Closed center system (CAT Industrial BackhoeTractor)

[CalG]

I helped a friend replace the rod and piston into the cylinder of his CAT tractor today. About a 6 inch cylinder, 4 foot rod, and over 125#. Fun work and it's all good after seal replacement .

But

My friend raised a question. When WARM, he has to open up a control valve (boom or bucket) to start the engine. There seems to be excessive load on the engine for it to turn over rapidly and start.

His description was:

Doesn't do it when cold
Didn't used to be that way.

I'm thinking some control element that is associated with pump output (I assume closed center on a CAT)

I know nearly nothing of closed center systems.

Looking for a place to start.
Sorry, I didn't catch the tractor model. My bad.

Someone suggested to him a "screen" somewhere in the system, but that makes no sense to me.

An aside,........Boy! A five gallon pail of hydraulic oil disappears fast into those larger pieces of equipment!

 

[mrmikey]

Possibly the low pressure standby may be too high or the compensator or swashplate may be sticking to full flow when warm, the pilot system may be calling for full flow? You check the pilot lines for any questionable collapsed or seedy looking lines that may be collapsed inside not allowing correct flow? They're probably small lines, #4 or thereabout.
I'm not sure of the pressures on a CAT but the systems I've worked on, Rexroth, the low pressure standby was 400 psi. The compensator is what controls the swashplate in the pump to control flow. If it's stuck to full flow position and there's nothing requiring oil it's going to be a bear to turn over at a good speed............Mike

 

[CalG]

Possibly the low pressure standby may be too high or the compensator or swashplate may be sticking to full flow when warm, the pilot system may be calling for full flow? You check the pilot lines for any questionable collapsed or seedy looking lines that may be collapsed inside not allowing correct flow? They're probably small lines, #4 or thereabout.
I'm not sure of the pressures on a CAT but the systems I've worked on, Rexroth, the low pressure standby was 400 psi. The compensator is what controls the swashplate in the pump to control flow. If it's stuck to full flow position and there's nothing requiring oil it's going to be a bear to turn over at a good speed............Mike

thanks

A pilot line check is certainly in order, the machine is of the age were many hoses are being replaced.

Thank you for your comments!

He has the full service and repair book. so using the correct terms should make identification simple.

 

[true temper]

I don’t know about Cat, but the old John Deere’s closed center systems had a destroking solenoid that actually turned off the pump when the starter motor was engaged.

 

[mrmikey]

I don’t know about Cat, but the old John Deere’s closed center systems had a destroking solenoid that actually turned off the pump when the starter motor was engaged.

A good idea.
There's negative and positive load sense as well, (think that's the correct name). One type uses a spring on the swash plate to stroke the pump to max flow when the pump is sitting there with the engine not running, the load sense pressure de-strokes the pump as required and the spring strokes it as flow is required. The negative pressure works the opposite way, I never had much to do with that type.......M