M998 Transfer Case in "H" Slips out of Gear

[Phoenix77]

I have a 1986 M998 HMMWV and when I try to put the transfer case into "H", it just makes a grinding noise and goes nowhere.

I adjusted the linkage and could get it to drive a little way (about 30 sec - 1 min) and then it would disengage, make the grinding noise, and coast to a stop. I can put it in HL and it moves again.

Any suggestions?

Thanks.

 

[bchoate66]

Sounds like an issue I had. Your range shifting fork is going bad and the ends that keep the transfer case engaged in gear are getting eaten away every time you hear the grinding noise.


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[Phoenix77]

Thanks for the quick reply. Did you replace the range shifting fork? How intensive is that?

 

[bchoate66]

I did. And depends on which case you have. I have the np218, and it was he’ll finding the fork to begin with. But I had to completely take the case apart to replace it. Hard to do with only a diagram to put it back together


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[Retiredwarhorses]

not understanding, the truck stays in H unless shifted into HL or Low...
all shifting of Tcase is done with engine off and trans in neutral...per the TM

 

[Phoenix77]

not understanding, the truck stays in H unless shifted into HL or Low...
all shifting of Tcase is done with engine off and trans in neutral...per the TM

When it would disengage and grind, I shut off the engine and shifted to HL to get it to drive again.

 

[Phoenix77]

I did. And depends on which case you have. I have the np218, and it was he’ll finding the fork to begin with. But I had to completely take the case apart to replace it. Hard to do with only a diagram to put it back together


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Do you have an NSN for the Shifter Fork Transfer Shift you used?

 

[Retiredwarhorses]

Sounds like a broken halfshaft or bad differential.

 

[Phoenix77]

Sounds like a broken halfshaft or bad differential.

Any advice on how I can determine which it is?

 

[bchoate66]

Part number C-13731 is all I have. That’s the cast number on the shift fork. Good luck finding one. I had the hardest time


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[Retiredwarhorses]

Any advice on how I can determine which it is?

start buy looking at your halfshafts at each wheel...when you lose a HS, the way you get going again is to put the Tcase in HL to force full time power through tcase, sends torque to all 4 wheels.

 

[Phoenix77]

Part number C-13731 is all I have. That’s the cast number on the shift fork. Good luck finding one. I had the hardest time


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Thanks for the info: I just found one via army surplus warehouse for $26 shipped. It says it is for HMMWV 1986-1990, so that's my HMMWV.

 

[Phoenix77]

start buy looking at your halfshafts at each wheel...when you lose a HS, the way you get going again is to put the Tcase in HL to force full time power through tcase, sends torque to all 4 wheels.

Probably a dumb question, but in "H", which wheels get the power? i.e., RWD or FWD?

 

[bchoate66]

Thanks for the info: I just found one via army surplus warehouse for $26 shipped. It says it is for HMMWV 1986-1990, so that's my HMMWV.

Good deal. The only thing I could find the do the tear down was in New Processes old catalog. If you find something better, please let me know


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[Coug]

Probably a dumb question, but in "H", which wheels get the power? i.e., RWD or FWD?

in "H" power goes to all wheels, but if for some reason one wheel loses traction, then all the power will try to go through that one wheel, and the truck won't move.
The "HL" and "L" modes lock the transfer case so that power will always go half to front half to rear. If one wheel loses traction then it only gets the power for the wheel on the opposite side lost through it, you will still have equal power from the opposite end (i.e. you lose traction front right, front left won't have power but both rear still get power)

 

[Retiredwarhorses]

Good deal. The only thing I could find the do the tear down was in New Processes old catalog. If you find something better, please let me know


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so your going to tear down a Tcase and replace a part based on a hunch?
more often than not...bad hal shafts of differential is the culprit.
i haven’t had a bad Tcase in the shop in so long i can’t say when I saw a bad one last.

 

[HMMWV1045]

So, my HMMWV is having the problem of the transfer case being "H" and the truck goes no where. All I get is a whirling sound. If I put it in "L" or "HL", the truck moves. I do have a broken right hand front half shaft. Are we sure that is the cause of the problem?

 

[Retiredwarhorses]

So, my HMMWV is having the problem of the transfer case being "H" and the truck goes no where. All I get is a whirling sound. If I put it in "L" or "HL", the truck moves. I do have a broken right hand front half shaft. Are we sure that is the cause of the problem?

any broken HS will act this way...

 

[Phoenix77]

So, my HMMWV is having the problem of the transfer case being "H" and the truck goes no where. All I get is a whirling sound. If I put it in "L" or "HL", the truck moves. I do have a broken right hand front half shaft. Are we sure that is the cause of the problem?

That's most likely it.

All my half shafts we're good, so I ended up putting in a new rear differential.

 

[AJB777]

OK, this thread is a little old but perhaps will get a response, thanks.

My 1987 m998 does exactly what has been described here. I was driving it one day and it just slipped out of gear. I had to go to L in order to get it to move. Drove it home at 10mph before I started messing with it. Of course I stopped engine to shift.

Prior to this happening I replaced both front half axles, all the steering components i.e Pinon, tie rod ends and follower arm (six pieces). everything torqued and lubed per spec.

With vehicle on the rack I can see both rear half axles look good. they both turn when wheels are turned.

When the thing pops into neutral You can hear what sounds like a penny bouncing around just between the seats.

My question is how can I test the half shafts or differential or transfer case to see what is causing this?

Interesting note: I was originally told that trans fluid was leaking into the transfer case thus overfilling it. So I did a Trans filter change and transfer case oil change. When I took the filter out I noticed that the O ring was on the filter side of the plastic tube that feeds the case. The diagram shows that the O ring should be placed on the case side (top) of the plastic tube. After doing this she ran for a month without a problem them bam it just started up again.

Any suggestions would be greatly appreciated.
ooops almost forgot. I have a very small slow leak from engine rear oil seal if that matters at all.