HMMWV No Power to TCM, No Transmission Light

[Nic - M1097]

Hey Guys,

I recently purchased a 1998 M1097. I knew the trans was in limp mode when I got it and the Trans Light will not illuminate.

I replaced the trans light. I replaced the Transmission Relay.

I checked the connections at the battery. I have power at the trans relay but when testing the pins at the J1 harness in the TCM I have no power.

I am assuming there could be more issues with limp mode but without being able to read the codes from the trans light (because it will not illuminate or flash codes when jumped) I am kind of stuck.

I did a visual inspection of the wiring harness under the vehicle. Nothing apparent.

I’m assuming it’s a bad ground? Possibly a bad 291D power wire?

I am looking for any suggestions on what to check and where to begin? I don’t want to rip out the entire wiring harness if it’s something simple, any thoughts?

Thanks in advance guys, this is a super informative thread.

 

[Mogman]

There are two 12v feeds to the TCM one switched and one not switched, you need to make sure you have both of them.
Here is the troubleshooting guide that will give you the schematics you need to troubleshoot.
You will need 24V to the relay coil with the ign. on

 

[williamh]

Did you check behind the battery box behind the passenger seat for the resettable circuit breaker ?

 

[Mogman]

If you do not have power going into the TCM box at 290/291 then check 537, if you have power there then you have a relay problem, ether the relay wiring or 24V feed to the coil.
If you do not have power to 537 then you have a general 12V feed/breaker issue.
This should be checked at J1 going into the TCM box with the TCM connected.

 

[Nic - M1097]

Where should I be able to test for 24V? I have 2-12V batteries wired in series but it seems like everything I’ve tested is throwing out 12V.

 

[Nic - M1097]

Did you check behind the battery box behind the passenger seat for the resettable circuit breaker ?

The circuit breaker had continuity so I don’t believe that was the problem. However, the circuit breaker didn’t have much play (if any). It doesn’t seem popped to me.

 

[Mogman]

You should have 24V across wires 29 and 57 at the transmission relay in the run position.
57 goes to ground and 29 goes to the run circuit.
This is the only place in the transmission circuits you should have 24V
This is shown in the picture above.
Did you check for 12V at 537?

 

[Nic - M1097]

What is the c

If you do not have power going into the TCM box at 290/291 then check 537, if you have power there then you have a relay problem, ether the relay wiring or 24V feed to the coil.
If you do not have power to 537 then you have a general 12V feed/breaker issue.
This should be checked at J1 going into the TCM box with the TCM connected.
View attachment 913245
[

If you do not have power going into the TCM box at 290/291 then check 537, if you have power there then you have a relay problem, ether the relay wiring or 24V feed to the coil.
If you do not have power to 537 then you have a general 12V feed/breaker issue.
This should be checked at J1 going into the TCM box with the TCM connected.
View attachment 913245

What is the correct relay wiring sequence? My humvee came with:

1 (57T)

2 (29T)

3 (290B)

4 (290C)

But I found an article online where 3 and 4 were switched.

Follow up, what is the correct sequence for the transmission circuit breaker:

290 (Top)

290A (Bottom)

Lastly, I have continuity through the circuit breaker. Does that mean the fuse is still good and not popped/blown? Just checking that is not the issue.

 

[Mogman]

You cannot connect the relay incorrectly, it does not matter which way 57/29 are connected or the 290/291 wires. one side has female and the other male connectors, same thing with the circuit breaker.
You need to start confirming if you are getting 12V to 537 and 290/291 at the TCM then if not you can start backing up to see why otherwise you are just going in circles.
The circuit is actually very simple to follow.

 

[Mogman]

Also again do all voltage tests with the equipment connected, use a sharp meter (or test light) probe to pierce the wire jacket, When I locate 290, 537 and 657 at the TCM I wrap a bread tie around them to make it easier to find the next time I am looking for voltages (or ground in the case of 657)
I use a bipolar LED test light and can go through the entire system confirming voltages in just a few minutes.
Also again disconnect the temp sending unit for the glow plugs while doing these tests so you don't fry your glow plugs.

 

[Nic - M1097]

Voltages tested with sharp meter at TCM J1:

291D : 11.84 V

537D : 11.84 V

657A : 10.30 V


Why am I getting any voltage across 657A and why is it low?

Also, should I swap the batteries? Is 11.84V too low? I know the diagnostics checks all call for greater than 12V.

Thanks in advance and sorry for the late reply. Been super busy at work and haven’t had a chance to mess with the 1097 until today.

 

[Mogman]

When looking at 657, it grounds to turn on the trans light, I have never looked for a positive voltage but it would be there if the TCM was not trying to turn on the trans light, the other lead to the trans light is hot (12V) all the time, so 657 would be "hot" until the TCM grounds it to turn on the light.
The way I test 657 is use a dual polarity lest light (green for forward and red for reverse), I put the normally grounded clip lead on the +12V source and prick wire 657 with the probe, if the TCM is trying to turn on the trans light the test light will light red.

It sounds like you have both sources of 12V but the TCM is not trying to light the trans light

I would make sure I see a good ground at 599A and 599B, if so I would suspect a bad TCM.

You can also unplug the harness from the TCM and ground 567 at the harness plug, this will light the trans light indicating a working trans light.

If you have the two source voltages, good grounds a working trans light and the trans light still does not light when not running but in run position just about the only thing left is the TCM or possibly the wiring in the TCM box.

Have you opened the TCM box to confirm there has been no water intrusion?

 

[HUMMER H1]

Tnx to this awesome forum and thread I was able to track down my ECV transmission not shifting issue, after checking all the power supply mentioned above, everything checked out good, last thing was to open the TCM box and check,
sure enough there was a loose connection, as soon as I tightened it, trans light started to work and shift,

The light is flashing when I did the test drive, I am thinking it’s loose ground somewhere, I touched all the wires while truck is parked and running, but it didn’t make the light flash,

Does anyone know where is the ground connection for the TCM?

thank you guys.