Problem with '91 4L80E

[Gary Gable]

Hi Folks,

I have a '91 C3500 GMC Sierra with a 4L80E with 70K original miles on the truck. Bought surplus from a nearby municipality at 56K. About 6 months ago I began experiencing a loss of 4th gear. I took it to 2 different reputable transmission shops. Since it was intermittent and evidently not throwing a code, neither could adequately shed any light let alone fix the problem. Over time it got worse, up until the point where it happens every time it's driven. Although I originally thought it was just 4th that was lost, I soon realized it was all gears except 3rd and reverse that went away.

Faced with this, and not particularly delighted with the service provided by the aforementioned shops, I chose to get a kit that replaced the internal wiring harness (with the '93 & later spec) along with the internal solenoids, force motor, temp sensor, etc, as well as both external speed sensors. I upgraded the external plug to fit this harness and was very careful to correctly splice onto the old harness. BTW: The oil the pan was clean, no debris or bits of warn parts to be found.

I still have the exact same problem. I'm still trying to find an OBD1 code reader, but using the paperclip method the check engine light flashed 24. According to the internet that indicates an output speed sensor issue, which is more or less what my ATSG manual says as well.

I have not replaced the wire from the OSS back to the ECM, and further my speedo has always worked fine. I mention the speedo as evidently this often comes with the problem I'm experiencing as there is a DRAC unit integrated into the instrument cluster. Apparently if the speedo goes and you have shifting problems you've likely found your culprit.

Has anyone had this exact same issue? What was your resolution? My next step is to replace the OSS wire(s) and see if it makes a difference, but any ideas you all might have would be much appreciated.

Thanks,

Gary

 

[tpass]

Try testing/swapping out the Throttle Position Sensor on the throttle body. I swapped motors in my 93 C3500 and thought the trans was going to need a rebuild after i drove it, had erratic/no shifting. I happened to have a spare position sensor, swapped it and it shifted great after that.

 

[Gary Gable]

I will give that a try, thank you!

 

[stutaeng]

I would think you could hook up a pressure gauge and figure out how to apply 4th gear manually (look at the apply table on that ATSG table and power the shift solenoids.) An advanced diagnostics computer will also do this, at least for OBD2, not sure on OBD1.

Hopefully you have adequate hydraulic pressure when you command 4th gear.

Or, remove the valve body. You can apply compressed air to the front-most hollow bolt...and listen for leaks. Perhaps the over-run/overdrive lip seals are leaking. The same clutches in that could be slipping, although OD clutches are not usually prone to damage. But you never know.

The TPS is easier, so start there.

 

[Schurkey]

Transmission has no first gear, no second gear, no fourth gear? Third and reverse work fine?

Does the Torque Converter Clutch engage?

I'd have expected second gear, but not third. Both shift solenoids are "off" in second. The 2-3 solenoid would be active in 3rd.

On my '88, the output speed sensor is wired to the instrument cluster. The circuitry in the cluster modifies the VSS signal to drive the speedo, odo, cruise, ABS, and ECM.

If yours is wired like that, the VSS, the wire to the instrument cluster, and at least some of the instrument cluster circuitry is working as it should, because the speedo works. I'd be looking for a partial failure in the cluster, or a pinched/broken wire between cluster and ECM.

 

[Gary Gable]

I would think you could hook up a pressure gauge and figure out how to apply 4th gear manually (look at the apply table on that ATSG table and power the shift solenoids.) An advanced diagnostics computer will also do this, at least for OBD2, not sure on OBD1.

Hopefully you have adequate hydraulic pressure when you command 4th gear.

Or, remove the valve body. You can apply compressed air to the front-most hollow bolt...and listen for leaks. Perhaps the over-run/overdrive lip seals are leaking. The same clutches in that could be slipping, although OD clutches are not usually prone to damage. But you never know.

The TPS is easier, so start there.

The first bit of diagnostics are a bit above my skill level, but the TPS isn't. Thanks!

 

[Gary Gable]

Transmission has no first gear, no second gear, no fourth gear? Third and reverse work fine?

Does the Torque Converter Clutch engage?

I'd have expected second gear, but not third. Both shift solenoids are "off" in second. The 2-3 solenoid would be active in 3rd.

On my '88, the output speed sensor is wired to the instrument cluster. The circuitry in the cluster modifies the VSS signal to drive the speedo, odo, cruise, ABS, and ECM.

If yours is wired like that, the VSS, the wire to the instrument cluster, and at least some of the instrument cluster circuitry is working as it should, because the speedo works. I'd be looking for a partial failure in the cluster, or a pinched/broken wire between cluster and ECM.

I'm liking the pinched broken wire scenario. My '91 is in fact the same as your '88. I'm also going to resume my search for used cluster to swap in to see if it makes a difference. All excellent help, Thanks!

 

[Gary Gable]

Update: I replaced the TPS, but it made no difference. Also, it appears I may be getting both 2nd & 3rd, not just 3rd.

Question to all of you: Does anyone know of a company that makes a replacement wiring harness? I called Painless wiring and they don't. I could probably create a (non color matched) harness myself, but I'd rather buy one that's actually correct.

Thanks!

Gary

 

[stutaeng]

I used this one when I rebuilt mine: https://www.google.com/amp/s/global...ransmission-internal-wiring-harness-24222798/

 

[tayto]

stop throwing parts at this and put a gauge on the trans and get some readings.

with pan off you can see the 4th clutches, next to the pressure regulator. can check to see if they are burnt. if you want to go one step further you can drop the valve body and air check the 4th clutches by putting shop air into the torx bolt with a hole. should hear the clutches engage and when you remove air they should release. if you hear air, there is a leak (most likely teardown to repair). can also check the 3-4 valve and see if it moves freely or is hung up.