Trouble Shooting 2001 Humvee with 4 speed trans in limp mode

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Tim Mullen

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Marco Island Florida
I have a 2001 Slantback with 900 miles very nice truck ran perfect for last 500 miles. I was driving it on interstate for about a hour at 70MPH and all of a sudden lost transmission, engine revved like it was in neutral . Also at same time all my gauges went dead. Truck is in limp mode I guess, because once I slowed down it started again in first gear but that is it and reverse works also. The gauges are ticking ever 10 seconds like trying to reset. No one knows anything about these trucks locally and I am trying to figure out what to do? Any advice I would really appreciate. Thanks Tim Mullen 239 404 1951
 

Coug

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If all the gauges and transmission acted up at the same time, I'd be thinking about troubleshooting the electrical system.
Have you done the supplemental ground harness on it?

I could be off, but electricity seems to be the one thing that is common between gauges and transmission control module. Transmission control module should be 12v, gauges 24v, so that would suggest a grounding issue.
 

Tim Mullen

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If all the gauges and transmission acted up at the same time, I'd be thinking about troubleshooting the electrical system.
Have you done the supplemental ground harness on it?

I could be off, but electricity seems to be the one thing that is common between gauges and transmission control module. Transmission control module should be 12v, gauges 24v, so that would suggest a grounding issue.

I am starting to think the same thing about electrical system. I have not done the supplemental ground harness. What is this?

I will forward the info you gave me to the guy I have just brought it to to look it over. Were are both a little lost with what is going on.

If the T C M went bad would that cause the gauges to all shut off at the same time it died?

I will update this post when ever we figure out the problem and the fix in case it happens to anyone else in the future.
 

BLK HMMWV

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Pasadena California
What engine is in your HMMWV?
What alternator is in it. 100 amp dual voltage ? 200 amp dual voltage
What TCU are you using?
Aftermarket or military?
Where is the TCU getting it's power from?

If OEM have you checked the breaker in the foot well of the rear passenger area ?
How old are the batteries?
What kind of electrical upgrades have been done to the truck?

Do you have a small yellow " Trans " light in deck near the shifter?
Have you tried getting the diagnostic code by jumping the pins?

Familiarize yourself with the Technical manual section of the board.
BH
 

Coug

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Olympia/WA
I am starting to think the same thing about electrical system. I have not done the supplemental ground harness. What is this?

I will forward the info you gave me to the guy I have just brought it to to look it over. Were are both a little lost with what is going on.

If the T C M went bad would that cause the gauges to all shut off at the same time it died?

I will update this post when ever we figure out the problem and the fix in case it happens to anyone else in the future.
This thread is a sticky and has all the info you need to either buy or build one and get it installed.


Basically, the HMMWV has some steel frame runner, aluminum body, and a few other metals thrown in.
All those different metals don't always play nice together, and can cause electrical issues on the ground side of things.
The supplemental grounding harness adds additional electrical ground paths at the places where the truck needs grounding but can sometimes have issues.

Even if it isn't the issue right now, eventually it could be, and it's a relatively inexpensive upgrade to do.
 

DREDnot

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Phoenix, AZ
The part where you say the gauges are trying to reset every 10 seconds...sounds like there may be a short somewhere that is popping a breaker and resetting and popping again. There is not much wiring in there so I would start with a visual inspection of your wiring harness front to back looking for damage/modifications and start there digging in to the harness looking for something obvious.

I had a similar issue in an old CJ. Found a power feed wire melted and fused to a few other wires in the center of the harness.

Start with a thorough visual survey.

DSCN8812.JPG

Another good place to look is all your signal light housings

DSCN3177.JPG
 
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