Transmission- or differential issue?!

[FMFHMMWV]

Just driving on city streets and I lose power in H. I have an m1123. I pull over and shut the truck off and shift in HL. the truck is then able to move but I get a clunking sound coming from underneath (possibly the rear), I thought maybe it was the transfer case, but I just took it apart and I don't see anything wrong with it, no shavings at all and the magnet is clean. (haven't looked at the internal transfer case differentials). took off the rear diff plate and didn't see anything wrong with the diff upon first glance. No busted parts at the bottom.

HOWEVER... while the truck is parked and all wheels are on the ground, I am able to spin the drive shaft input to the rear diff. parking brake on, all 4 wheels on the ground and chocked, but I am able to spin the yoke from the drive shaft into the rear diff. I didnt think this is right but some dude from a transmission shop said on some cars I can.

million dollar question. is my problem the transfer case or the rear diff?

 

[Coug]

transfer case is possible though not likely.

Most commonly when we see someone post about only being able to move in HL it's a failed half shaft/CV joint. Not uncommon for the bolts holding the shaft to the differential flange/brake rotor to back out and shear off.

I've only heard of a couple times where it's something internal to the differential.

 

[NDT]

Rear differential.

 

[FMFHMMWV]

transfer case is possible though not likely.

Most commonly when we see someone post about only being able to move in HL it's a failed half shaft/CV joint. Not uncommon for the bolts holding the shaft to the differential flange/brake rotor to back out and shear off.

I've only heard of a couple times where it's something internal to the differential.

both front and rear shafts were taken off and are in good condition. am i supposed to be able to spin the pinion yoke while all of the trucks wheels are on the ground and parking brake is up?

 

[NDT]

both front and rear shafts were taken off and are in good condition. am i supposed to be able to spin the pinion yoke while all of the trucks wheels are on the ground and parking brake is up?

Nope.

 

[FMFHMMWV]

Rear differential.

just tried to spin the front yoke and it wont move. wont budge a bit. but the rear one I can freely spin. so Im thinking that the only reason why I was able to get home in HL is because I was front wheel drive due to the front and rear interlocked.

 

[NDT]

just tried to spin the front yoke and it wont move. wont budge a bit. but the rear one I can freely spin. so Im thinking that the only reason why I was able to get home in HL is because I was front wheel drive due to the front and rear interlocked.

That is correct. Nearly every time this happens, it’s a blown cv joint, but not if you have the ebrake holding the rotors tight.

 

[FMFHMMWV]

Nope.

see thats what I thought. The front pinion will not spin freely with both tires on the ground but the rear will. I took the rear diff cover off and didnt find anything looking out of the ordinary. I would have expected to see busted gear teeth or something.

 

[FMFHMMWV]

That is correct. Nearly every time this happens, it’s a blown cv joint, but not if you have the ebrake holding the rotors tight.

yes the e-brake is holding the rotors

 

[FMFHMMWV]

That is correct. Nearly every time this happens, it’s a blown cv joint, but not if you have the ebrake holding the rotors tight.

so if I have the parking brake on, its the differential and not the cv joints, correct?

 

[osteo16]

Sounds like a sheared pinion... Not very common I'd say, but if you didn't see anything unusual about the gears then that would be my next guess. See if you can remove driveshaft and pull yoke forward out of case...

This is assuming you can turn yoke freely without feeling any gear resistance.

 

[Mogman]

It would be possible to turn the pinion shaft with the P brake on if all the bolts were missing from one of the inboard CV joints.
But that should be pretty obvious.
If you did shear a pinion the input yoke would certainly have some slack in it (in/out)

 

[REF]

If the rear cover is off, just turn the pinion if the ring gear rotates the problem is in the center section or outputs. If the ring gear does not turn then its the pinion.

 

[FMFHMMWV]

Sounds like a sheared pinion... Not very common I'd say, but if you didn't see anything unusual about the gears then that would be my next guess. See if you can remove driveshaft and pull yoke forward out of case...

This is assuming you can turn yoke freely without feeling any gear resistance.

Ill have to try that after I get home. I had the rear drive shaft off and was able to spin the yoke but I have not pulled on it to see if it would pull out.

 

[FMFHMMWV]

It would be possible to turn the pinion shaft with the P brake on if all the bolts were missing from one of the inboard CV joints.
But that should be pretty obvious.
If you did shear a pinion the input yoke would certainly have some slack in it (in/out)

Ill have to check on the status of the cv joints when I get a chance. This is all very good information and I cant thank you guys enough!

 

[Maxjeep1]

You sheared your pinion off… if you can spin the driveshaft freely that’s the only outcome. I think it’s a very robust pinion and I’m surprised that it failed.

 

[REF]

I have seen sheared pinions but only in things with much more horsepower than a HMMWV could dream of.
If it is the input i would bet its more likely the yoke got lose and stripped out it's splines. with that in mind if the diff turns when you turn the pinion I would look at the output flanges and their splines.

 

[Maxjeep1]

I have seen sheared pinions but only in things with much more horsepower than a HMMWV could dream of.
If it is the input i would bet its more likely the yoke got lose and stripped out it's splines. with that in mind if the diff turns when you turn the pinion I would look at the output flanges and their splines.

Yoke that got loose would be leaking before it stripped the splines? Seems pretty easy to diagnose if we were hands on

 

[Maxjeep1]

AMC 20 is almost ford 9” strong. I don’t think op is trouble shooting this correctly and we are getting half of the info to help him

 

[FMFHMMWV]

had to work the last two days but will be able to provide more info on tuesday. there is some resistance from the yoke but i can spin it with one hand. is there an internal clutch in this diff?