UNIMOG Colorado
Rocky Mountain Moggers
RMM Tech Tips

Changing the Transmission output seal on a Unimog 404.1

Assuming you have the correct seal and gaskets (you might want to pick up some permatex and gasket material in case you have to make your own) the manual pretty much walks you through it.

Drain the transmission (unless you like gear oil all over everything, which adds to the fun).
Remove the spare tire.
Disconnect the front differential lock linkage.
Remove the corrugated rubber boot from the ball flange housing and torque tube.
Disconnect the shocks from the upper mounts.
Remove the lower coil spring retaining bolts.
Disconnect steering arm from drag link.
Disconnect sway bar from frame (I think they call it a wishbone).
Raise the frame to clear the coil springs (be careful, and use HD jack stands).

The book has you separating the brake line. No problem as long as you don't mind bleeding the brakes when you are done. We figured out that if you disconnect the brake line from all the retaining clips, and carefully cut the clip where the line is connected to the torque tube, the front end can be rolled forward enough to perform the task. Once everything was back together, a quick shot with the welder tacked the clip on the torque tube back together.

Another deviation we had to take was once everything was disconnected, was to disconnect the torque tube at the front differential as well. For whatever reason, when the front axle rolled forward, the ball flange housing would not clear the transmission gear case. Once the torque tube was separated from the front differential, the front driveshaft would come off the differential spline and the rest would come right off. Perhaps we did not have the correct angle.

Now you have the front axle completely out of the way, and the driveshaft hangs from the output flange on the gear case.

Unbolt and remove the driveshaft from the output flange. There should be retaining clips on all the bolts.

Remove the slotted nut that holds the output flange in place on the output spline. Look closely and you'll see a nasty little retaining ring that stakes the slotted nut in place. You'll need to bend the tabs back in order to remove the nut. If you don't have the socket spanner #401589000700, carefully use a pin punch, engage the 4x4, and tap the nut around until it can be removed.

Now for the fun. You'll need a good gear puller to remove the output flange. It is pressed into a bearing, and onto the output spline. We managed to use hardened bolts as jack bolts to run the flange off, because even with my brothers extensive complement of tools, we couldn't find a puller that worked.

OK. So now the output flange is off. Remove the small screws that hold the bearing flange in place, and remove the bearing flange. This piece holds your seal. Remove the old seal, throw it in the air, and blast it with your 12 gauge.

We found that the correct seal (red material and proper direction arrow) was installed, but was not installed correctly. It wasn't seated properly. We'll get to this in a moment.

Clean all your parts, inspect everything for wear, and prepare all the surfaces for reassembly. Take your time her, because you won't want to do this job again.

Here is the most important part, seating the seal. If you can find a bearing press, use it. The seal goes right up against the snap ring that hold the bearing in place. The output flange has a raised surface that the seal rides on. If the seal is not seated properly, that raised surface will pass right through the seal and there will be a gap to let the oil out. Do a few dry runs with the output flange and seal to see what I mean.

Once the seal is installed, you can now press the output flange back in the bearing race. Again, proper seating is critical. Press the flange into the bearing race until the flange shoulder contacts the inner race shoulder (no more movement possible). Try turning the output flange in the bearing flange, you should feel the resistance of the seal.

Before you go to install the bearing flange, take a look at the output spline. You should see a bolt with a hole in it screwed into the end of the spline. This hole allows oil into the output flange cup, and into the universal joint. It's a neat arrangement. Our bolt was missing, so we had to make one.

This is important - When you go to install the output flange on the spline, be sure that the oil hole on the flange lines up with the oil hole on the spline. This allows lubrication of the spline area and bearing. The output flange should fit on the spline with moderate blows from a hammer. Protect the output flange surface from damage with a soft faced mallet or block of wood.

The only other thing that is worth mentioning before you put back together in reverse order, is to make sure that the surface between the output flange and universal joint flange is clean and straight. Remember that the gear oil comes out from the hollow bolt in the output spline. If that 'cup' (between the universal joint flange and output flange) that holds the oil that feeds the universal joint leaks, you'll have oil under the truck again. We made sure everything was clean and straight, and installed a new gasket.

Things went back together very well. The 30 mile drive home from my brothers garage produced no leaks.

I know that we did some things that would get a Unimog mechanic yelled at, but we did what we could. The net result is we are no longer leaking oil, and we have a re-enforced respect for the design of the 'mog. I have always enjoyed working on Mercedes-Benz vehicles, and the Unimog is no exception. They are well laid out with maintenance in mind.

Now all I have to do is clean the undercarriage of all the old residue.

It took the two of us about ten hours (we split it up over the weekend) and we weren't killing ourselves to get it done. We took our time to investigate and clean as we went.

I would not recommend doing this all by yourself. An extra pair of hands is needed for some tasks, and it helps to have an extra pair of eyes. Besides, someone has to mop up all the oil on the floor when you are done.

Good luck!

Drip free in PA
Dean E. McCahan
1963 404.1

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