4L80E Limp mode, Trans fuse blowing, unable to clear DTCs 81, 82, 83 - Diesel Place : Chevrolet and GMC Diesel Truck Forums
 
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4L80/85E Transmissions. 6L90 Transmission Discuss the 4L80/85E transmissions found in the 6.2/6.5L equipped vehicles and Duramax equipped vans. 6L90 discussion.


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Old 10-28-2019, 01:54 AM   #1 (permalink)
redshift9
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4L80E Limp mode, Trans fuse blowing, unable to clear DTCs 81, 82, 83

Hey all,

This is long winded. If you're not super interested in armchair troubleshooting of 4L80E bugs, punch "eject" now and save yourself a migraine. Lol.

After exhaustive searching on here, and a fair amount of troubleshooting, I remain stumped about this problem. Trans and electrical is all stock/original afaik, aside from a new ignition switch, which did not seem to affect anything. Fluid looks normal and level is perfect, as it has been for a year at least. No leaks, no other issues at all. No unusual speedometer or TPS behavior whatsoever.

All grounds have been gone over recently and are in great shape, verified with multimeter. With the possible exception of the "instrument panel ground" as shown in the service manual (screenshot attached below), which does not appear to exist at all on this truck. Whatever the little relay-type box is that is shown in this diagram, I don't have one, and I'm 99% certain that the truck never did have one, or the ground pigtail shown adjacent to it. No sign of holes or anything where it would have been on the firewall. Afaict, the PCM grounds to one of the two studs at the back of the intake on the passenger side. If the 94 trucks have a ground for the instrument panel that affects the PCM in any way, I'd sure love to know where it lives.




I'm going to list the whole chronology here, in hopes that this may end up being of use to others once I manage to track down the gremlin. I've found a dozen posts citing similar issues, but only two of them reported back after solving the problem. (I promise not to leave y'all hanging once I crack this riddle.) One of them was resolved with a new ignition switch, the other was due to cracked insulation between wires at an external harness plug. The others all remain a mystery.




This all apparently started fairly suddenly a few weeks ago (no previous transmission problems whatsoever in the 15k miles or so that I've put on it since buying it) with the truck stumbling as if it had some air bubbles in the fuel line when I took right turns with anything but the lowest of speed. It would sputter a few times over 3-5 seconds, then return to normal. A couple of times it actually died, but restarted immediately and drove fine until the next moderately hard right. No DTCs, no limp mode, no blown fuses, so it seemed like a fueling issue.


No air bubbles at the clear return line. I recently installed a new Delco lift pump and checked the FFM and screens, so I figured I try changing the fuel filter as it was probably about due anyway. Checked the FFM bowl and screen while I was there and all looked clean. I was a little surprised that the new filter didn't seem to improve the problem at all.

On the next drive, it went into limp mode (2nd gear only) within a few blocks and after a little poking around I realized that the Trans 10A fuse had blown. Replaced the fuse but it kept going into limp mode either immediately or soon after starting to drive. Still no CEL but when I hooked up GMTDScan it gave me historic AND CURRENT codes of 81,82,83, and 87. These translate to Shift Solenoids A & B, TCC Solenoid, and High Ratio Error. Cleared the codes but they came back fairly soon after, along with the limp mode again. But no blown fuses. And still no CEL despite the DTCs. Weird.

After some reading here, I checked the voltage at the trans fuse socket and saw about 0.5v to battery ground on one side of the fuse socket and around 0.1v on the other. That had me shaking my head. WTF? No other blown fuses or DTCs to indicate what was amiss.

That sent me in the direction of the ignition switch, and despite the fact that checking it at the connector showed the expected 12.5v at the pink wire leading to the fuse, I figured the switch was the most obvious suspect, so replaced it with a new AC Delco unit from Rock Auto.

After replacing the switch I got 12.5 with key on but not running, and 14+ with the engine running, but the DTCs were still setting immediately and knocking the truck into limp mode again. So pulled the trans passthrough connector and it was not dry, but not dripping either. Cleaned both sides with WD-40 first, then TF solvent (contact cleaner), then blew dry with canned air. Checked voltage at pin E in the harness and got similar readings to above. Reconnected and went for a test drive but it went into limp mode again within blocks.


