Volvo EC 18 C Hydraulics.

[STEVE69]

Hello all.
Have a mini digger here with a rexroth servo controlled spool valve, if only one spool is
activated and the ram dead ended, only half relief valve cracking pressure is reached.
If two or more are activated, boom in bucket curled at the same time, maximum system
pressure is reached. Same with tracks and blade.
Servo pressure is up to spec, right to the spool, teed in pressure gauge to test full spool
travel. Only one pump, swash plate type. Stripped spool block all free and clean.
All rams hold up no internal leakage.
The only thing that changes the pressure when operating one spool is to slacken
the cap on the flow control valve, this improves it slightly.
Would be very great full for help on this one. Steve

 

[nikthewho]

Your symptoms point to a pump flow/pressure compensator issue, not the spools themselves. Here's what's happening:

With one function: The pump's load-sensing system sees low demand and limits displacement/pressure. The flow control valve you loosened likely affects the LS (load sense) signal. So, when you cracked it open, you have altered that feedback.

With multiple functions: Combined demand forces the pump to full stroke, building proper pressure.

Check these: Load-sense line: From the valve block back to the pump—any restriction, leak, or air in this line causes the pump to "think" there's no demand. Pump compensator/LS piston: Could be sticking or worn, not responding correctly to single-spool demand. Priority/unloading valve: May be dumping flow prematurely when load is light

Test: Block the LS port at the pump temporarily and run one function—if pressure comes up, the LS circuit is faulty.

 

[STEVE69]

Hello Nickthewho,
Many thanks for your swift reply and information.
On this system there is no LS feed to pump, only three hoses
attached to pump, inlet from tank, outlet to spool block, and case drain
back to tank. LS pressure follows pump pressure as spools are operated.
Is LS load sense or load sharing .
Thanks again for your help. Steve.

 

[nikthewho]

LS is load-sensing in this context, but you're right - your setup is different than I assumed. If there's no LS line to the pump, then your pump is likely pressure-compensated only (maintains set pressure, not flow-on-demand).
Your issue then points to:

1. Pump compensator spring/setting: Not stroking fully under single-function load. When you dead-end one spool, the pump may be hitting its compensator before reaching relief pressure. Multiple functions create more resistance, forcing full stroke.
2. Internal pump wear: Worn swash plate servo or control piston - can't maintain displacement under light load, but works better under heavier combined demand.
3. Pressure-limiting valve in spool block: May be partially open or set incorrectly for single functions, dumping flow before relief cracks.

That flow control valve cap: Likely adjusts cross-bleed or anti-cavitation flow - loosening it increases resistance slightly, helping pressure build.
Test pump outlet pressure directly (before the spool block) with one function stalled. If it's low there, pump compensator is the problem.

 

[STEVE69]

Hello Nickthewho.
Have at last had time to sort the troubled system, removed and stripped the pump.
This one is new to me, swash plate held at an angle with the assistance of quit a
strong spring. One of the swash plate pivot rollers had broken loose, was held in place
by a 5mm roll pin. Renewed roll pin from out of a blind hole.
Three , 3mm pins worn down
they sit in the revolving section drive splines, renewed them.
Pleased to say all good for now, Thank you for pointing me in the right direction and
a very welcome win on this one. Steve

 

[nikthewho]

Happy that i could help!