4l80e head scratcher.. need help

[Cluongo]

Gentlemen,

I've been fighting an issue with my 1999 m1123 for months now. The 4l80e had an intermittent issue where it would randomly go into limp mode. That was usually remedied by a simple off/on key cycle and it was back to normal. That has since progresses into the truck will not come out of limp mode at all. I had several ideas as to what the cause could be and installed an auxiliary ground harness.. installed all new ACDelco solenoids and internal wiring harness inside the transmission.. used the troubleshooting manual to check continuity in the harness leading from the tcm to the transmission.. and finally sent the TCM to module experts in Jacksonville Florida for testing to the tune of $270. They informed me a "main component" in the TCM is bad and is not able to be repaired. They said they do have remanufactured replacements for an additional $670... so I bought the replacement and the truck is still in limp mode.

With the original TCM the trans light would provide code 73 when in diag mode.. with the new TCM the trans light stays illuminated and will not enter diag mode. I made a custom harness to adapt the cannon plug to an obd1 and was able to communicate with the old TCM via an OBD1 connecter hooked to a snap on modus scanner. The live data made no sense which makes me believe the old TCM was actually fried. I am unable to communicate with the new TCM at all.. it was confirmed that the TCM is getting 12v at the plug.. I'm just not sure where to go from here. Any insight would be a huge help. This truck was my beater for a year and a half without any issues. I'd love to have her back on the road.

Thanks,
Chris

 

[Cluongo]

This is the original TCM

 

[Cluongo]

This is what module experts sent me

 

[Retiredwarhorses]

Sounds like you went down a rabbit hole with this so called “module expert” you’d have been better off cont a hmmwv centric shop and had some discussions with them about possible solutions. Do you have the old TCM?

 

[springer1981]

I don't have first hand knowledge on the TCM but in general that type of intermittent issue is probably in the power circuit or some wiring. The TCM can go bad but if it intermittently works that is likely a wiring. Also any time the transmission loses control signals the default is 2nd gear AKA Limp Node. If you unplugged everything from the transmission it would be in 2nd gear. I would find the TM with wiring diagram for any 4 speed with TCM and start monitoring the power circuit and see if you lose power when it goes into Limp Mode.

 

[Cluongo]

Unfortunately I did not get my old TCM back. I Have a call in to them and am waiting for a call back. Just to clarify, I replaced the solenoids and internal wiring harness after the transmission issue went from intermittent limp mode to limp mode all the time. It threw diag code 73 (PCS circuit) and I read these transmissions have had issues with the internal harness going bad. So I replaced everything internal just to rule it out. When that didn't work I checked the external wiring and couldn't find any issues. I made a scanner harness based off a thread i found which allowed me to access live data from the TCM. The data was all over the place and didnt make sense. That's what led me to believe the TCM was bad. I guess at this point I just need to confirm whether or not Module experts sent me the correct TCM for my setup. They claim this was a plug and play TCM. Is that a load of crap?

 

[springer1981]

Unfortunately I did not get my old TCM back. I Have a call in to them and am waiting for a call back. Just to clarify, I replaced the solenoids and internal wiring harness after the transmission issue went from intermittent limp mode to limp mode all the time. It threw diag code 73 (PCS circuit) and I read these transmissions have had issues with the internal harness going bad. So I replaced everything internal just to rule it out. When that didn't work I checked the external wiring and couldn't find any issues. I made a scanner harness based off a thread i found which allowed me to access live data from the TCM. The data was all over the place and didnt make sense. That's what led me to believe the TCM was bad. I guess at this point I just need to confirm whether or not Module experts sent me the correct TCM for my setup. They claim this was a plug and play TCM. Is that a load of crap?

Where you already have everything on your truck for the stock TCM and wiring it's hard to suggest going a different route. A full aftermarket TCM and wiring harness for your transmission would cost $750. I have personal experience with the Quick4 TCM and like it a lot. If you are handy at all and have a little mechanically ability it is an easy conversion. It also gives you the ability to program it and change things that are not so easy to change in the stock system. If you made harness and changed out the internal wiring in the transmission it would be an easy install for you.

