Help! Unimog 406 hydraulic pressure adjust

[forestMog]

Has anyone adjusted pressure on the front (belt drive) hydraulic pump? I put a gauge on one of the quick connects and measured about 1900psi where it should be over 2500psi. After a couple seconds it drops to about 1000psi.

The v-belt has the right tension.

The SEE tractor has a Unimog 406 front hydraulic system. Thanks to RockyMountainMoggers I was able to find a hydraulic system test procedure in TM 5 2420 224 34 pages 2-15 to 2-22 to try to isolate the problem. It details a test for flow to measure pump efficiency at least 75%. However, I don't have a flowmeter so I am wondering if I can substitute a tank for the reservoir and weigh it between steps to get a flow accurate enough to use in the test. The SEE hydraulic control valve appears to have an adjustment I don't find on the 406. The picture shows where the relief valve is located (see the lead seal on safety wire) but I don't see an adjustment.

From my search on the internet this does not seem to be a common problem on Unimogs. Hopefully there is someone here with deep knowledge of Unimog/SEE repair history.

Thanks to all
Paul

 

[Wolf.Dose]

Too choises: take out the pump, disassembel, check for wear and tolerances, try to get the neccessary parts and reassembel the parts. SECOND IS JUST BY A NEW PUMP: Caution, Unimog prices! are truck prices (not Pick up truck prices!).
Wolf

 

[tennmogger]

Having pressure at first then losing it sounds like the intake filter (inside the reservoir) might be plugged. This starves the pump for fluid.

Have you changed your hydraulic filters? Take out the bolts on the aluminum contraption on top of the reservoir and lift it out.

Bob

 

[forestMog]

Too choises: take out the pump, disassembel, check for wear and tolerances, try to get the neccessary parts and reassembel the parts. SECOND IS JUST BY A NEW PUMP: Caution, Unimog prices! are truck prices (not Pick up truck prices!).
Wolf

Thanks Wolf. Unfortunately I have not been able to find information on dissassembly OR tolerances in the service manual. My internet search has not found any service information on this pump. I think I can get a used pump for about $500 where a new pump is about $2500.

Not having access to a flow meter I simply tried to measure the pressure by putting a 3000lb guage on one of the quick couplings and got less than 2000lbs. This does not isolate the relief valve as the procedure in TM 5 2420 224 34 does. That procedure calls for removing the inlet to the control valve and substituting a 'flow meter' as shown in the schematic on the top of the graphic.

One thing I have not tried is to put a high pressure valve on the output of the pump as a load valve and run it to a 5 gallon bucket to check flow and pressure as shown in the lower part of the graphic. That would isolate the problem to the pump or the relief valve since the relief valve would no longer be in the circuit.

This will require enough hose to reach inside the cab, a stopwatch and measuring stick to determine gallons per minute. Reservoir only holds 7 gallons and flow should be up to 11 gallons per minute so will have to make it quick.

The filter looked good but I replaced it anyway.

Comments? Better ideas?

Paul

 

[spicergear]

If you're not circulation pressure, ie; taking a pressure reading just off of one hydraulic drop, that may be the reason for the large pressure decrease. Again, check the intake filter. Also, I didn't see you mention it anywhere but the pumps are rated 2,500psi at a specific rated rpm. I *think* it's 2,800rpm. I looked through my manuals and found that info when I was setting up my hydraulic drive front winch.

Oh, and you should definately be able to measure flow by the weight or volume of what the pump moves. Figure out what the weight is per gallon and use a good scale. We do stuff like that at work...usually ounces though.

 

[NC_Pinz]

I second the checking of filter before going any further.

 

[forestMog]

The filter looked good but I replaced it anyway

I repeat, filter was good. Changing it had no effect.

Has anyone replaced the pressure relief valve?

 

[Wolf.Dose]

These pumps in Germany are usually exchange parts, so at a Unimog shop they change a mullfunctioning pump to a new one / rebuilt one. So the price is lower than mentioned by you. However, in the States there might be no factory rebuilt part available, so they charge the new price.
By the way, so just beside all other, is it the power steering supply pump? The power steering pump for the LS6 steering should be at 180 bar max. (or some 2500 psi), so from that point of view, it should be the power steering pump.
One more thing came to my mind: Probably the pump did somewhen run dry, so excessive wear, means finally no correct pressure (if the pump ever was able to bring the correct output).
To my accumulated experianse I have to add, that hydraulic pumps alsmost never fullfill the theoretical requirements. However, Unimog Mercedes is an excemption in the case of the steering pump. Usualy their parts fulfill the specs. It is an other world, the Unimog / Mercedes World.
So, as a firend in Japan said to this case, you have to choises: Buy or die. (Or try to live with the lower pressure and harder steering.).
Wolf

 

[spicergear]

Wolf.Dose, Forest Mog is looking for the belt driven pump under the engine for the hydraulic system. I think his steering is okay. I'm saying this because his first picture shows the WABCO or Westinghouse hydraulic valve above the steering column beside the air filter.

 

[forestMog]

It is not the power steering pump. It is the belt driven hydraulic pump at the bottom center of the belt driven accessories. I have been too busy with other things to break the output connection and test the pump without the relief valve. I suspect wear in the pump. The engine has about 7000 hrs but runs strong.