John Deere 401C Hydro trouble

[Tx Jim]

Priority valve on JD ag tractors has less hyd lines attached to it than JD industrial tractors. Look closely at the photos Zebra5 posted in his reply @ # 56 in this thread

 

[85bj60]

Priority valve on JD ag tractors has less hyd lines attached to it than JD industrial tractors. Look closely at the photos Zebra5 posted in his reply @ # 56 in this thread

I've seen that, the 401D PCV has no big hole but there's a O-ring for it(to make a seal). Where does the extra psi escape to?

 

[Tx Jim]

Due to my very limited JD ind tractor hyd experience & no access to a JD ind tractor tech manual my guess that oil flow you refer to travels via the extra hyd line that ag tractor doesn't possess.

Technically there should be no extra psi because JD closed center hyd system psi is determined by stroke control valve in frt hyd pump not by pressure control(priority) valve.

 

[85bj60]

I've ventured into a unknown hydraulic world when I took possession of this JD 401C, it's a real eye opener :shocked:
I'm slowly learning the ropes because I'm slow learner

 

[Tx Jim]

JD closed center hyd systems are unique. I was employed by a smaller JD dealership & helped diagnose/supervise repair hyd problems for a large majority of the 21 yrs I was there. I've learned even more since I left dealership. Just remember "good diagnostics is a must" before any parts are replaced.

It would have been wonderful if the tractor discussion forums had been available back in the 60's-late 80's !!!!!

 

[Zebrafive]

I pinpoint the culprit of my FEL and steering woes on the Pressure Control Valve. Local dealer is unable to get one or kit and says they're discountinued. Any other source to find one. What's the correct part #? I could fabricate another style into the system by blocking off the transmission two holes with a plate and maybe using one hole for return of over pressure.https://www.tractorbynet.com/forums/asset.php?fid=584263&uid=344904&d=1603539926

IIRC, the two holes in the transmission case are for feed to the rockshaft valve and a bleed passage at the rear end of the valve to prevent hydraulic lock of the internal valve.

On your PCV where does the swivel female connector line go to? AG tractors do not have this connector. The one above it is oil in from the front pump. The one at the rear is oil out to SCVs.

 

[Zebrafive]

I've bypassed it and everything was working like it suppose to. I reconnected it after taking it apart to make sure no dirt was in it. I also plugged the rear selector to take it out of the equation and it's back to the woes again.

What do you mean by "plugged rear selector" ? Did you plug the rear fitting on the PCV?

 

[Zebrafive]

My knowledge of JD "utility tractors" is limited BUT on AG tractors with a similar pressure control valve this PC valve doesn't control system pressure. As I previously stated PC valve provides hyd priority pressure to power steering. The stroke control valve in frt hyd controls system hyd pressure. Does your 401C have a 3 pt hitch??

You are correct the PCV does not control pump pressure.
What it does is close oil flow to rockshaft and SCVs if pressure drops below 1700-1800psi (AG tractor) to give priority to steering (all flow goes to steering) until pressure builds back to above 1700-1800psi.

 

[Zebrafive]

Isn't 3 pt hitch supplied hyd oil via a galley in trans case supplied by hyd pressure from pressure control valve?

Yes, internal passage to rockshaft cover

 

[Zebrafive]

Could the small hole of the transmission feed the rear selector is my question?
I'm gonna tap my new pressure regulator into the existing defective pressure regulator to direct over pressure flow and T the working flow towards the rear selector if needed.

By rear selector do you mean rockshaft/3point hitch? Or SCV?
One hole feeds the rockshaft, internal passage. The other is bleed off for PCV's internal valve. Should be the rear most hole.