JCB 1550 weak backhoe : relief valve?

Hi guys,
I've been a lurker for a while and you've helped me on a bunch of projects, but I've come across a problem I can't solve with the search function. On the bright side, it got me to finally sign up!

So here's what I've got, I'm hoping it's actually a common issue to you hydraulic gurus...

1985 JCB 1550 3C serial # 308711
2wd with Wain Roy twist-a-wrist clamshell hoe

Backhoe always seemed weaker then it should be, but I don't have a lot of experience and I could get the job done eventually. Loader always seemed fine.

I finally got a gauge hooked up at the loader valve test port. It has a 2 stage main relief valve to separate loader (2600psi) and backhoe (3000psi)

I can't get the backhoe over 2000psi even after adjusting the relief all the way to the stop. It will adjust pressure down to zero if I go the other way.

The loader adjusts to 2600 easily and can get over 3000 if I wanted, so I don't think it's the pump.

I pulled the relief valve apart and cleaned it. No obvious worn out bits. Changed the o-rings as much as possible (didn't have one tiny one).
No change in numbers.

The high pressure sensing line seems to have flow as the port leaks when the line is removed. I don't have specs or fittings to test that currently.

So, sorry for the book. I hope there's something obvious I'm overlooking. I really appreciate any help

Just thought to add some more info. I've changed filters and fluid, and cleaned suction
screen. Also fixed a couple leaking cylinders. I recleaned/polished up the relief valve.

I'm trying to figure out how the backhoe can have a lower PSI than the loader. They share a 2 stage relief. They share the same poppet(?) and spring, the difference is the loader uses neutral circuit pressure and the backhoe uses a higher pressure that's injected when the hoe is engaged.

So why can't I even get the pressures to match? Much less get the hoe higher than the loader?

Is my relief valve shot?
Bypassing somewhere in the hoe?
Bad pump?
Another relief valve somewhere?

Thanks again

You have to set the mrv in 2 stages have you got a smalll steel pipe going upto a steel block on top of the mrv.
Normally the o rings are shot you need the correct ones ans also on assembly they are easy to damage as the valve block has a sharp edge inside

jcb jeff said:

You have to set the mrv in 2 stages have you got a smalll steel pipe going upto a steel block on top of the mrv.
Normally the o rings are shot you need the correct ones ans also on assembly they are easy to damage as the valve block has a sharp edge inside

Click to expand...

Thanks for the reply. Yes I have the small step pipe to steel block.

The o rings that the block slips over looked ok, maybe a little stiff, so I replaced them with some out of my metric kit. There was no change in the pressures before or after the change.

I am setting the pressures as I understand the book... But I'm sure I could still have it wrong...

Rpms ~ 3/4 throttle (full throttle and idle make miniscule differences)

Back off the loader pressure screw, then tighten the threaded body for the backhoe pressure. It's supposed to hit 3k psi, but I can't get over 2k.

Then tighten the loader pressure screw to 2600. This one dials in easily every time.

I'm getting pressure by retracting a cylinder to full stop.

I guess I'll pull the block off again and see if I damaged the o rings when installing.


In reading the book I see there are ARVs for each backhoe function, but it seems hard to believe that all of those failed to 2k psi evenly at the same time. Is there anything else common to the whole back end other than the MRV?

Thanks again

Hi
Have you pulled all the mrv to bits out of the front loader valve block .The jack leg or stabaliser is the ram to use to set pressure on back end as no arv in use and front loader arm down is to set front loader but can be a fiddle to get it to set up from scratch you need to juggle both parts until you can raise pressure sometimes.
Jeff

jcb jeff said:

Hi
Have you pulled all the mrv to bits out of the front loader valve block .The jack leg or stabaliser is the ram to use to set pressure on back end as no arv in use and front loader arm down is to set front loader but can be a fiddle to get it to set up from scratch you need to juggle both parts until you can raise pressure sometimes.
Jeff

Click to expand...

Sorry to disappear for a while there. I ended up wrecking a spring in the MRV with all my messing around and had to order a new one from the UK (through the local JCB dealer).

Got the spring today and reassembled the MRV. O-rings all appeared good (I had replaced them previously). I did nick a metal corner on one bit outside of the o-ring seal area doing something

No real change with any pressures. A couple of times it reached 2100 psi then quickly popped down to 2000 psi and held there. I used the stabilizer ram at first, then checked all other functions again. Everything on the back end still stops at 2000 psi. The front loader still adjusts easily to 2600.

I still can't figure out how the backhoe is a lower pressure than the loader. They use the same spring. Could something be bypassing in the backhoe?

Hi Again
It is a fiddle to get the pressure balanced especially as you have had it all to bits ,just to clarify
you have pulled the complete unit apart not just the top part all the guts from the under side as well.

jcb jeff said:

Hi Again
It is a fiddle to get the pressure balanced especially as you have had it all to bits ,just to clarify
you have pulled the complete unit apart not just the top part all the guts from the under side as well.

Click to expand...

Yeah sorry I had the underside apart once, cleaned it. Checked the tiny hole in the poppet, then put it back together. I've had the top side apart a few times since. Is there something in the bottom i need to revisit? I'll try playing with the adjustments a bit more.

Jlap530 said:

Yeah sorry I had the underside apart once, cleaned it. Checked the tiny hole in the poppet, then put it back together. I've had the top side apart a few times since. Is there something in the bottom i need to revisit? I'll try playing with the adjustments a bit more.

Click to expand...

Hi
When you push the spool up from the underside its easy to tear the o ring , may need to revisit it just to check.

PROBLEM SOLVED!

Well, I pulled the bottom of the relief valve off, everything checked out... Took a nice bath in fluid... Any tricks to not taking a shower Everytime I pull something apart?

While I was under there I looked over the rest of the system. My machine has a Wain Roy rotajaw bucket. Turns out it has a separate valve and relief for the extra functions... And all the backhoe fluid runs through that valve after the loader valve block and before the backhoe valve block.

1 turn of the 2nd relief, then I could adjust my pressures on the main valve properly. 3000psi and the machine has come to life!

Do I need to be worried about anything since I blindly adjusted a downstream ARV, but set my MRV properly? I definitely don't want to blow anything up at this point

Thanks for all the help this far!

Hi
Your main releif will protect you machine if set correctly the additional valves will never get over the mrv if that is what you saying it is.
A ARV protects each circuit for shock loading such as external forces acting on the m/c equipment.