As an aside here, I have increasingly had trouble getting GMTDScan to connect to the PCM over time. It has never connected consistently, and tends to drop it's connection before long whenever I use it. I've suspected that my ALDL cable was the problem, but I'm not entirely convinced of this. I haven't found anyone else complaining about this here, so I tend to doubt that its a problem with the software.




After pressing "connect to truck" 50x with no joy (see above), I started to wonder if maybe the PCM itself might have something to do with both the connection failures and the trans problem. I swapped it out with a (previously tested good) spare and it seemed to connect to GMTDScan a little more readily, and a quick test drive seemed to have solved the problem! Until two minutes later when the trans fuse blew and the same DTCs set again. Since then, test drives have suggested that the fuse is more likely to blow on right turns or rough road, but it's blown far more often at random moments with no obvious cause. I've prob gone through 20 fuses by now. I've tried 15a fuses and they blow just as readily. Tried a 25a fuse once as well, blew just as quickly. So I don't think its a matter of a deteriorated or failing solenoid drawing a little more current than expected.

Since then, the situation is this: I can operate the 1-2 and 2-3 shift solenoids just fine using GMTDScan on a laptop, until it loses it's connection to the PCM or until the 10a trans fuse blows. This works despite the limp mode and DTCs. I am definitely getting all four gears just using the up/down buttons in the software. I have also noticed that I cannot clear the DTCs using GMTDScan or by removing power from the PCM no matter how many times I try. I suspect that this may be due to the electrical condition (short?) persisting. Maybe the PCM is continuing to set the DTCs every cycle since its seeing the problem every cycle, thus ignoring the clear command? DTC 87 sets occasionally, but 81, 82, and 83 are constant, both in history and current, and refuse to be cleared, even when the fuse is holding fine. Doesn't it seem like if the voltage is steady and within range and the fuse isn't blown, then those DTCs should be able to be cleared? Maybe this is a software or PCM fault unrelated to the any intermittent short circuit?

Here is a typical DTC scan result. Ignore the FSO solenoid DTC #13, that solenoid has been gutted and DTC #13 is a constant.

blob:https://imgur.com/2c979635-9348-4a78-89f2-edc945b779be


I do seem to have consistent 12v at the fuse and at the trans passthrough with the key on, and 14v with the engine running since changing the ignition switch, so maybe that helped somewhat. But it seems apparent that there is an intermittent short somewhere between the fuse and the PCM quad driver ground. I'm going to try to at least determine whether it's internal or external by unplugging the passthrough connector and seeing if I can get the fuse to blow while wiggling as much of the external harness as I can reach. I also have not checked the resistance of the solenoids yet due to lack of appropriate probes to get into the pins, but I'm going to try using paperclips and alligator clamps and see if I find anything out of spec there. Guessing that may not yield much info given the intermittent nature of the problem. But worth a try. (Once the hellacious winds die down, that is. This whole damn state is on fire right now. Crazy.)

My best guess at this point is that it may be the internal harness (assuming the solenoids check out ok). I've ordered replacement solenoids and harness, but while I'm waiting I think I may pull the pan and inspect the internal harness for damage and see if I can find any problem using a multimeter. Has anyone done this and found a problem area? Or have thoughts about particular points where a short or wiring problem might be likely? I've heard that failure of the internal harness is not uncommon, and as there are boatloads of replacement harnesses available online, I'm guessing this would not be an unusual failure mode.

If anyone has solved similar issues or has ideas about anything else worth checking (or methods) I'd love to hear them. Am I overlooking anything obvious?

If you actually read this far, beers are on me next time you find yourself in California. Seriously, thanks for any suggestions you can offer.

