4L80E Transmission troubles, DTC 86

[LeafDriver]

Hi, first time poster long time lurker.

I've been sifting through many threads and the Gear-report 4l80e troubleshooting guide on this particular problem but I've not found a solution yet so I'm posting this SOS before I resort to pulling the entire transmission out.

My Hmmwv is a 2003 M1123 (converted to fastback) 6.5 N/A 4L80e
Problem: DTC 86 (limp mode), occasionally stalls at stop when in drive/reverse. Engine running well, still drive-able at low speeds.

Here's the order of events:
On the way back from a long trip I suddenly noticed the transmission would not lock the TC into overdrive, checking the fluid I didn't notice anything unusual and stopping/starting again after 30 minutes the problem seemed to resolve itself, tried pulling codes, got only 12 on repeat. Went for a test drive the following morning I found the truck seemed to be stuck in Limp mode, 2-3 no TC lock no 1st gear, stopped there found the fluid had turned dark red and had a different odor, DTC 86 first appeared here. Checking the Transmission harness pass-through connector I found fluid inside the plug, so I thought this was the cause.
Dropped the pan, fluid was dark red, a light smell of burnt rubber, some dark sludge, no metal chunks in pan or on magnet.
After some fix hunting and wrenching I decided to replace the internal harness as well as all the solenoids, the TISS/TOSS and also the TPS.
I got DTC 22 while I was adjusting the TPS but cleared it once I got the correct adjustment.
Put it all back together, added new fluid and cleared codes again.
Drove it the same night and everything worked beautifully, it seemed to be fixed. Observed a smooth 1-2-3-4 shift/TC lockup at highway speed no problem. That seemed to be it yay.
Unfortunately, the day after I went for another test run and it was DTC 86 all over again. No up/down shift, TC dragging in drive/reverse occasionally.
Back to square 1. Nuts

Just about at my wit's end on this, anyone have any ideas?

Here's a nice picture for interest.

 

[Dieselmeister]

Have a look at the attached.

 

[NDT]

What’s going on with the TC lock switch at the brake pedal? Seems like something is erratic there.

 

[LeafDriver]

What’s going on with the TC lock switch at the brake pedal? Seems like something is erratic there.

I'll hook up a multimeter to it and check, also have a spare lying around. Could just swap it out.

 

[Mogman]

You are not the first to have issues with the TC locking up at idle pulling the engine down, I do not remember if any solution was ever given.
It SHOULD not matter what the brake switch is doing as the TCM should never try to lock the converter unless it is in ether fourth or in third or fourth depending on the way the TCM firmware was set up but confirming the switch is operating correctly would certainly not be a bad idea.
As far as the code 86 goes this sets when the "command" (from the TCM) is for 1st or 2nd and the trans for some reason is in 3rd or 4th.
Both of these problems could be caused by a defective TCM so if you can get your hands on one that would be my next move.

 

[LeafDriver]

As far as the code 86 goes this sets when the "command" (from the TCM) is for 1st or 2nd and the trans for some reason is in 3rd or 4th.
Both of these problems could be caused by a defective TCM so if you can get your hands on one that would be my next move.

From driving it around for the past 3 years I know the TC should only lock up on 4th D(O). I think worst case the TC is burnt. Yet from the one day where I swapped the internal Trans electronics everything seemed to work fine for a short while so it doesnt seem to be the TC specifically.

I do have the Black Dog Customs TCM swap kit which I was planning to drop into the truck eventually to upgrade from the mysterious 90's computer technology and unobtanium diagnostic tools.
Looks like I'll be doing that sooner rather than later.
Will return with results.

 

[LeafDriver]

It's definitely not the brake switch. Moving on to the TCM swap

 

[Retiredwarhorses]

From driving it around for the past 3 years I know the TC should only lock up on 4th D(O). I think worst case the TC is burnt. Yet from the one day where I swapped the internal Trans electronics everything seemed to work fine for a short while so it doesnt seem to be the TC specifically.

I do have the Black Dog Customs TCM swap kit which I was planning to drop into the truck eventually to upgrade from the mysterious 90's computer technology and unobtanium diagnostic tools.
Looks like I'll be doing that sooner rather than later.
Will return with results.

I Work on these trucks everyday, there is nothing mysterious about the TCM in these Hmmwv’s, there are hundreds of thousands in use in the military and millions of the same TCM’s still in use world wide.
Diagnostic tools? A paper clip is Pretty easy to find….and any “good” transmission shop knows how to work and run diagnostics on this setup.

 

[TNDRIVER]

From driving it around for the past 3 years I know the TC should only lock up on 4th D(O). I think worst case the TC is burnt. Yet from the one day where I swapped the internal Trans electronics everything seemed to work fine for a short while so it doesnt seem to be the TC specifically.

I do have the Black Dog Customs TCM swap kit which I was planning to drop into the truck eventually to upgrade from the mysterious 90's computer technology and unobtanium diagnostic tools.
Looks like I'll be doing that sooner rather than later.
Will return with results.

In the civilian world the 4l80e is capable of locking up in any gear when it detects an over heat condition. I've never been able to find any definitive information on the military TCM as to it's programing. Seeing it has no "Haul/Tow" mode, and its intended use , I think the way mine acts it is in a sort of permeate tow mode. The temp sensor is in the wiring harness, did you replace that also with your electronics replacement?" BUT" it should still unlock when the input speed drops to a low enough level. If the TCM doesn't cure the problem I would think the torque converter might be your second guess (diode / sprage clutch or the plate its self). My 2 cents.

 

[LeafDriver]

The temp sensor is in the wiring harness If the TCM doesn't cure the problem I would think the torque converter might be your second guess (diode / sprage clutch or the plate its self). My 2 cents

Yes, I got a brand new wiring harness kit along with all 4 solenoids and the 2 speed sensors. Still hoping its not the TC itself.

After researching, pulling all 350lbs of the torque converter+transmission+transfer case out and in again may not be something I can do in the driveway.

 

[Mogman]

Yes, I got a brand new wiring harness kit along with all 4 solenoids and the 2 speed sensors. Still hoping its not the TC itself.

After researching, pulling all 350lbs of the torque converter+transmission+transfer case out and in again may not be something I can do in the driveway.

I have seen TC's slip but have never seen one lock up on its own, seems even more doubtful a mechanical failure in the TC would cause it to lock randomly