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So I have a 1991 U1250. I know that it has the 8 speed gearbox with the splitter engaging between fourth and fifth. The issue I have is that I'm missing first and second gear. I know this since I can only engage two gears before the splitter kicks in and then I go through the final four gears. The other issue is that it doesn't like to stay in fifth and sixth gear which is essentially first and second. It never really seems to slide in; I have to hold it there. I want to know if this is a problem with the linkage or if it could be a lot worse? How would I diagnose this?
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5 ex ADF U1700’s and 1 ex RAAF U1750. Some would say too many.
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Given that gears 1,2 low are the same gears for 5,6 high, and 3,4 low is same gears for 7,8 high. Then the issue cannot be the gears or
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selector forks in the transmission. It is unlikely to be the the switch assembly, I would suspect the issue is with the adjustment of the gear linkage, ie: the gear stick will not move to the left far enough for the switch assembly to align with the selector fork.
 

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1990 Unimog U1200, 1989 Unimog u1200, 1971 Unimog 421, 1978 Case MB4/94, 1963 Land rover Series IIa,
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its definitely in your linkage adjustment. I had the same thing going on, I had to think logically and the very slight adjustment makes just the difference.
 

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2015 Rubicon Unlimited (Let the shame be upon me!)
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But you responded now, so that's okay.
 
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1991 Unimog 1250L wdb4271111w173869
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Discussion Starter · #9 ·
If it's left hand drive, I posted the measurements and adjustments a while back for the linkage, I'd start there.
I'm guessing that this is the forum that you were referencing @tkv000

I found first by adjusting the linkage. I also noticed that the clocking of the U-joint made quite a difference. I still have an issue with grinding into gear or not being able to shift into a gear for gears 1,2,5, and 6. The same gears in the high and low range. To give a little more light on the situation. I have a hard time putting it into any of these gears at a stand still, and these gears like to "pop out" of gear if I'm not holding the shift lever in place. It's possible to shift up into 5th and 6th, but I can't get back into those gears unless I rev up or go back down to 4th and shift up again. My guess is bad syncros. I read this thread (1250 Transmissions breakdown), thanks @NhBen, but I'm still not sure. I understand that there are bad syncros, but I don't know why it wouldn't stay in gear while it is there. I looked around a little, and it could be my linkage still, bad fluid (doubt it but it's worth changing anyways), and maybe the syncros. I get the feeling I'm going to have to take the transmission apart.... Thanks for the help.
 

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1250v
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I would not suspect the linkage is the issue since it's both 5th and 6th now that I'm thinking about it, if the linkage was out if adjustment I think it would be inclined to stay in one gear and pop out of the other.

Another possibility would be that your shift fork is worn, if the fork is to thin it wouldn't engage the gear enough to keep it in place. Unfortunately you can take that out without taking apart the transmission.
 

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1991 Unimog 1250L wdb4271111w173869
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Discussion Starter · #11 ·
I am reviving this since I never finished it. So, I found all 8 gears awhile ago by adjusting the linkage. I still have issues with 1/2 and 5/6. I always have an issue getting into 1st and 5th, but it gets in. I need to slide it in slow and hold it there; otherwise it will grind and never go in, or pop out. 2nd and 6th rarely go in, and I have to hold those as well. I finally took someone's advice on here and took the plate off. From initial glances, it looks okay. I was expecting to find some pretty worn gears from the sounds and feels of it. If anyone has any thoughts, I'd be open to it before I take it apart. With all of the seal leaks I have though, it might be worth it. It looks like someone took it apart previously and did a poor job of putting it back together. I'll attach a few photos from taking the cover off. Also, someone mentioned that some of my leaks could be due to it being pressurized from having a deep water kit installed 🤷‍♂️?

Last, I have ben going over @NhBen 's thread about his rebuild religiously.
 

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U1000Ag
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May or may not be relevant, but whilst you are in there - check the rivets on the selector plates.
 
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1991 Unimog 1250L wdb4271111w173869
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Discussion Starter · #13 ·
I’ll check that out for sure. It sounds like it caused issues for certain gear sets like I’m experiencing. I wish I could more obviously see an issue though. That would make my life a lot easier! I took the bed off, so I may pop the transmission out. I have a USB workshop manual. It’s pretty vague. Are there any other resources that you guys have?
 

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Unimog U1250
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I've attached the ADF repair manual, it has a section on removing the transmission. The risk of chopping your fingers off is very real when you are unbolting the propshafts. I decided to remove the front and rear axles, as I needed the space as I didn't want to take the camper off. The U1250 chassis is a lot narrower than a U1300 or U1700, and you need to tilt the transmission over to get it out the bottom. Taking it out the top with the bed removed would be easier.



