M1123 transmission

[kingConan]

Hello,
This is my first post. I have an 04 M1123 with a 6.5 NA engine and a 4L80E transmission. It was running great but is now stuck in limp mode and when I put it in reverse it stalls the engine. I am thinking it is a control issue with the computer. If anyone has dealt with this and has a solid solution please let me know. My truck does not have a transmission light so I cannot pull the codes yet. Light is ordered. I have access to another TCU. Any help would be appreciated.

 

[Mogman]

Welcome to the SS forums!!

 

[NDT]

Often the problem is wiring in the battery box or the relay right behind the batteries. No power to thr TCU means limp mode.

 

[frauhansen]

I had that too. But luckily mine had a trans-light. And this always stayed off with me. Can also be with you so, because you do not see it. But you could measure it with a multimeter. Whether the multimeter shows 0V or 24V or the lamp is lit does not matter. 24V are 24V!
My first tip would be to see if the ecu gets power at all. If not, then the trans-light remains dark!
For this it has a fuse behind the front passenger seat. In the middle behind the seat runs a cable on a crossbeam from left to right. And there is also a small box in the cable with a fuse that you can push back in with a button.
I had to do this once. After that... never happend again.

 

[kingConan]

I had that too. But luckily mine had a trans-light. And this always stayed off with me. Can also be with you so, because you do not see it. But you could measure it with a multimeter. Whether the multimeter shows 0V or 24V or the lamp is lit does not matter. 24V are 24V!
My first tip would be to see if the ecu gets power at all. If not, then the trans-light remains dark!
For this it has a fuse behind the front passenger seat. In the middle behind the seat runs a cable on a crossbeam from left to right. And there is also a small box in the cable with a fuse that you can push back in with a button.
I had to do this once. After that... never happend again.

Thank you. I'll check it out.

 

[Mogman]

Dying in reverse would be a red flag to me.
If it were me while I waited for the trans. light I would disconnect the 12V power to the TCM, it is one of the two wires connected to the positive post of the rear battery (connection between the batteries) the other goes to the regulator.
Then I would try reverse again, the only relatively easy to fix problem I can think of that would cause it to die in reverse would be the TCM for some reason is sending a lockup signal to the torque converter, removing all power from the TCM would show one way or the other.
If it continued to die in reverse with no power to the TCM I would be very concerned.

 

[kingConan]

Dying in reverse would be a red flag to me.
If it were me while I waited for the trans. light I would disconnect the 12V power to the TCM, it is one of the two wires connected to the positive post of the rear battery (connection between the batteries) the other goes to the regulator.
Then I would try reverse again, the only relatively easy to fix problem I can think of that would cause it to die in reverse would be the TCM for some reason is sending a lockup signal to the torque converter, removing all power from the TCM would show one way or the other.
If it continued to die in reverse with no power to the TCM I would be very concerned.

Thank you. Is there a difference between the two wires between the TCM and the regulator? I'm at work and can't take a look at it. Side note, I had a shop install a 12v stepdown, 12v circuit board and a relay so I could charge phones etc. The day before I took it in, I was driving and it felt like it slipped out of gear while in 2nd. (High RPM and no power). I pulled off of the road and ran it through the gears. The next day it drove fine. Thank you again for the advice.

 

[Mogman]

According to the manual attached page 141 the wire should be marked 290 and or 291 and or 537.
I am curious what you mean by you do not have a transmission light, the one thing everyone with the 4L80 really needs is a working transmission light, it can help tell you if you have a power issue, and of course you need it to get the codes, unfortunately most give it no thought until they have a transmission issue and have no idea that they should have been paying attention to it when the transmission was working correctly.
Also have you checked the fluid level and when was the last time it was serviced?

 

[kingConan]

According to the manual attached page 141 the wire should be marked 290 and or 291 and or 537.
I am curious what you mean by you do not have a transmission light, the one thing everyone with the 4L80 really needs is a working transmission light, it can help tell you if you have a power issue, and of course you need it to get the codes, unfortunately most give it no thought until they have a transmission issue and have no idea that they should have been paying attention to it when the transmission was working correctly.
Also have you checked the fluid level and when was the last time it was serviced?

Thank you. Transmission was serviced in April of this year. Fluid is full. The light was removed either when the transmission was removed or by someone else. I just received the replacement light from eastern surplus. Any idea where it plugs in on the transmission? The pigtail is about 8 inches long. Once I put the light in I will try to get some codes out of it. I have access to another TCM, and spoke with the mechanic that did the stepdown wiring. He went hot off of the front battery and ground off of the rear battery for a true 24v. Should it be run off of only one battery?

