Analysis of Transmission Troubleshooting Results Help

[Battlefield Flag Co]

Hello,
I got my 1991 M-1079R1 with 2000 miles on it a few months back. Changed fluids, new batteries, tire maintenance and things going great.
Went out a couple weeks ago, thing won't back up when put in R. In park it is fighting itself against going forward, in reverse the same things, in neutral it goes forward slowly. I can put it in neutral with engine off and it rolls like it is in neutral, so that makes believe linkage is fine.

• Changed filter and fluid, no change. Everything looks great from bottom side of transmission valve body.
• Trans light on all the time.
• Went through the 4L80E troubleshooting book.
• Jumped pins A&E, no codes will flash, light remains steady.
• Disconnected J1 and light goes out, indicating MIL not shorted. Checked wiring anyway, all looks good.
• Followed procedures for Chart A-2 MIL on steady.
  • Both results say to replace the TCM (more to follow before I just replace that I think)
• NOTE: Pulled cover off of the TCM box today, TCM is warm and the truck hasn't been running for 24 hours. (possibly relay stuck closed charging TCM all the time?)
• With all wiring disconnected from transmission and TCM it still behaves same, not going backwards when in reverse, etc.

Mechanical issues.? Seems like it.
Electrical issues.? Seems like it too.

Looks like possibly two problems, while possible, it's strange and before I take the plunge on a big bill at the shop I am asking for your analysis and input please.

Thanks,
Jeffrey

 

[Mogman]

Well the first thing is to get the codes, if you jump the pins and get no codes that is a big problem, even with "no codes" you should see a repeated 12 code.
with the truck not running anything but park will allow the truck to roll, so that means nothing.
But no reverse with no TCM connected is bad news, it is something in the transmission, ether the clutches are fried (would likely have seen some stuff in the pan) or a valve body issue.
It seems you unfortunately have multiple issues

 

[Retiredwarhorses]

No TCM equals high pressure, thus you should be in 2nd gear, and reverse works fine…
I have an M1151 I just received in, No reverse, forward is Ok…my transmission shop said the reverse band is probably broken, probably from some jar head slamming it into reverse while driving down the street, we had another a few weeks ago where the park assy inside the trans was grenaded, the case is shot now because of all the damage.
I’ve now replaced 4 or 5 transmission’s in as many months on trucks that run after arriving from GP.

 

[Battlefield Flag Co]

No TCM equals high pressure, thus you should be in 2nd gear, and reverse works fine…
I have an M1151 I just received in, No reverse, forward is Ok…my transmission shop said the reverse band is probably broken, probably from some jar head slamming it into reverse while driving down the street, we had another a few weeks ago where the park assy inside the trans was grenaded, the case is shot now because of all the damage.
I’ve now replaced 4 or 5 transmission’s in as many months on trucks that run after arriving from GP.

I think I am going to pull the pan again and check the solenoids (found the ohm values they should read out if good), maybe one failed and is stuck.? (I don't know, just not ready to take it to the shop yet).
I am worried about repairs costing more than trying to find a used replacement.

 

[NDT]

I would make sure that the shift lever positions are in sync with the internal transmission detents.

 

[Battlefield Flag Co]

In an effort to try and do everything I could think of I took the pan back off.
All the shift detents look good when shifting, the manual valve appears to work properly. See pics.

I also put an ohmmeter to the 4 solenoids and they all measure great.

Mechanically:

• Any other thoughts before I yank the transmission?
• I do have a lead on a new 4L80E from a friend, cost near zero.

Electrically:

• Any thoughts?
• Appears I need to replace the TCM.? Thoughts? That is not cheap.

Thank you.

 

[Westo]

In an effort to try and do everything I could think of I took the pan back off.
All the shift detents look good when shifting, the manual valve appears to work properly. See pics.

I also put an ohmmeter to the 4 solenoids and they all measure great.

Mechanically:

• Any other thoughts before I yank the transmission?
• I do have a lead on a new 4L80E from a friend, cost near zero.

Electrically:

• Any thoughts?
• Appears I need to replace the TCM.? Thoughts? That is not cheap.

Thank you.

Don't know anything about the transmission side, but I had mine with a TCM light stuck on all the time. Does it come on only when the start switch is on run, or is it on all the time the batteries are connected? Mine was on all the time at about half brightness and turned out to be a blown capacitor inside the TCM that had shorted the light in that way.

 

[Battlefield Flag Co]

Don't know anything about the transmission side, but I had mine with a TCM light stuck on all the time. Does it come on only when the start switch is on run, or is it on all the time the batteries are connected? Mine was on all the time at about half brightness and turned out to be a blown capacitor inside the TCM that had shorted the light in that way.

Thanks for the feedback. The light is only on when switch is in run position, however the battery does drain down and when I opened the TCM case the other day, the TCM was warm and it had been parked for 24 hours without the engine running. That makes me think the relay is possibly bad still sending power to the TCM all the time.
When jumping the pins in J2 the light stayed on steady, no flashing at all. And further testing ruled out a short on the wire to the light.

 

[Mogman]

DELETE, my mistake

 

[Battlefield Flag Co]

What "not enable serial data" means is it may not talk to the diag tool you do not have, not likely your issue.
You need to not have the same conversation taking place on multiple threads.
If there are no codes it should flash 12 four times then stop, if you have codes it will flash 12 three times to say it is in diag mode then flash each code three times, after all the codes have flashed it will flash 12 one last time.

I don’t have this thread anywhere else.

 

[Mogman]

DELETE, AGAIN, my mistake!

 

[Battlefield Flag Co]

Your not having this same conversation on this thread?

GM truck TCMs in HMMWVs

I have an M1114 and recently found that my tcm has a bunch of burnt out components on the board. The part number listed in the TM for my truck is the same as a GM part used on 6.5 turbo trucks with the 4l80e. If I get one of these units and replace the EPROM with the one from my TCM, will it...

www.steelsoldiers.com

No, I don't have my issue posted anywhere else. I would never do that and I would have appreciated the benefit of the doubt before being accused.
Even after I told you I didn't do that you come back with a link and ask me if I did that.

 

[Mogman]

No, I don't have my issue posted anywhere else. I would never do that and I would have appreciated the benefit of the doubt before being accused.
Even after I told you I didn't do that you come back with a link and ask me if I did that.

Sorry, my bad...