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OK, I now have the rig (1979 406, 20 speed) at my place and have had more time to fiddle with it. Symptoms are:
6 speed shifts fine in all gears

Reverser feels a little weak, very short throw, doesn't really seem to go all the way into F or R. "pops" out of F&R under virtually no pressure. Will grind if you try to hold it in F or R. 6 speed stays in 1&2 fine.

3 thru 6 work fine.

So I think I am at a point where at least the top of the tranny has to come off, hopefully just a shifter or shifting fork issue...hope.

It looks like the cab tilts if I remove the 2 rear cab mount bolts and possibly pull the steering coulmn. Can anybody shed any light on either the tranny issue, tilting the cab or both? Thanks, Doug
 

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BenzWorld UNIMOG statesman
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Lots of discussion in here on cab tilting. You need the two special front brackets (which you may have) to tilt the cab.

I suggest you do a search of this forums using various ways to stay "cab tilting" the wild card of * can help with searches too.

Also check the "Ultimate Sticky" and to get to the FAQ/DIY. There may be some stuff in there on cab tilting. I can't recall off the top of my head if that is so or not.
 

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Upon further fiddling, it seems that I may have 2 problems. One with the reverser, mainly in reverse and another with 1st and 2nd. It is like the shifter does not go over to the driver enough to fully engage the 1st and 2nd gears.
 

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The shift plate on a 406 transmission can be complicated. I suggest you search the archives to understand what you may find when you get in there. It takes a lot more force then one would think to accomplish the transition through the gate between 1/2 and the rest of the pattern. That *might* be the problem with your 1/2 gears.

I don't recall if the forward/reverse lever is part of the shift plate logic or not, but I do know that it's a pretty short throw and the difference between engaged and not engaged is minimal.
 

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The F-R lever actually does two functions, it selects forward or reverse (duh) , and it locks out the 3-6 gearing from being accessed when in reverse. From what you have mentioned it may be a problem with the lock out side of the linkage which is either bent, worn or out of adjustment ( if there is any) that is keeping you from fully engaging either the F or R gearing. I am not sure how this is accomplished as I have not hade one apart but is probably a pin or detent of some type that has to slide into a matching hole to allow or prohibit a shift.
Dave
PS. The above is missing part of the linkage logic (?). There is also a lock out of the reverse function controld by the position of the main shift lever. This is the reason for the left or drivers side neutral gate. Once you are fully into this position you can shift to reverse. Once in reverse the F-R lever locks you from leaving this gate and going to the 3-6 side on the gearing until you shift out of R.
 

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I have some pics of the shift logic of the 6 sped plate, and the 8 speed. We have been working on converting the 6 to 8, which in my opinion, gives you a more user friendly group of different gears. I'll take a look at one of the 6 speed plates to try and identify what on the plate could be causing your problem. Excluding a problem with the shifter mechanism, I'd guess at a shift fork of synchronizer. Von on this board has experience with these transmissions, PM him. I have a spare 6 speed, as I will be stepping up to a 20 speed in the future, if yours is not econmically fixable. I think there was someone with a similar dilema on the http://mogmsgexch.com a year or so back, try searching the archives there, it may lend a clue. Good luck,
 

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First, make sure your shift lever bolts on the transmission cover are tight. I had a problem with it popping out of gear and it was just loose bolts where the shift lever enters the transmission cover.

I recently raised my cab for the first time. It's an engineering marvel how everything fits. First, make sure no one has installed wiring, hoses, etc. that go from the cab to the frame. Everything has to pass from cab to frame right at the front. You'll need to the tilting brackets that mount at the front of the cab and they raise the cab about four inches. There are several things that have to be disconnected (shop manual has the list). Make sure the emergency brake housing isn't stuck - it splits and the lower part has to pivot down as the cab is raised. Make sure there are no brackets or accessories mounted on the front of the truck - the cab will impact them (don't ask how I know this). Finally, you'll need the support rod and safety cable for when the cab is up. The rod bolts between the cab and frame mount points to hold it safely in the up position (safely is the key word here). The cable is to prevent the cab from going over frontwards (which would be disastrous). I almost lost mine when I put it down and unhooked the bottom of the support rod - the cab started tilting forward on its own but I was able to grab the rod (top was fortunately still connected) and stop it.

Good luck.

Gary

1984 406
 
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