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Thread: 1992 Chevy P30

  1. #1
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    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.

  2. #2
    Fuel Injected! MO LS Noobie's Avatar
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    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

  3. #3
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    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.
    Last edited by boba; 3 Weeks Ago at 05:01 PM.

  4. #4
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    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.

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    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...

  6. #6
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    Quote Originally Posted by boba View Post
    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.

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    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...

  8. #8
    Fuel Injected! MO LS Noobie's Avatar
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    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.

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    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.
    Last edited by boba; 2 Weeks Ago at 12:50 AM.

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    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?

  11. #11
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    Quote Originally Posted by boba View Post
    I have some discrete electronics with me, can this thing be repaired?
    Anyone have s schematic?
    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

  12. #12
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    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.

  13. #13
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    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.
    Last edited by boba; 2 Weeks Ago at 07:39 AM.

  14. #14
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    Quote Originally Posted by boba View Post
    Pretty well frustrated with it for right now, but I'm not quite ready to give up on it just yet.
    Don't give up the fight. We're all pulling for you.

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    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.
    -Carl

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