Attached Images
File Type: png Instrument Panel Ground.png (78.1 KB, 19 views)
File Type: jpg Bad connector in trans wiring harness behind intake (1995).jpg (31.4 KB, 0 views)
File Type: png GMTDScan typical connect failure 2.PNG (33.3 KB, 20 views)
File Type: png DTCS typical.PNG (35.1 KB, 20 views)
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94 GMC Sub K2500 SLE - 170k
33" ATs on 17" H2 whls (minimal wheelwell trim rqd)
PMD cooler to frt bumper;
2/0 Btty cbls, 4ga Alt & grounds
4" DiamondEye Exhaust; K47 Airbox
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Old 10-28-2019, 01:52 PM   #2 (permalink)
Glagulator
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You have harness problems. I have rebuilt the 94 harnesses more than any other.
One of GM's harness contractors was using inferior wire on the 94's.
Some are OK and some look like yours. Based on the engine harness connector in the photo,
if you open up the harness you will find wires fused together just barely shorting, corroding, etc.
The engine harness under the intake will be bad also.
Here is a photo of a short in the transmission part of a harness,
(not a 94), any year can have these problems.
You can remove the complete under hood harness without cutting any wires, open up each section to make repairs, then replace it.


https://www.dieselplace.com/forum/at...1&d=1572281082

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File Type: jpg Shorted-Wire-02.JPG (118.9 KB, 2 views)
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99 CREW LONG SRW C3500 work truck 199k+ mi. orig. motor & trans 4.10, modded engine harness, new oil cooler & lines, all new front end parts and poly bushings, trans poly mount, all new brakes.
96 SUBURBAN K2500 wife's truck 210k+ mi. orig. motor & trans, 3.73...
95 C2500 Suburban 6.5td HO w/p, timing set, HB & pulley, all rubber hoses, modded engine & main harness, converted to OBD2, PMD to bumper...
95 C1500 Suburban 6.5td rebuild tranny, convert to DB2, replacing as many parts as I can.
96 C2500 Suburban 7.4L converting to 6.5td w/ DB2, 4L80, 3.73
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Old 10-28-2019, 06:15 PM   #3 (permalink)
redshift9
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Just to clarify, the photo I posted of the cracked wires was from another member's report of tracking down a similar failure. I included it as an example of one possible failure mode, which might help others searching for possible sources of these symptoms.
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94 GMC Sub K2500 SLE - 170k
33" ATs on 17" H2 whls (minimal wheelwell trim rqd)
PMD cooler to frt bumper;
2/0 Btty cbls, 4ga Alt & grounds
4" DiamondEye Exhaust; K47 Airbox
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Old 10-28-2019, 08:25 PM   #4 (permalink)
redshift9
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Quote:
Originally Posted by Glagulator View Post
You have harness problems. I have rebuilt the 94 harnesses more than any other.
Thanks much for this. I'll check the external harness over carefully before pulling the pan and checking the internal wiring.

Do you have a preferred source for replacement wiring harnesses (int/ext), or for connectors and high-grade wire, should I decide to fab a DIY replacement? Rostra seems to supply many aftermarket trans parts houses, but they're surprisingly cheap compared to many others.. makes me dubious about the quality. AC Delco replacement internal harness runs around $150 afaict, Rostra more like $40. Other off-brands run $60-$90. Normally I'd spring for the ACD part, but if the OEM harness was crap, Im not sure it makes sense to pay 3x for an OEM replacement. Are there other/better choices out there?

Incidentally, in keeping with your comment about the shoddy original wiring supplier, I've recently replaced the engine wiring harness entirely to resolve an intermittent open that I could not track down after several years of trying. So far so good. Haven't had that particular problem again since.

Does anyone know the route of the main trans harness from firewall to trans? I've scoured the manuals and cant find a drawing for the '94. Closest I've come is the 4x4/TC routing, but it doesn't show the main trans bundle. I have the schematic, but that doesn't tell me where the harness is routed.

