Cab Hydro Controls only work when engine is full throttle. Any less RPM=Nothing will operate / No track movement or bucket movement

[aongheas.macask]

Update

Local Hydraulics tech had the following input. If the pressures meet specs at full throttle there probably isn't anything wrong with the pump(s). He explained how there was a hydraulic system on top of a hydraulic system in this Lieberr. He explained in more detail. We are trying to find the valve, or vane that adjusts pressure depending on throttle position. Does this sound familiar to anyone?

The hyd pressure is set by the main relief valves not by engine speed, the hydraulic pump output flow will reduce as revs drops but if the pump is good pressure should be maintained until near stalling point.
A worn pump will usually not maintain pressure as the rpm drops

 

[NORTHCOASTLLC]

Understood.

The diagnostic detail I noticed is how the pressure "behaves"

Full throttle = Full pressure on test gauges

Slightly drop rpm's by 20% = All gauges go from full pressure to 0 pressure immediately.

All lower throttle positions = 0 pressure / nothing works


Would a worn pump build pressure as the throttle goes down proportionally?

Seems like a red flag that the pressures go from great to zero immediately below 80% throttle.

I have attached a few photos of the main assembly with the tank cover off.

After removing the cover we ran a magnet through the bottom of the case looking for any evidence of broken parts. Nothing. Completely clean

 

[aongheas.macask]

How are you measuring pressure, you need to end stop a cylinder and hold it end stopped to see what pressure the main relief valve opens at, as you slowly reduce the engine revs the pressure should be pretty constant ,if the pressure drops at the same time as the revs this is usually an indication that the pump is worn.
Have you checked the main relief valves ?

 

[NORTHCOASTLLC]

PRV inspection a few weeks ago. Everything looks good. No obvious damage.

 

[NORTHCOASTLLC]

I agree. Pressure should drop proportionately with engine rpm.

It doesn't.

It drops suddenly from good pressure to no pressure

 

[NORTHCOASTLLC]

I agree. Pressure should drop proportionately with engine rpm.

It doesn't.

It drops suddenly from good pressure to no pressure

I meant to say the pressure should not drop proportionately with engine rpm. typo

 

[NORTHCOASTLLC]

Update.

I decided to dive in.

After taking the pump apart I found a few issues.
1. Bad O-rings
2. One of the pump cylinders turns too easy.....feels loose / wore out?

See photos

 

[aongheas.macask]

Update.

I decided to dive in.

After taking the pump apart I found a few issues.
1. Bad O-rings
2. One of the pump cylinders turns too easy.....feels loose / wore out?

See photos

The pistons look to be scored,, maybe it's just the photograph,what are the barrels like inside,can you see or feel any scores of marks?

 

[NORTHCOASTLLC]

Everything feels smooth. No scores.
Got new O-rings
One pump hub was a bit loose. Tightened to specs.

I'm getting ready to put it all back together.

 

[aongheas.macask]

Everything feels smooth. No scores.
Got new O-rings
One pump hub was a bit loose. Tightened to specs.

I'm getting ready to put it all back together.

On the bottom of the piston barrels there should be a brass running surface, ensure there are no scores on them as this runs against the swash plate and any marks or scores will cause a drop in pressure

 

[NORTHCOASTLLC]

I checked the brass surfaces. Nearly perfect condition. Almost no signs of wear.
However one of the pump barrels will spin by hand much easier than the other. Can I assume the loose spinning barrel is a sign of wear? It appears to be adjustable.

 

[aongheas.macask]

I checked the brass surfaces. Nearly perfect condition. Almost no signs of wear.
However one of the pump barrels will spin by hand much easier than the other. Can I assume the loose spinning barrel is a sign of wear? It appears to be adjustable.

Sorry I don't know about that but best of luck anyway, keep us posted after you rebuild the pump.

 

[NORTHCOASTLLC]

Both pumps are within specs. Pressure is within specs.

Could really use a flow chart. That would clear up a lot of questions.
Can we buy one from someone? anyone?

JH

PS. What are the two air valves for in the cab? Something important? See photos

 

[aongheas.macask]

Both pumps are within specs. Pressure is within specs.

Could really use a flow chart. That would clear up a lot of questions.
Can we buy one from someone? anyone?

JH

PS. What are the two air valves for in the cab? Something important? See photos

I think the air valves are for swing and travel brakes

 

[NORTHCOASTLLC]

We are looking everywhere for an operators manual. Anyone have one for sale?

1973 Liebherr 945