 

[Mogman]

Thank you. Transmission was serviced in April of this year. Fluid is full. The light was removed either when the transmission was removed or by someone else. I just received the replacement light from eastern surplus. Any idea where it plugs in on the transmission? The pigtail is about 8 inches long. Once I put the light in I will try to get some codes out of it. I have access to another TCM, and spoke with the mechanic that did the stepdown wiring. He went hot off of the front battery and ground off of the rear battery for a true 24v. Should it be run off of only one battery?

The supplement I posted above has the wire numbers, look on page 140 it is called a MIL for "malfunction indicator light"
It is an LED so it likely is polarity specific, plug the "A" wire into the "A" connector on the harness.

 

[kingConan]

The supplement I posted above has the wire numbers, look on page 140 it is called a MIL for "malfunction indicator light"
It is an LED so it likely is polarity specific, plug the "A" wire into the "A" connector on the harness.

Also, when the truck is cold everything works great. At about 160 degrees or operation temp it goes into limp mode.

 

[Mogman]

Also, when the truck is cold everything works great. At about 160 degrees or operation temp it goes into limp mode.

It may have been trying to warn you for some time.
Have you inspected the TCM for signs of water intrusion?
You just need to get where you can read the codes, if it works cold it is less likely it is a power issue and more likely it is ether a bad sensor/wiring or a bad TCM.
EDIT, or possibly a mechanical issue like low pressure when hot, reading the codes will certainly help.

 

[kingConan]

It may have been trying to warn you for some time.
Have you inspected the TCM for signs of water intrusion?
You just need to get where you can read the codes, if it works cold it is less likely it is a power issue and more likely it is ether a bad sensor/wiring or a bad TCM.
EDIT, or possibly a mechanical issue like low pressure when hot, reading the codes will certainly help.

I will check this out. My goal is to dive into it this weekend. The truck sat for about 5 years. Had to replace the motor, but I figured there would be other things that would pop up. Hands down the most fun vehicle I've ever had.

 

[Mogman]

Great, that gives you a few days to go through the troubleshooting guide.
Another thing you can check is unplug the main harness from the transmission and see if there is any oil in the connector.
Disconnect the batteries before plugging or unplugging anything, safest to pull both the front battery positive and the rear battery negative.

 

[kingConan]

Great, that gives you a few days to go through the troubleshooting guide.
Another thing you can check is unplug the main harness from the transmission and see if there is any oil in the connector.
Disconnect the batteries before plugging or unplugging anything, safest to pull both the front battery positive and the rear battery negative.

Odd. Truck is running fine now. Granted, it's cold in the NW, but I haven't done anything other than not turn on the step down.

 

[Mogman]

That sounds like a clue!
Try it again with it on and at the same temp.

 

[kingConan]

So far I haven't been able to diagnose the issue. The transmission light arrived, but under the truck there is not a plug in for it. Chased every wire. Odd. And now the truck is running fine cold and at temp. Any ideas? I drive it every day and don't want to be side lined with an issue.

 

[Mogman]

Look at the pages I referred you to, you can use a SMALL 24V test light OR your new trans light between the 12V positive and pin U on the TCM connector (page 140), then put a jumper from pin EDIT A to E on the J2 connector (page 60) and get the codes.
IF you are POSITIVE the cables have been cut you can tap into the wire at pin U and run it to the lamp mounted near the shifter and run the other wire permanently to the 12V connection at the battery as it was originally meant to be as referenced on page 140 with a fuse of course.

 

[Mogman]

Maybe someone else can come in here and say exactly where the wires should be, I would think they would be somewhere between the left rear of the engine and the shifter but really don't know.

 

[Mogman]

I do not want to seem like an alarmist but you really need a working trans light, not having one can cost you a complete rebuild when the problem could have easily been taken care of before it got out of hand.
There are problems that do not even set the MIL light but do set codes, having a working light and knowing how to get the codes the very first time the transmission indicates any problem can save you big bucks, a jumper to get the codes should be the first "spare/repair" part anyone with a 4L80 should have handy.
Since you have had problems I would not drive it until you can read the codes or take it to a shop that has the old school tools to read the codes directly off of the transmission, you do not have an OBD1 port to even use the older "scanners"