Mercedes-Benz Forum banner

1 - 12 of 12 Posts

·
Registered
1991 1300L Doka
Joined
·
300 Posts
Discussion Starter #1
Ok ready for a non-technical description?

From my basic understanding my SBU has a dump valve (lets call it dumpy 1)thats located just in front of the air tanks) that lets go at around 18 bar. It then chatters away trying to maintain the same pressure. This I think is normal.

What isnt normal is the other dump thingy (call it dumpy 2) located right beside dumpy 1. Dumpy 2 is plumbed into the lower air tank. For some reason it has decided to leak at the bottom where there is a set screw with a nut. I took dumpy 2 apart and did a bit of cleaning but now I am not sure how to set the set screw. If it is screwed in too much it dumps the air before dumpy 1 reaches 18 bar.

Can you tell I have children? :dunno:
 

·
Unimog Moderator
250GD Wolf
Joined
·
13,266 Posts
Do you have a picture? My manual lists a troubleshooting procedure, some exploded diagrams ect, but my scanner is down, I can take a picture with my camera but I'd need to know which exact piece.

I think probably your speaking of the "Pressure regulating Valve" ? Or "Pressure Reducing Valve"?

EDIT: Added picture

Pick a number...any number...
 

Attachments

·
Registered
'91 U1250 '02 U500NA
Joined
·
3,439 Posts
Yes you have.

Now, not long ago I serviced both of those valves (this spring and summer).
First was #2 that was leaking.
Ended up getting repair kit from Eurotruck Importers.
Not long after dumpy #1 started to leak.
Same procedure - repair kit fixed the problem.
As it comes to screwing around, I didn't touch the screw,
they were set right, compensating the pressure built up and
the strength of the springs in the valves.
Cleaning and replacing used O-rings, pretty much did the job.
Build up gum inside the valve usually creates extra resistance for the piston
sliding inside, messing up with the minimum and maximum pressure allowed in the valve has been set for.
 

·
Registered
Sold .....1987 U1250 Unimog
Joined
·
1,005 Posts
Lost1

Do you have a U1300 ?

If I understand correctly the first thing is your unloader valve, basically the air compressor always pumps air, when the tanks are below say 18Bar it send the air to the tank, once it reaches 18 bar it pops and expels the air to the outside. And yes it sounds like it is working correctly.

Thing 2 could be a few items but it sounds like your PSI regulator for the truck side air. Basically the truck needs 7 bar to work, brakes, 4x4, lockers, shifter ETC, but the tanks on your truck are small .. solution is to store 18 bar worth of AIR pressure in the tanks and use 8 bar to make it all work... So on supply side you have 18bar on the truck side 8bar. thing 2 limits its output to 8 bar

There are 2 kinds of air systems on Unimogs one is like yours and then a completely 8 bar setup. Basically everything is 8 bar but the tanks are much larger same volume as 2 smaller tanks at Higher PSI"

IF thing 2 has about 6 lines going to it it’s your 4 way protection valve .. in a nut shell it separates the 2 tanks in case of a major air leak but when things are OK it allows you to obtain air from both.
 

·
Registered
1991 1300L Doka
Joined
·
300 Posts
Discussion Starter #5
Uh oh. Looking at the diagrams I think it's 29.4. This means I could risk overpressurizing the 8 bar system by fiddling with the setscrew
 

·
Premium Member
Unimog U1250
Joined
·
1,677 Posts
I have heard from on fellow Unimogger that when that device broke out in the field (similar problem to what you are describing IIRC), he just bypassed it - everything seems to work he said, and the shift cylinder was lightning fast, brakes were better etc. Probably not good in the long term but for a "get you home fix" his seems to work. I am not sure if he has replaced the pressure regulator yet.
 

·
Registered
Sold .....1987 U1250 Unimog
Joined
·
1,005 Posts
Uh oh. Looking at the diagrams I think it's 29.4. This means I could risk overpressurizing the 8 bar system by fiddling with the setscrew

I don't know for sure but I would assume the screw is a fine adjustment Maybe 1 bar worth of ability with its desired output somewhere in the center of its ability to change. over say a course adjustment of multiple Bars
 

·
Registered
G500, Unimog 2450, Pinzgauer 712M
Joined
·
615 Posts
Here is a photo of one disassembled. I got this one working just by cleaning it and relubing. You might want to get a repair kit as well, as there's a few of these on the SBU and they tend to leak. Hard to find these leaks, because if you try spraying with soapy water, the leak on these valves is too far up inside to allow any visible bubbles to form. You have to wait until it get so bad you can hear it.
 

Attachments

·
Registered
UNIMOG, Gelaendewagen
Joined
·
922 Posts
Lost1

Do you have a U1300 ?

If I understand correctly the first thing is your unloader valve, basically the air compressor always pumps air, when the tanks are below say 18Bar it send the air to the tank, once it reaches 18 bar it pops and expels the air to the outside. And yes it sounds like it is working correctly.

Thing 2 could be a few items but it sounds like your PSI regulator for the truck side air. Basically the truck needs 7 bar to work, brakes, 4x4, lockers, shifter ETC, but the tanks on your truck are small .. solution is to store 18 bar worth of AIR pressure in the tanks and use 8 bar to make it all work... So on supply side you have 18bar on the truck side 8bar. thing 2 limits its output to 8 bar

There are 2 kinds of air systems on Unimogs one is like yours and then a completely 8 bar setup. Basically everything is 8 bar but the tanks are much larger same volume as 2 smaller tanks at Higher PSI"

IF thing 2 has about 6 lines going to it it’s your 4 way protection valve .. in a nut shell it separates the 2 tanks in case of a major air leak but when things are OK it allows you to obtain air from both.
Hey thanks David, I learn something new about my dear Mog every day, I swear!
 

·
Premium Member
U1000Ag
Joined
·
2,373 Posts
U1000 unloader valve

OK, call it what you may - foofy valve, or whatever. But I thought for completeness I would record the unloader valve on the U1000 here as well.

Original is Bosch 0 481 039 101 and no longer available from Bosch. It unloads at 8.1bar. The direct replacement is from Wabco - 975 303 474 0. Same mounting arrangement and coupling dimensions. No number plate on the new Wabco - seems they are cutting costs as well. Looks like it was made in a 3rd world country - very rough casting and poor finishing - not that it necessarily detracts from its functioning - but still. Wabco name is cast into the housing, so one assumes it is in fact Wabco and it came in a Wabco box - but no country of origin info.

Original seems to have worked well - gunk in the inlet port, but none in the outlet. So bad stuff seems to be successfully expelled through the bottom valve when it pops.
 

Attachments

1 - 12 of 12 Posts
Top