Unimog 404 drivetrain issues

[kingjon3rd]

Newbie here looking for some help!

I picked up a 1967 404.1 unimog and it's been sitting for the past 13 years out in the weather(it ran and drove when they parked it there). Before I dump too much into it I want to get it rolling and running.*

With the vehicle in 2 wheel drive, the 6 speed trans in nuetral, the dif levers disconnected, the rear tires will only spin with both rear tires lifted and in opposite directions.* When one of the rear tires is on the ground the rear tire won't spin.* If I jack the front left tire up, it spins freely in either direction.

I decided to put the truck in 6th gear with the rear lifted and spin the tires by hand to see if it would it would spin the engine. I was expecting it just to lock up but nothing changed and the fan wouldn't spin.

Any thoughts of what I should check next to see if I can get this chassis rolling?

 

[tennmogger]

From your testing, the diffs are both unlocked as you stated, but transmission may not be in neutral as you thought because, first: if the front end allows one tire to spin with one tire on the ground then that verifies the 4wd gears in the rear of the transmission are not engaged. Therefore, if one rear tire will not spin then the driveshaft is not spinning and the transmission is not spinning (not really being in neutral is 'easy' conclusion to that test). These transmissions are easy to get hung up between the 1-2 side of the gears, and the 3-4-5-6 side. Try reshifting across the "Gate" a few times to try to unlock it. This is tricky without power so use care to not force the shifter.

I am not sure how to interpret " I was expecting it just to lock up but nothing changed "

In 6th, as you tried to turn the engine, it should turn, but with effort. Engine must be seized. How bad is an unknown. You should pull the plugs and put in some penetrant to soak the rings. Please do not try dragging with another vehicle, or anything rough like that, because rings might break.

Soak the rings a few days then 'bump' the jacked up rear tire back and forth (trans in 6th). That shakes the crankshaft and pistons, gently working the rings. Bet it will free up.

Bob

 

[kingjon3rd]

Bob-
Thanks for the feedback.

I can shift into all 6 gears, but I will double check by shifting back and forth across the gate to ensure it is in neutral next time I'm at the truck.

I would agree that the motor could be seized if I saw no change with the truck in 6th gear or neutral with the rear lifted, however, if I jack only one rear tire up with the truck in neutral the tire should still roll in either directions, which it can't do, making me think its something in the drivetrain. The rear torque tube and collar was very rusty due to a ripped boot, do you think that rear universal could ever freeze up?

Since it has been sitting for a while, I have already pulled all the plugs and put a cap full of marvels mystery oil down each one.

 

[tennmogger]

You have already done the things I thought of! Being able to shift into all six gears rules out the lockup due to crossing the gate.

Even with a bad torque tube boot (the one behind the transmission) there should not be a locked U-joint. If the grease fitting on the rear torque tube swivel ball was ever lubed, that makes a pretty good water seal around the ball end. forcing water to run off instead of penetrating.

Is the rubber boot exposed to rain, or is it underneath the bed or box? If it is torn on top, not torn on the bottom, and exposed to rain, I supposed it could cause the entire torque tube to fill with water. Water could pass seals and go toward the transmission and affect output bearing. Check transmission for water.

Water in the torque tube would leak down to the pinion and could lock up the pinion bearings. Any water that got that far would accumulate in the differential, giving you another spot to check.

What's in the diff? If there is no water there, there could still be water in the torque tube but that should not lock the driveshaft. There is a seal on the lower end of the driveshaft, just ahead of the splines on the pinion shaft. Those seals do leak most of the time though.

Another dumb thought, the brake shoes on that jacked up wheel are free, aren't they? Obviously if the brakes were locked there would be zero play on the wheel so you would know immediately.

 

[kingjon3rd]

The rear torque tube collar grease fitting will not accept any grease and that whole area is rusty(where the collar mates with the torque tube). The boot was torn on the top and not the bottom so rain water could get in especially since the whole wooden deck was rotted away. I wish I had pictures I could show. I did check the fluid in the rear differential. There was about a cup of water before oil started coming out. I'll have to check the transmission for water next.

