Transmission Light Issue

[hrbergeron]

Hello all,

I have a M1025A2 HMMWV that has had an issue with the transmission light since I received it. The problem is that the transmission light is illuminated after the vehicle is turned off. This leaves the TCM on and eventually kills the batteries. Once the vehicle is turned on, the transmission light goes out, and the transmission shifts as designed.

What can cause this? I was told in the past that it was an issue with my glow plug box, but I have replaced it with another glow plug box that I know works and I still have the issue. The glow plugs cycle properly as designed. Someone sent me a link to a US Army 4L80e troubleshooting TM but I can not find it any more.

 

[papakb]

The T-case lock LED is powered from the switched RUN power to the instrument cluster, down thru the shifter switch to the T-case lamp and then it grounds to the left cylinder head. The control box isn't in the circuit anywhere. Someone has been mucking around with your wiring or has possibly Tee'd something where they shouldn't have. Take a look at the connectors down by the shifter and make sure they're connected properly and didn't get switched around.

Neutral start switch are wires #14

Backup lights are wires #467

Gear lamp is wire #40

T-case lock are wires #511 & 511A

 

[Augi]

Sounds to me like the TCM power relay in the battery compartment is stuck closed or bypassed.

When I shut my truck off the warning light illuminates for about 1 to 2 seconds, I presume because the engine speed or some pressure sensor suddenly reads 0. Once the capacitors drain in the TCM the light goes out.

If your relay isn't disconnecting the TCM then the light won't go out and the TCM will drain the lower battery only since it's connected to 12v.

Augi

 

[Retiredwarhorses]

The T-case lock LED is powered from the switched RUN power to the instrument cluster, down thru the shifter switch to the T-case lamp and then it grounds to the left cylinder head. The control box isn't in the circuit anywhere. Someone has been mucking around with your wiring or has possibly Tee'd something where they shouldn't have. Take a look at the connectors down by the shifter and make sure they're connected properly and didn't get switched around.

Neutral start switch are wires #14

Backup lights are wires #467

Gear lamp is wire #40

T-case lock are wires #511 & 511A

he is talking about the trans lamp on an A2 truck.
Reset your TCM by unplugging it.

 

[aghumvee]

More than likely the relay behind the battery box. Mine had the same issue. Followed wiring diagrams and it pointed to the relay.

 

[Retiredwarhorses]

Trans relay has 4 wires, 2 are 24v, the other 2 are 12v to TCM.
upon turning the start switch to run, it sends 24volts to the relay, the 12v relay in turn closes, sending 12v to the TCM.
when it’s stuck on after you turned it off, there is power still getting too the TCM, thus the trans lamp is on.
ive seen several things cause this, the relay, the start switch, bad TCM...some times jjst unplugging it all will clear it.

 

[hrbergeron]

Still have the issue. Swapped the transmission relay with a brand new one and one off my m1123 and the transmission light is still illuminated with the ignition off. I swapped the circuit breaker and it still has the same issue. Cleaned all connections on the transmission. Unplugging the wires from the relay will turn off the light, so it is getting power from the relay.

What's next?

 

[Augi]

Still have the issue. Swapped the transmission relay with a brand new one and one off my m1123 and the transmission light is still illuminated with the ignition off. I swapped the circuit breaker and it still has the same issue. Cleaned all connections on the transmission. Unplugging the wires from the relay will turn off the light, so it is getting power from the relay.

What's next?

The relay has 4 wires:

1. Ground (57T)
2. Ignition switched 24v to the relay coil (29T)
3. Switched 12v out to the TCM (290C 291C)
4. 12v in from the battery (290B 291B)

If you disconnect wire #3 and the TCM light DOES NOT go out then you have a wiring problem in the truck somewhere. Someone probably bypassed the relay and wired 12v power directly to the TCM.

If you disconnect wire #2 and the TCM light goes out then you have a wiring problem in the truck somewhere. That wire should only have 24v on it when the starter switch is set to "Run" so use a multimeter to check for that. You should also hear the relay click as you apply and remove power from wire #3.

There is a third test you can do to test for a bad relay that I will tell you later if you need it.

Augi

 

[Retiredwarhorses]

Did you swap TCM’s?

 

[hrbergeron]

Did you swap TCM’s?

No I have not. Don't really want to either. but I am running out of options.

Tried a different ignition switch with no luck.

 

[Retiredwarhorses]

Why not? You said you had another truck, it’s one cannon plug

 

[ebs]

Hello, I just bought a M1025A2 and it has the same issue. This is a great thread! Thank you! I am a little mechanical and understand very basic electronics but this problem looks a little overwhelming to me. I am going to start with disconnecting the unit and hope that it re-boots.

 

[hrbergeron]

I am going to start with disconnecting the unit and hope that it re-boots.

The TCM has nothing to do with it. Somewhere in the A2 harness it gets the 24v switched ignition signal that tells the relay to turn on, but I don't know where. I think something with the special forces wiring of accessories and whatnot they tapped into something that they shouldn't have and that's the issue.

 

[Milcommoguy]

They don't call them Special Forces for nothing, CAMO

 

[Retiredwarhorses]

The TCM has nothing to do with it. Somewhere in the A2 harness it gets the 24v switched ignition signal that tells the relay to turn on, but I don't know where. I think something with the special forces wiring of accessories and whatnot they tapped into something that they shouldn't have and that's the issue.

of course the TCM has something to do with it, it’s part of the circuit, if the TCM is not seen as present or defective, it will show up at the trans lamp...not that the TCM aid his problem, but I’ve reset many and fixed issues.

 

[hrbergeron]

of course the TCM has something to do with it, it’s part of the circuit, if the TCM is not seen as present or defective, it will show up at the trans lamp...not that the TCM aid his problem, but I’ve reset many and fixed issues.

If the ignition is off, the transmission light should not be illuminated.

The TCM doesn't get power until the relay sends it. Swapped in other tcms that work and got the same issue. That next makes the issue the relay, which is independent of the TCM. The problem is that the relay is working right, but the 29T wire is always energized, when it shouldn't be. The ignition is working right. I ran a new switched ignition wire to the relay and it fixed the problem. I just don't know where exactly in the a2 harness where the switched ignition wire goes.

 

[ebs]

Thank you for the replies and patience! So to be clear, I should run a new wire from the ignition to the relay?

 

[Retiredwarhorses]

29t connects to the Circuit breaker on the trans panal, comes from the trans relay.direct to 24v.
57T is the ground from the trans relay.
the other 2 connections from the Relay go to the trans check lamp, TCM, ignition power.
ive always wondered if the schematic is wrong and is backwards...I say this because 24v should energize the relay from the ignition, closing the 12v relay in the trans relay, thus powering the TCM...which is 12v, the schematic I believe has it backwards.

 

[ebs]

Thank you for the breakdown! First opportunity I will trace it!

 

[ebs]

Just wanted to thank everyone for the help! I ordered and replaced the transmission relay and that seems to have rectified the issue.