Sluggish Differential

[Crux444]

Out for a nice Sunday drive yesterday and my M151A2 became very sluggish. The engine is running like a top and purrs like a kitten, but shifting up through the gears, it is very sluggish in 1-2-3 gear and the rear diff feels jerky. Getting it into 4th allowed me to limp home gently, but the jerkiness was still there, just less pronounced.

Any thoughts before I drag it down to the shop?

Thanks,
Sean

[Rickf]

If you are sure it is the differential did you check to see if there was any oil in it? Was it glowing red? Don't laugh, it happened to me when mine spit out a seal as I was driving and it dumped out all the oil. Once I tore that diff down years later I was surprised that the damage was not really that bad, besides water damage from years outside in the scrap pile. But jerking can be a bad axle, broken u-joint, bad wheel bearing, bad bearing in differential. Although diff bearings on these are pretty stout as long as they have oil. Could also be loose bolts on rear diff mount. Certain years of A2 had problems with frame cracking around rear diff mount and there was a support kit made to fit around the diff mount to fix that.

[Crux444]

All of the seals on the diff are intact (new last summer), drain plug is in and no signs of leaking. Just checked and the bolts are properly torqued on the diff mounts and no cracks. Axles look to be in good shape and there isn’t any wobble in the rear wheels. I had a similar issue last year, but that didn’t include the sluggishness in the torque for gears 1-3. It seemed to fix itself as one day it didn’t do that anymore. Temperamental little beast…

Any further thoughts?

[Rickf]

Are you sure it is in the driveline? If you push in the clutch does it drag down like something is binding? Sure it is not the engine? I notice you are the one that had the issue with the 4 wheel drive that would not stay in after the transmission rebuild. I am wondering now if there are not more issues inside that transmission. Was the transmission shimmed correctly during the rebuild? There are different thicknesses' of thrust washers to be used.

[muttguru]

.....that didn’t include the sluggishness in the torque for gears 1-3. It seemed to fix itself as one day it didn’t do that anymore. Any further thoughts?

One of the urgent messages sent out by the military to all mechanics was the possibility that the 2-4 wheel driver shifter snap-ring(s) could be broken, preventing the shifter from disengaging the 4-wheel drive.

As you reported recently that you were having trouble with the shifter and now you are experiencing a "sluggishness" throughout all the gears, is it possible that your shifter is going into 4WD without you realising it? One of the novelties of the M151 transfer is that 4WD can be engaged without any need to slow down or de-clutch.
The torsional windup of 4WD used on hard surfaces will give you exactly that sluggishness.

Ken

[Crux444]

So, tried to see what the issue was today - started the engine and it blew oil out of the tailpipe. That immediately got me thinking blown head gasket which would mean the sluggish jerky motion when it drives is in fact low compression and not a differential problem at all.

I will get a compression tester and further diagnose. Looks like I’m grounded for the season as if will require a teardown. Boo…

[Rickf]

Did it blow oil out or water vapor? Water vapor on cold start is perfectly normal. Oil is bad rings or valve seals, head gasket will be sweet smelling antifreeze. You will also have bubbles in the radiator. A blown head gasket will cause a drop in power evenly across the power band, not in one spot that feels like binding. I lean more towards what Ken was saying especially considering the issues with the transfer case shifter. I think you may be stuck in four wheel drive either all the time or off and on. If that is the case and you keep driving it like that eventually you will load it up in a turn with the gears halfway engaged and things will get ugly fast.

[Crux444]

It spit out black oil, not water vapour. I checked the oil dipstick and under the filler cap (no milky colour) and no coolant smell or bubbles, so that eliminates a head gasket issue I guess.

I am going to do a compression test tomorrow to see if there is an issue there. I tried to pull the plugs to see if they are oiled up, which would likely be a valve seal(?) but I can’t get my deep socket to stay on the hex collar of the plugs. It feels that there just not enough hex collar showing to allow the socket to seat properly. it a 3/4” socket like most? Special tool?

I was also going to pull the head to see if it is a ring or burned valve. While the engine is out I will go over the transfer case again and make sure the transfer and output shaft gears are meshing properly.

Bummed that I am likely down until spring 2023…

[hambone]

It's a 13/16", go to the local auto parts or hardware store and get a universal plug wrench, like used on motorcycle or small engine.

[Rickf]

A regular spark plug socket will not work because the insulation in the socket will not fit over the threaded top of the military plug. I simply use a deep 13/16 socket. And the stuff coming out the exhaust is more than likely a mix of water vapor and carbon which is normal for these old engines. Especially if you have a fording kit. They will trash anything behind the vehicle. These engines tend to run rich at idle and soot up the exhaust so when cold and the normal condensation mixes with the soot you will get nasty oily black crap coming out. Once it warms up that should all be gone. A leakdown tester will much better pinpoint engine issues. You can hear bypass air in the valve cover if it is rings, exhaust if you have a leaky exhaust valve, same with intake valve hearing it through the carb. You can get a leakdown tester at Harbor Freight.

[Crux444]

Compression test shows no leakage, so I think I can rule out an issue with the engine (sort of). When I pulled the flywheel housing last winter I discovered oil on the flywheel - not a lot, and chalked it up to the pan seal which I knew leaked. Main seal on the crankshaft looked fine and wasn’t dripping, so I didn’t change it out. Now I’m thinking that there could be the possibility the seal is failing and there is oil on the face of the clutch plate which would mean it has trouble gripping the flywheel and causing the shuddering every time I up gear.

I am going to pull the drain plug on the flywheel cover to see if I get any oil out.

[Crux444]

…and lo and behold - motor oil drained from the bell housing drain plug. Time for a new main seal and clutch plate methinks.

[Rickf]

Was that plug installed the whole time?! That should only be installed during fording and stored in the holder next to it all other times. Either way oil there is not good. Next question is it gear oil or motor oil? A rear main seal leak will generally leak on the back of the flywheel and be spun off into the bell housing and run out never touching the clutch. But if the plug was in then it may have filled up if you have the very rare case where the inspection cover seals are all good. Otherwise, if the bell wasn't full of oil then it is gear oil coming from the front seal of the transmission and that will go right on the clutch. If the clutch is not slipping in 1'st or 2'nd gear I find it hard to believe it would be slipping in the higher gears where there is far less power and torque. I am not trying to be critical but I really think that transmission rebuild needs to be revisited.

[m-11]

Just went through this on my A-1 and found that my 4 wheel drive was engaged even though it was not selected. I replaced the torn boot on the selector last year and for whatever reason now it started preventing the lever from moving back far enough to disengage. I jacked up all four corners and started it to verify it was indeed engaged. I removed the small cover for the parking brake and transfer case lever boot and the 4 wheel drive disengaged. I ran it through several times with the cover removed to make sure it wasn't inside the transfer case. I reinstalled the cover with the boots but relieved the tension on the transfer case boot by eliminating the forward screw on the cover from holding the boot. The forward part of the boot is now sandwiched between the little cover and trans cover. This now gives it enough movement in the rubber so that when I move it out of 4 wheel the rubber isn't applying opposite pressure to the transfer case lever thus preventing it from holding it in 4 wheel. Got to love after market crap parts.