I've tried to remove the collar on the torque tube (using lots of kroil!). I was able to move it back about a 1/4" but now the collar moves with the torque tube rearward as an assembly. How easily do they come off so you can access the universal joint, the manual shows it just slides back? I know i'm going to have to remove the rear but I would like to get it rolling the easiest way possible without removing the rear so I can flat bed it home and deal with it there.

I loosened all of the cams on the brakes shoes. There is some play in the tire, but only about an 1" play.

I appreciate the help!

 

[tennmogger]

Good planning on the brakes.

Even if you remove the collar (two halves) on the torque tube, there is still no access to the U-joint without moving the entire torque tube and axle assembly rearward. The U-joint is inside the ball of the torque tube. The axle has to move rearward far enough to expose the bolts that hold the forward flange of the U-joint assembly to the output shaft flange of the transmission. That means supporting the frame on jacks so the springs are relieved of weight. Once those bolts are out (and there are locking plates on them, and the drive shaft has to rotate to get to them all!) the axle has to go further back, torque tube lifted, then the shaft with U-joint slides out of the torque tube to expose the U-joint. You might even have to unbolt the rear springs to move the axle that far.

The 'collar' is probably stuck to the torque tube ball by rust. There is a two piece 'cup' inside the collar that provides the round surface to match the torque tube ball. It also sets front-back slack of the torque tube using shims. Those cups were was originally cast iron (nylon on later rebuilds). Those can rust and hold the collar on, unless you split the collar (it is two piece also). I would take the Zerk fitting out of the collar and screw a grease gun hose directly into the hole. If memory serves me, it will fit. Anyway, the goal is to force lubricant of any kind into the ball assembly to free stuff up.

Honestly, I doubt the drive shaft is the stuck culprit. Wish I were nearer to assist.

Getting into that driveshaft is not a trivial task. It's best to eliminate all other possible causes of locked up driveshaft/transmission first. If you know for sure the transmission is in neutral and clutch not stuck(like by turning the engine), you can get more aggressive trying to get the rear axle and wheels to turn.

You have a good puzzle there!

 

[kingjon3rd]

Thanks for the feedback! I agree with the approach with ruling out some engine and trans issues before dropping the rear. Any other thoughts on how to rotate the engine? With the fan on the crank I didn't see any good spots to put a wrench on?

 

[kingjon3rd]

I was able to turn the motor over by putting a wrench on the camshaft pulley bolt with the truck in neutral.

Next I'm going to drop the rear so I can slide the rear out enough to remove the torque tube to see if its something in the driveshaft or rear dif.

 

[tennmogger]

Great news that the engine turns over. There is a mode of failure in the U-joint area we have not mentioned. If any of the 6 bolts on the transmission output flange have backed out, considering they are inside the ball of the torque tube, a loose bolt can lock the driveshaft. Never heard of it actually happening on a 404 but it certainly does on a 406 (more torque to loosen the bolts). If a bolt gets jammed between the U-joint crosses and the inside of the ball, it locks the driveshaft.

Do you have any idea why the truck was sidelined to begin with? Obviously a loose bolt and locked driveshaft does not happen just sitting. That failure could have been the cause of shutting down the truck back then. On the other hand, rusting from water intrusion into the shaft, the differential, or the rear output bearing of the transmission certainly could cause freeze up while sitting.

In this case, with the engine known to be free and with transmission known in neutral, I would try getting more aggressive to make the driveshaft/wheels turn. With the gear boxes at the wheels taking the force, and able to handle it, it is very difficult to hurt anything in the drive train by dragging it around. Tires will slide before anything breaks.

 

[kingjon3rd]

It was my father in laws. He bought, it drove it around a few times and then got a fuel issue and never fixed it.

I tried dragging it about 1 foot forward and didn't break loose. I was thinking of taking an impact gun to the rear hub bolt. Since they are torqued to 750 ft lbs, I could try loosening the left rear with the tire up in the air and see if the impact breaks the rust free.

 

[tennmogger]

There's too much mass in the wheel, hub, portal gears, and axle for the impact gun to do any good loosening the diff pinion or rear of transmission. Dragging the truck back and forth will be more successful. This is a head-scratcher.