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Shifting just fine and then gets stuck in 1st gear. Help!

Paul M998

Paul M998

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Hi, new member here. I have browsed the forums for a while but never need to post until now. I have an issue I was wondering if I could get some help on.

I have 1986 m998 humvee.

Humvee drives for about 5 - 10 minutes shifting just fine. All of a sudden It will down shift into first gear and stay there. I can turn if off start it back up 30 minutes later shifts just fine and then does the same thing again. Engine temps are normal. Not even high. Fan not needing to kick on yet.

I put 2 new batteries in thinking it was that. Did not fix the problem.

Anyone experience this? Any help would be appreciated.

Paul
 
Mogman

Mogman

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Just a thought, did you service the transmission when you changed ALL the fluids and filters?
That is different though
 
NDT

NDT

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Suggest you check the operation of the mechanically operated transmission modulator. Maybe getting stuck in WOT/kickdown.
 
Milcommoguy

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Paul M998 said:
Hi, new member here. I have browsed the forums for a while but never need to post until now. I have an issue I was wondering if I could get some help on.

I have 1986 m998 humvee.

Humvee drives for about 5 - 10 minutes shifting just fine. All of a sudden It will down shift into first gear and stay there. I can turn if off start it back up 30 minutes later shifts just fine and then does the same thing again. Engine temps are normal. Not even high. Fan not needing to kick on yet.

I put 2 new batteries in thinking it was that. Did not fix the problem.

Anyone experience this? Any help would be appreciated.

Paul
Click to expand...
MY guess... so here we go. Guess #1 you have a 3 speed transmission? There are many reasons... guess # 2 check the TV cable assembly and adjustments. It a little strange it can be a start - stop thing. Guess #3 Check the kick down switch on the passenger side of IP. I could be sticking in the closed position position. (mechanically or electrically. worn out mess) It could be wiring too. It activates a solenoid in the transmission and if stuck (closed intermittent) can get it into first gear and not switch up to second.

If this condition is pretty much repeatable... One can pull the wire to the kick down switch or easier pull the one wire going into the driver side of tranny feeding the kick down circuit. If it clears up... work it back. AND it could be a goofed up tranny ???

Also note it feed the TDM (time delay module) might pull that connector to see if it clears the problem.

The above make NO quickie fixie. Going to have to dig in a little and think it through with thought. More info would help, pictures too.

The manuals or TM would be a good place too get a feel of the goings on too.

I used up my three guesses for the day, CAMO
 
Mogman

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Nether the kick down switch nor the modulator cable (TH400 does not have a TV cable) should completely override the mechanical governor and cause this issue, I am of course assuming you have taken up to an RPM in first that would normally force a shift, those are not bad places to start but think about it for a second or two,,, even with your foot to the floor which would both pull the modulator cable to full and switch the kick down switch on will prevent it from shifting. (this is actually NOP for a three speed truck)
I would think this is a pressure issue caused when the transmission heats up, if not already done (which it should have) I would change the fluid and filter.
 
Paul M998

Paul M998

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Hi thank you for the replys! This gives me a place to start. My humvee does have a 4 speed transmiasion. It had a transmission swap with a humvee with a 4 speed.
 
Mogman

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You are probably going into limp mode, could be a very simple problem like a bad TPS
 
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Mogman

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You need to read the TCM codes, if they did not install a trans light when they did the conversion you will have to rig something up first to get the codes.
As CAMO said we had bad information to begin with, you should have lead with the fact it had been converted to a 4 speed.
 
Mogman

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You never said if they installed a trans light in your dash or not, if not the first thing you should do is get one and install it, the schematic is on page 49 of the guide I posted for you, that is how you get the "codes" out of the TCM
 
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Paul M998

Paul M998

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Mogman said:
You never said if they installed a trans light in your dash or not, if not the first thing you should do is get one and install it, the schematic is on page 49 of the guide I posted for you, that is how you get the "codes" out of the TCM
Click to expand...
I checked this morning. I do not believe they installed one. I will get a test light and check the codes. Thank you!
 
Mogman

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Be careful, the light circuit is only designed for an LED so it will not sink too much current, a small automotive test light should be OK.
Also plan on installing one, the correct one, if you had it you would have known if the TCM was pissed off.
 
Paul M998

Paul M998

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Mogman said:
Be careful, the light circuit is only designed for an LED so it will not sink too much current, a small automotive test light should be OK.
Also plan on installing one, the correct one, if you had it you would have known if the TCM was pissed off.
Click to expand...
Okay, thank you!
 
Retiredwarhorses

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Mogman said:
Be careful, the light circuit is only designed for an LED so it will not sink too much current, a small automotive test light should be OK.
Also plan on installing one, the correct one, if you had it you would have known if the TCM was pissed off.
Click to expand...
Not necssarily, the trans lamps does not come on just because your in limp mode.
a faulty trans relay, dead TCM with no power produce Nada…
 
Mogman

Mogman

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Retiredwarhorses said:
Not necssarily, the trans lamps does not come on just because your in limp mode.
a faulty trans relay, dead TCM with no power produce Nada…
Click to expand...
While that is true, if you are in limp mode and there is no light you would immediately suspect the TCM has no power.
 
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