Here are the 1994 4x4 harness routing drawing



and the 1994 4L80E PCM schematic.

https://i.imgur.com/Erv0A3O.png


I think I see some of the trans harness shown faintly in the first dwg, but not 100% certain that's it. Regardless, it falls a little short of telling me exactly where to track down the near end of the trans harness.
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94 GMC Sub K2500 SLE - 170k
33" ATs on 17" H2 whls (minimal wheelwell trim rqd)
PMD cooler to frt bumper;
2/0 Btty cbls, 4ga Alt & grounds
4" DiamondEye Exhaust; K47 Airbox
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Old 10-28-2019, 08:28 PM   #5 (permalink)
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Meant to include the above dwgs as attachments as well, in case the image host drops them in the future.
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File Type: png 4x4-TC Harness.png (65.5 KB, 18 views)
File Type: png 1994 4L80E PCM Wiring Diagram.png (71.4 KB, 20 views)
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94 GMC Sub K2500 SLE - 170k
33" ATs on 17" H2 whls (minimal wheelwell trim rqd)
PMD cooler to frt bumper;
2/0 Btty cbls, 4ga Alt & grounds
4" DiamondEye Exhaust; K47 Airbox
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Old 11-04-2019, 08:45 PM   #6 (permalink)
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Well, I finally disconnected the trans harness at the transmission, and after a LOT of swearing and climbing back and forth from on top of the engine to underneath the trans, I was finally able to free the harness and pull it up into the engine compartment. In the process, I noticed that the loom was pinched between a body seam on the trans tunnel and the bellhousing. Sure enough, once I got it free and unwrapped and inspected, there were some wires worn down to bare copper right where the pinch was. None were broken, and no frayed or loose copper strands, so I taped them up carefully and replaced all the plastic spiral loom as the original stuff was so brittle that it pretty much fell apart as I pulled the wiring out of it. Less than 200k on this truck. Seems kind of crazy that the plastic would be that far gone, but I've noticed the same with many of the Metripack connectors as well. GM seems to have used spiral loom all over the engine compartment for abrasion protection on A/C, fuel, and other rubber hoses as much as the electrical wiring. Almost all of it appears to be ready for replacement. One more thing to add to the list.

I was pretty stoked to have found what was likely the cause of all this headache. I pulled the PCM fuse for half an hour or so before starting the truck. It now no longer blows the trans fuse (duh). At least, it hasn't yet. But I still can't clear the DTCs, and the damn thing is still in limp mode. I'm not sure if there's a more complete way of resetting/rebooting the PCM, or if the trans would work properly once the codes were cleared. I fear that I've damaged the PCM by running it with the wiring shorts...

Has anyone else encountered the issue of GMTDScan failing to clear DTCs no matter how many tries/restarts/resets ? I hate to buy another PCM without verifying somehow that it's malfunctioning. It is also still difficult to get GMTDScan (or the basic version, same problem) to connect to the PCM... takes many tries before connecting. Is this common? Exact same issue with both PCMs I've tried now, so it's not likely a problem with the PCM.
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94 GMC Sub K2500 SLE - 170k
33" ATs on 17" H2 whls (minimal wheelwell trim rqd)
PMD cooler to frt bumper;
2/0 Btty cbls, 4ga Alt & grounds
4" DiamondEye Exhaust; K47 Airbox
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Old 11-05-2019, 04:07 AM   #7 (permalink)
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Thanks for posting what should be the problem.


Don't know about why your computers won't let their codes be cleared, other than making sure everything (the computers and the odb port) have good power and ground connections.
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Old 02-05-2020, 06:02 AM   #8 (permalink)
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One more addendum.

Failure to clear the DTC's turned out to be a malfunctioning ALDL cable I was using to communicate between GMTDScan on the laptop and the ECM. Using the brake/gas pedal method cleared them right up. ECM survived all the electrical madness and seems to work fine. Whew.
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94 GMC Sub K2500 SLE - 170k
33" ATs on 17" H2 whls (minimal wheelwell trim rqd)
PMD cooler to frt bumper;
2/0 Btty cbls, 4ga Alt & grounds
4" DiamondEye Exhaust; K47 Airbox
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Old 02-05-2020, 06:08 AM   #9 (permalink)
redshift9
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PS: Massive thanks to Glagulator, who nailed it right off the bat with this comment: "You have wiring problems." LOL

Seriously, thanks so much for all the help.
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94 GMC Sub K2500 SLE - 170k
33" ATs on 17" H2 whls (minimal wheelwell trim rqd)
PMD cooler to frt bumper;
2/0 Btty cbls, 4ga Alt & grounds
4" DiamondEye Exhaust; K47 Airbox
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