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Unimog 404
1 Posts
Discussion Starter #1
hello all, a newbie here writing my first post,
I'm having trouble with my 404 transmission. the low range gears "1 and 2" don't seem to be engaging, in both forward and reverse. the high range gears "3-6" appear to be functioning as normal and the shift seems to go in as normal in all gears, but although it feels like the selector has chosen 1 or 2 theres no sign of movement, and a very mild grinding occurs when lifting foot off the clutch.
have looked through the side plate and the synchros are free and no apparent damage to the gears. any ideas on what the problem could be?
if anyone knows of any threads on the low range or has had this problem before any info would be greatly appreciated. cheers

1963 404.1 TLF 8/8 Firetruck
101 Posts
Wondering if one of your shift forks is bent. I have heard that can be a problem sometimes. From what I have read, they have some weak spots that are prone to cracking or bending under the right conditions. I don’t think it takes much drift to cause the gears to shift improperly. The only other thing I can think of is an issue with the mechanical “logic” in the top cover that provides the foreward/reverse lockout feature for 1st and 2nd. Can you clearly see both forks do their thing with the side plate off?

495 Posts
Well.. from your description, it sounds like the problem is limited to either the high/low syncros or the forward/reverse sliding gear (subject to revision when we get more details). So here goes a blind diagnostics...

The 404 is just a 4 speed transmission at its heart, with a leading input reduction step and optional secondary reduction step (crawler box). 1st/2nd are the exact same gear set as 3rd/4th, with the addition of a leading high/low reduction step from the input shaft. Reverse 1st/2nd is the same leading reduction step, with power feed from the reduction gear back, thru a reverse idler, and back into the main gear set.

In forward low range, 1st and 2nd gear, power from the input shaft (middle left) flows:
- into the reduction shaft (bottom left)
- thru the low range syncro
- thru the reduction gear in the forward position
- into the main shaft (via what would normally be 5th gear)
- thru either the 1st or 2nd gear set
- into the secondary shaft (opposite side of the transmission, behind everything)
- thru the secondary/output shaft gear set (not shown)
- and out the back of the transmission

In high range (forward only), 3rd thru 6th gear, power from the input shaft flows:
- thru the high range syncro
- into the main shaft
- - thru either the 3rd, 4th, or 5th gear set
- - into the secondary shaft
- - thru the secondary/output shaft gear set
- - thru the 6th gear syncro (not shown, just a coupling between the main and output shafts, not an actual gear set)
- and out the back of the transmission

The reduction shaft and forward reduction gear just spin in high range and do not transmit any power (unless driving a PTO).

In low range reverse, 1st and 2nd gear, the power flow is nearly identical to that of low range forward, with the exception that the reduction gear is now moved from the forward position (feeding thru 5th gear), to the reverse position (feeding power into the secondary shaft and back to the main shaft thru 5th gear).

From your description, there are a few possibilities:
1) The high range syncro is not fully disengaging (either because the syncro is stuck or the fork is bent), preventing the low range syncro from fully engaging.
2) The low range syncro/collar is fragged.
3) The forward/reverse fork is bent/broken (or its shifter linkage), causing the forward/reverse reduction gear to sit between the forward and reverse positions.
4) The forward/reverse reduction gear is stripped.

From your description I'd guess #3 followed by #2. The forward/reverse fork is in the bottom of the transmission, with a linkage that runs up the side wall (not really viewable from the side access panel).

Some more diagrams of the transmission internals and power flows:

885 Posts
Make SURE you are shifting thru the DOUBLE D-TENT. I cannot count the number of times people will go thru the first detent and force 1st,2nd or 3rd,4th from the middle of D-tent #1. A sloppy tranny will allow the shifter to feel like it is going in, but create problems like you describe.

On the shift from 3rd to 2nd......

Up to Neutral - 1 hit with palm of hand left towards you (detent #1), 2nd Hit with palm of the hand left towards you (detent #2) then shift 1st or 2nd.

In tech support with customers this is a recurring theme for new operators unaware of the 2 detentes between 1/2 and 3/4.

Assuming the shifting is correct, then it would be opening up the side plate to visually inspect what is going on as you are trying to shift.


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