Now I'm not saying that is your answer but it is an option. When I did a 4L80 conversion on my truck it is what I installed and I did a write up of it starting here https://www.steelsoldiers.com/threads/my-new-1992-m998-avenger.184818/post-2408610

 

[Cluongo]

Thanks for the input guys. Aftermarket has definitely crossed my mind through this whole mess. I'd just hate to deviate from the factory setup. One thing I noticed is the new TCM has eprom BKUL installed. From what I can tell that's for a 95-99 4l80e HD w/ 6.5l. The original eprom was a CZSU which is for a 2001+ 4l80e w/ 6.5l. Could this be the issue?

 

[Mogman]

Thanks for the input guys. Aftermarket has definitely crossed my mind through this whole mess. I'd just hate to deviate from the factory setup. One thing I noticed is the new TCM has eprom BKUL installed. From what I can tell that's for a 95-99 4l80e HD w/ 6.5l. The original eprom was a CZSU which is for a 2001+ 4l80e w/ 6.5l. Could this be the issue?

Not likely, all the transmission control points are the same, if you cannot get codes out of the new TCM and the light just stays on but you could read the old one I would suspect the "new" one is defective.
I would likely try a civi TCM from any 4L80E just to see what happens they are a dime a dozen on fleabay, (over 1000 for sale the last time I looked)
the only differences would be the shift points and if you still had the old eprom you could switch it.

EDIT If someone was to send me an eprom I could clone them, this would make using a civy TCM a snap and much cheaper..

EDIT, there should be a number on the TCM something like 24234274, 24242391 etc search for that number and at least you will get the right hardware.

 

[Cluongo]

Ok, thank you. That answers my question. I just talked to a technician at module experts. They are going to send my original TCM back so I can swap eprom's but like you said it probably won't make a difference. But at least I will have all the original equipment back for the truck and I can start from scratch to figure out what is actually going on with this thing. All of the wiring in the truck is original except for the internal wiring harness inside the transmission. And I actually have that harness and the original solenoids on my work bench if needed.. Who can I contact for some diagnosis help with this thing? I'm located in Lancaster PA. I would happily pay someone for their time.

 

[Cluongo]

I received and reinstalled my original TCM today. The trans light flashes like it used to, it connects to the modus scanner/displays live data and enters diagnostic mode when I jump pins A and E. The "remanufactured" TCM from module experts would not do any of the above. I swapped the eproms just for the heck of it and it made no difference. What do you guys think? I'm essentially at square one with a truck stuck in limp mode. Where should I start in the diagnosis?

 

[Mogman]

What codes are you pulling from the old TCM?

 

[Cluongo]

No codes present when using the modus and the live data makes no sense. I'll post a pic of some of the readings.

 

[Cluongo]

 

[Mogman]

As I said before if I suspected a TCM failure I would buy a couple off of Fleabay, I see tons of them for $20 shipped.
I would try them as is and then if it did not shift correctly I would swap the eprom.
It would not surprise me that the Live data would be all screwed up because allot of the data comes from the ECM which is not present.

 

[Cluongo]

This is the adapter I made to go from the 6pin cannon plug on the tcm housing to an OBD1 port.

 

[Retiredwarhorses]

I received and reinstalled my original TCM today. The trans light flashes like it used to, it connects to the modus scanner/displays live data and enters diagnostic mode when I jump pins A and E. The "remanufactured" TCM from module experts would not do any of the above. I swapped the eproms just for the heck of it and it made no difference. What do you guys think? I'm essentially at square one with a truck stuck in limp mode. Where should I start in the diagnosis?

You should be bypassing the TCM and using the snap on tool to act as the TCM, thus eliminating the transmission as the Culprit

 

[Mogman]

Good example, even when looking at the TCM the RPM comes from the ECM which again is not present, I do not think a scanning tool would be of much use.

 

[Mogman]

Again what code/codes is the TCM throwing?

 

[Cluongo]

I would definitely buy another TCM from ebay if I could find one cheap. The cheapest I've found are $600-$1000.