1999 u90 transmission

[Unimog66]

Hi
I just bought a 1999 u90 unimog and the 1st and 2nd gear shift smoothly but the 3rd grinds and won’t go in. Any ideas what could be wrong
thanks

 

[Mogman]

Hi
I just bought a 1999 u90 unimog and the 1st and 2nd gear shift smoothly but the 3rd grinds and won’t go in. Any ideas what could be wrong
thanks

Do you know how to double clutch? very common necessity on the early Unimogs

 

[Unimog66]

Do you know how to double clutch? very common necessity on the early Unimogs

Yes I tried that

 

[Mogman]

Yes I tried that

Can you shift into 3rd while sitting still, engine running clutch pushed in?

 

[Unimog66]

Can you shift into 3rd while sitting still, engine running clutch pushed in?

No it grinds

 

[Mogman]

No it grinds

Then the clutch is not completely releasing, you may want to check the adjustment.

 

[Mogman]

Here is one thing to try, start the truck in 3rd gear, clutch depressed, then slip it into neutral and then see if you can slip it back into third gear keeping the clutch depressed the entire time.

 

[Unimog66]

Then the clutch is not completely releasing, you may try to check the adjustment.

Ok why would 1st 2nd 4 th and other gears shift good but just not the 3rd

 

[Mogman]

Ok why would 1st 2nd 4 th and other gears shift good but just not the 3rd

Third likely has poor or non working synchronizers You may be able to detect the very slight pulling down of the RPMs when stationary and going into gears that "work" that is the synchronizers stopping the clutch (transmission input shaft) that is not fully disengaging. the exorcise listed in post #7 can detect one particular type of problem.

 

[Mogman]

Actually third having the synchronizers "burned out" may make perfect sense, I have no knowledge of the U99 but if it is like the 404 you would normally start out in third as 1-2 are low range, so over time as the previous owner drove it with a defective clutch they would naturally burn out the third gear synchronizers as the gear used to stop the input shaft while stopped would create much more wear than the other gears.

 

[Unimog66]

Third likely has poor or non working synchronizers You may be able to detect the very slight pulling down of the RPMs when stationary and going into gears that "work" that is the synchronizers stopping the clutch (transmission input shaft) that is not fully disengaging. the exorcise listed in post #7 can detect one particular type of problem.

Is this an easy fix? do You think it is very costly? Would you have a diagram of that transmission that would help me visualize it more easily

 

[Mogman]

In any case you should be able to shift into a gear with no synchronizers or working synchronizers while stationary as after short time the input shaft should stop and would not grind any gear, the fact that you grind the third gear while stationary shows you have a clutch that is not working correctly.

You need the TM for that truck and go through the procedure to adjust the clutch, if that procedure goes as shown in the TM and you still have problems then you may have to replace the clutch, if it goes as as detailed in the procedure and you can shift into third while stationary then you have fixed the clutch issue but still have bad syncros in third gear and can probably can live with it by double clutching.

If there is a problem adjusting the clutch as outlined then you may have a linkage issue, I would assume a 1999 Unimog would have a hydraulic clutch so there may be issues with the master cylinder/slave cylinder.

There are likely others that can guide you through that process much better than I as I said I have no experience with the U99

 

[Unimog66]

Thank you very much for your help