1986 M998 HMMWV Dead Rear End

[Phoenix77]

Recently my 1986 M998 converted itself to front wheel drive only. Even in "HL", which is supposed to lock the rear diff (right?).

I have recently replaced 3 of 4 half shafts and none of them were broken (hindsight).

I jacked the rear end up, chocked the front, put it in gear, and gave a little throttle to see if the rear wheels would turn.

Nothing.

I can turn the rear drive shaft with my hand about 1/2 turn, and the front shaft moves too, so I don't think it is the T case.

Thoughts?

Thanks

 

[Mogman]

When you say the rear wheels will not turn are you saying the engine will rev or does it seem like something is locked?, HL does lock the differential in the transfer case, so far everything you are describing sounds like a transfer case issue, but we will need more info, if you put the transfer case in neutral can you turn the drive shaft?
The first thing I think I would do is pull the inspection cover on the rear end, easy enough to do, any oil leaking out anywhere?
Does the transfer case seem to be shifting correctly, low range?

 

[NDT]

So simple. Put it in H rev it and look underneath to see what is and is not spinning.

 

[Phoenix77]

So simple. Put it in H rev it and look underneath to see what is and is not spinning.

I looked; rear drive shaft spins in "H"

 

[Phoenix77]

I revved the engine, the front wheels were ready to go, but the rear did nothing. I have had this thing see-sawed on a dirt mound and seen the rear wheels spin, trying to autocrawl.

In neutral, I can rotate the driveshaft easily.

I noticed grease by the driver's rear half-shaft. I just installed this one. Maybe needs tightening?[/QUOTE]

 

[Phoenix77]

Other than the splattered grease, no visible oil leaks (all aftermarket dried mud still installed on undercarriage).

 

[NDT]

Please do this: Put it in H, rev the engine, report back if the driveshaft or either brake rotor or halfshaft is spinning.

 

[Phoenix77]

Please do this: Put it in H, rev the engine, report back if the driveshaft or either brake rotor or halfshaft is spinning.

I put it in "H", started it up, and the rear drive shaft spins, but no movement from the rotors or half-shafts

 

[NDT]

You are about to buy a rear differential. Drain it to see what comes out to confirm.

 

[Phoenix77]

You are about to buy a rear differential. Drain it to see what comes out to confirm.

What do I not want to see come out?

 

[NDT]

Teeth, shavings, ground up metal

 

[Mogman]

I revved the engine, the front wheels were ready to go, but the rear did nothing. I have had this thing see-sawed on a dirt mound and seen the rear wheels spin, trying to autocrawl.

In neutral, I can rotate the driveshaft easily.

I noticed grease by the driver's rear half-shaft. I just installed this one. Maybe needs tightening?View attachment 809336

[/QUOTE]
Some of that greese looks to be from old half shafts, but the grease on the caliper appears to be new and it looks like grease coming out of the new one, looks like the band cut the rubber, still does not explain the issue.

 

[Phoenix77]

Teeth, shavings, ground up metal

Gotcha.

Looking at a replacement rear diff for the M998: is the ratio 2.73 for all M998s?

 

[Coug]

Gotcha.

Looking at a replacement rear diff for the M998: is the ratio 2.73 for all M998s?

Not necessarily.
2.73 is for the 4 speed / A2 trucks, 3.08 is the REV and newest gen of trucks, the A0 and A1 should be 2.56. There might or might not be a tag on the differential with the gearing numbers stamped into it.

 

[Mogman]

I think you are getting ahead of yourself, you need to actually find the problem, Do not continue to do what you have been doing,,, just replacing parts.
The suggestion to drain the rear dif. is a good one, also removing the inspection cover will give you a clear picture of what IF anything is wrong with the dif.
There is not allot of room for crap to float around in the differential a catastrophic failure of a differential is usually very obvious.