Automotive tire Motor vehicle Hood Automotive exterior Automotive wheel system
 

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1250v
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From your description it sounds like you need synchronizers for at least first and second gear. Unfortunately your pictures don't show the synchro, just the main gears.

For the section you are tackling I would look at leaving the transfer case and axles alone and just pull the 4 speed main transmission. That will cut your weight down a lot, and get you access to what you need. If you pull the entire thing together splitting off the 4 speed will be one of the first things you need to do anyhow.

You may have already mentioned it, but it's possible the hard shifting is oil related. With heavy oil in mine it was almost impossible to shift. The lighter oil made a noticeable difference in the shifting. I don't believe that will have any effect on popping out of gear though.
 

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1991 1250L Doka Unimog, 2002 ML320
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Iain those PDFs are the bomb. Much better/easier to understand and follow than my shop manuals. Don't suppose you have any specific to the 352a or are they mostly the same for repair? Sorry for the misdirection Taylor.....

Taylor I think NhBen has the easiest idea about leaving the main structure/transfer box in the truck and just pulling the 4-speed box off. Somewhere here it talks about doing it in a narrow chassis while supporting the T-case passenger side mount/support. Scott from Expedition Imports talked me through it.

Chas
 

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1991 Unimog 1250L wdb4271111w173869
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Discussion Starter · #18 ·
From your description it sounds like you need synchronizers for at least first and second gear. Unfortunately your pictures don't show the synchro, just the main gears.

For the section you are tackling I would look at leaving the transfer case and axles alone and just pull the 4 speed main transmission. That will cut your weight down a lot, and get you access to what you need. If you pull the entire thing together splitting off the 4 speed will be one of the first things you need to do anyhow.

You may have already mentioned it, but it's possible the hard shifting is oil related. With heavy oil in mine it was almost impossible to shift. The lighter oil made a noticeable difference in the shifting. I don't believe that will have any effect on popping out of gear though.
Thanks @Iain_Unimog. That should come in handy. I noticed it had some of the parts for the air system as well. When it gets cold here, it seems to freeze something in the system and not want to build pressure past 7 bar (mine is a 20 bar system). So this should be handy in this respect as well.

@NhBen I think I'm going to try and leave the planetary gear and transfer case in place if I can. If I put a borescope in there, do you think I could see the syncros. I may give changing the oil a shot. I think I've seen some people mention stuff about using 10W engine oil rather than the heavy dino oil for the tranny. I don't really want to spend 400 on the Syntrans or whatever just to try a different oil haha. Also, do you have to support the transfer case if I'm just taking out the 4 speed transmisison?

@Chas Stricker you're more than welcome to misdirect this any way you want for how much you've helped me out haha.

Last, I took the bed off. I'll be taking it out with a cherry picker.
 

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U1450L DOKA
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I had problems with slow air pressure building when it got cold here. I removed the cartridge on the Air Dryer, and packed it in a bag loaded with silica gel, in an effort to pull any moisture out of it.
I also sat it in ‘warming range’ of the wood stove for a few hours.

When I reinstalled it, the pressure building was normal (+/-).
I ordered a new canister and that fixed the problem.
 

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When looking for the synchros, I’d suggest looking at the bottom of the box, and sweeping it with a mildly attractive magnet. A rare earth magnet is too strong, it’ll stick to whatever it hits and that’ll be that. A bore scope should show you random bits that may have fallen to the bottom of the case, and you might have better luck with a grabber than a magnet.

The thing is, if the scope shows bits at the bottom, you’ll be pulling
Things apart anyway, and then you’ll be able to pick the pieces out with your fingers.

The synchros have a habit of having their shoes, or pucks, or whatever they are really called, falling out of their slots on the synchro rings and winding up at the bottom of the case.

If everything is in order, they are captive in their pockets on the synchro rings, and the overlap of the gears keeps them there.

However, they can escape under certain conditions.
I’m not sure why, but I have two thoughts -
Wear, to the point that the pieces are smaller than spec. This might be able to happen.

My better crackpot theory is that the shifting forks are moving the gears farther than they should be, and this eliminates the overlap, and the pucks and balls are able to escape.

On the SBU, the gear lever is pretty stout, so you’d really have to Ape it to bend it to the point where it moves the forks too far, but maybe there is something in the mechanical chain of command that does bend a bit, giving the forks the ability to slide the gears a little further than they are meant to go. It could be the forks themselves that bend out of shape.
If the pucks and balls are in place, it is hard to see them, as the gears all but cover them up.
I’ll pull some photos, so you can come up with your own theory.
 
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