a worry with an unknown PCM, but it still gave me an MIL after just a short time and idle rpm changed.
Looked through the driveability manual and found a reference to check the neutral switch, I did that and it was out of adjustment (how I don't know) but I reset it and things seemed to smooth out after that.
Took it out and ran errands in town all afternoon and it was fine in mixed driving, highway and in traffic. I was thinking once again it was good to go...and then it gave me another MIL and a high idle. (I've not been able to get a code out of this thing the whole time, when it goes bad I also get no-com with the scan tool.)
On the way out of town it went to neutral again but I was ready for it this time and got off the gas, the trans dropped back into gear without me having to stop this time and I got it back to camp Ok.
But I'm wondering if I've got a bad DRAC module and might be losing the VSS signal? To my mind that could do weird things to trans operation.
I can put a scope on it tomorrow and check a little further.
Pretty well frustrated with it for right now, but I'm not quite ready to give up on it just yet.

Quote:

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Originally Posted by boba

Pretty well frustrated with it for right now, but I'm not quite ready to give up on it just yet.

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Don't give up the fight. We're all pulling for you.

Hiya's
Have you tried the BAMM memcal? It's 3.73 vs your 4.10 rear end and probably a few other things. Once you lose scanner data stream, it means the eprom is no longer communicating. Those early 7060 memcals were notorious for pin problems.

Thanks for the reply's - both.
@ In-Tech I'm running the BAMM now, It's on a Moates adapter with a custom prom for the trans from Sinister performance.
Honestly I had no idea what I was putting in there when I got it, but it works as well as the AWUM the rig came with, which is also the chip I had in the original ECU when these hicups started.
I didn't know there were pin problems with the memcal's, all I can say is I'm getting the same driveability weirdness with either one.
@MO LS - thanks for the support, I'm trying to get somebody to rocket-freight a DRAC (CBU 16124435 if anyone's interested) to me by Friday. (I need to be gone by Monday next week).
Reason being, I did the jumper re-calibration for tire size on the DRAC I have, and noticed it now "bumps" the speedometer needle at low speed, maybe related - one of the last times I drove it with with the old 7060 PCM the speedo needle flipped over all the way to the right.
My current working theory is that the trans going to neutral at 50~60 mph, the pulsing speedo needle at low speed, and the speedo flipping are all related, and all point to a bad DRAC. Also if the DRAC is passing 12v to an ECU pin that looks for a 5v square wave signal it could be the source of the electrical aneurysm this thing is having now.
I wish I could get an actual circuit board schematic for the DRAC, I'm thinking it may just have a bad diode and it's putting 12 volts to the ECU on the VSS input to the PCM.
I have diodes and a soldering iron.
I could fix that here and be done with this drama!
(sorry if my grammar and context are scrambled this morning, it's early and I'm pretty cranky over this...)

Quote:

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Originally Posted by boba

@ In-Tech I'm running the BAMM now, It's on a Moates adapter with a custom prom for the trans from Sinister performance.

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First thing is get rid of that adapter and just try the memcal alone, Those are a HUGE failure point regardless of brand. Report back ;)

I found and ordered a replacement DRAC, my testing approach today to start with will be to put my scope on both trans output speed and VSS input to see what I get.
Question on ditching the adapter though, you say the BAMM is for a 3.73 gear, so if I remove the adapter it will control the trans, I'll just be getting a different speed input?

Quote:

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Originally Posted by In-Tech

First thing is get rid of that adapter and just try the memcal alone, Those are a HUGE failure point regardless of brand. Report back ;)

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I changed back to the original AWUM memcal, still had a MIL & low idle, re-seated the chip but didn't bottom it out in the receiver and it was suddenly all good.
I don't know what to think of this now, except it's going to be hard for me to trust it. I'd do more if I had a work environment where I could, but that isn't the case for me right now.
I should get the DRAC in a few days, I have no idea now if that will change anything.
Wasn't able to get any scope patterns out of the current DRAC today, but that might be my fault - I need some more working time with the scope I have.

Hiya,
I'm not sure if you have internet on your vacation. How's everything going?

I'm full time in this thing In-Tech, it sucks to have my house break down on me, but I'm doing the snowbird thing so at least I'm in good weather.
Yes I do have internet (most of the time) I've had to learn to adapt on that like everyone else who does this. It's workable.
Updates since I've posted last - the DRAC module I ordered was a kerfuffle, seller sent the wrong part so I couldn't compare it to the one I had. He does have the correct unit and he's a good guy so if do end up needing the part I'll know where to get it.
I've also had a chance to open up the rest of the wiring harness and can say it's been thoroughly checked out and I haven't found any damage, some discrepancies compared to the schematics but I'm sure that's due to manufacturing changes that GM made during production, and it has been working fine up 'till it didn't.
Long & short is that I'll be putting it in a trans shop ASAP and getting the 4L80 rebuilt.
It's still going into limp mode, even though it's running better - and I don't want to drive 1200 miles back to home base in 2nd gear.
What really bugs me about the whole thing is that when It throws the MIL and goes into limp mode I get a no com with the ECU, can't read a darn thing, no codes or data all.
That and it doesn't store any codes either. This is consistent with both PCM's that I've had in it over the last six weeks and I have no idea why.
I get an MIL and the PCM totally sh*t's the bed. It's frustrating at levels I probably shouldn't try to describe, but you get the picture.
Other symptoms that lead me to suspect the trans are my speedometer needle is now pulsing at low speeds. I don't remember that from any time in the previous five years I've owned this thing.
In looking through some 4L80E rebuild videos I've seen rebuilders point out weak sprag clutches and torque converters along with everything else a manufacturer does to make their stuff just good enough to get through warranty, so my best guess is a failed rear sprag and a cracked drum that may have deformed the output tone ring.
This rig weighs in at a lithe 6 tons and it's got 125k miles so I'm probably due anyway.
I'll put that all on red and take it with me to the transmission shop casino.
'Cause baby needs a new pair of shoes!

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.

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.

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.

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.

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...

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.

Hiya Bob,
Did you make it home ok?

Still waiting for the replacement ECU Carl, I'm beginning to think some higher power doesn't want me to get up north this year for some reason.
Maybe all the problems go away if I just turn around and go south?
I dunno either...
It turns out I have enough spare parts with me to build a Speeduino, I've been working on that.

So it's finally fixed, did 700+ miles yesterday and today and got the rig and myself home.
Theory as to why this was such a headache - the only real failure found was a bad fuel injector randomly finding ground through the case. (hot or cold, wet or dry, day or night, it didn't matter - it could show up at any time, and just as easily behave normally for a day and fool me into thinking I'd found the problem somewhere else).
My theory as to how that caused PCM failures is that the injector was randomly firing in response to the intermittent ground, building up an inductive voltage, and when a timed ground from the PCM was present it was grounding not only the 12v feed for the injector, but whatever induced voltage the injector had already built up.
Odd that it didn't blow the injector fuse, but two new injectors and an undamaged PCM and the rig ran as good as it ever has and did a few days of driving without a hiccup.

Good job. :rockon::rockon::rockon: :thumbsup:

Thanks Carl, it would have been a significantly harder adventure without the interest of the people here, and I do hope it's information that will help someone else should they have this kind of trouble.
:thumbsup:

Yes, it's easy to get super cranky when you don't have anyone else to bounce Ideas off. Glad you got it sorted.
I'd still like to read that stock AWUM to have for my archives. If someone else has a copy, please post :)

So glad it's fixed and you made it to your destination. In 45 years of fixing cars I've seen shorted injectors (esp. Multecs) but not one that destroyed PCMs. One to add to what is left of my memory bank.

Jeff