300U Power Steering Troubleshooting Please

I have a 300U with a loader and a car-style 3GPM 1400PSI Saginaw pump mounted in front that only runs the power steering. Everything is mechanically O.K., including the steering box and gears, power assist cylinder rings and rack, and front end steering parts, but there's something wrong in the control valve assembly-I think.

Yes, I know the loader is outside design specs and is very hard on parts and yes, I have spoken to the very kind Mr. Jackson about my problem and about his excellent $olution. He indicated my stock system should work reasonably well - as it has in the past - with the pump I have.

Yesterday the rebuilt pump tested at 1400 PSI on the bench at the rebuilder's. I brought it home, put the gauge in a T at the high pressure outlet with everything hooked up and got only 500PSI when steering either way and no real action at the wheels unless I lifted the front a little with the bucket, in which case you can spin the steering wheel with one finger from lock to lock and it feels great - full hydraulics - but no force.

I have had the assist cylinder apart and it seems fine - both rings and the bore are good. Not a lot to go wrong. The valve body in the steering wheel column has been cleaned and reassembled- per the parts book - but it seems there's something bypassing or leaking in there - to the tune of 900PSI.

My plan is to measure the output pressure from the valve body to the assist cylinder and if it's no good, to tear the valve body out again and put it back on the bench. But I don't have a clue what to do differently.

Does any one know where to look? My guess would be there's a bypass or overpressure valve in there somewhere and it's releasing too early, but I don't remember seeing one.

I have ordered the PS manual but this is making me crazy and I'm sure the manual won't show up for a week or so.

Andy, as I recall, the 300U uses the same power steering setup as the 460U which I own.

One quick check you can do is to make sure the steering wheel raises and lowers on its axis as you turn it from lock to lock. The total displacement isn't a lot, on the order of 1/8" or so, but enough to pick up with a discerning eye.

As I understand the system, the control side of the circuit deals with that axial movement of the steering shaft up and down...which moves a spool off its center. That off-centering sends fluid to the "power" side of the circuit...which is the double-acting piston (a boost cylinder really) that acts on the "helper rack".

I think what I would do is to crack the lines going to the boost cylinder and see if fluid is being sent there (to both sides) by the control side of the circuit. Then you can draw a theoretical line between the control and the power side of the circuit...if my explanation has made those abstractions clear enough.

The 300U steering cylinder is a lot larger than the equivalent actuator in a car. Your automotive p/s pump might not be able to put out the volume you need. You say the pressure is 500 psi while you're steering, what is it when the steering wheel is static?

Zero

Looking at the manual, 300U and 460U are same, I believe zero pressure with steering at rest is OK as the system is "open center". In other words, with the steering "at rest" all fluid entering is discharged.

If you are building pressure with the steering wheel turning and not getting any fluid to the assist cylinder (check as above), then there's definitely something wrong in the control spool...which looks a little sensitive. The spring plungers which recenter the spool (IOW so the front wheels don't go full lock when you ease the steering wheel one way or the other) look to be a very important part of the assembly.

I'm waiting for the manual to show up, but I suspect there's a preload or clearance issue with the spool stack. It would seem from the difference between what I know the pump will put out (1400psi) and what the system is demanding of it at full lock (500psi) that something is in full bypass. Question is, what?

What I would look for next is what happens when you give the steering wheel a slight "crowd" in either direction with wheels fully engaged with the ground.

What I would expect is pump pressure would rise as the spool moves and forces the boost/assist cylinder to move, then as the spool re-centers, the pump pressure would then drop back to zero (open-center/spool-neutral condition).

Another mental abstraction to think about (or from the viewpoint of) is that the system doesn't exactly "demand" pressure in my thinking. In retrospect, it "creates" backpressure (what the pump-output gauge is reading) by virtue of creating an effective restriction to fluid flow or presenting the ever-adding positive-displacement volume of fluid against a mechanical load (as in the case of a hydraulic cylinder).

Just a practical example of above, with a functioning system I would fully expect pump backpressure to be significantly lower when turning the tires on a film of slick mud vs. when they are half-buried up to the axles. In other words, even though the pump can produce 1400psi when thoroughly dead-headed, the pressure is "matched" to the mechanical "resistance" load applied in the opposite direction.

Problem is, with front wheels down, there is only a tiny twitch of the tires when the 500psi comes on...and no larger movement of the wheels, no matter how long you hold the wheel over and keep the 500psi on the gauge. Same to each side. This says to me that I am lacking pressure to the cylinder, as 500psi isn't getting it done. If I have (the potential of) 1400psi at the pump and 500psi at the cylinder, where's the rest of it going?

O.K. Waited for the 300 Power Steering manual to come from ebay... and it's for the wrong system. I didn't realize there was another 300 PS system that had a hydraulic motor in the valve body on the column.

So anyway, the gauge went on both outputs from the valve body on the steering column this morning and I got 500psi at the pump and at both outlets...from a 1400 psi pump, so I went hunting for the bypass. Clearly the system was bypassing while it should be working.The spool seems to have minimal wear, has a really tight smooth fit, but on the bench with 200psi of air going in, it was letting a lot return when moved to either extreme. We decided to experiment before sending Mr. Jackson his check and are in the process of turning .075" off the outer O-ring face of both spool retainers...or whatever they're called.... the top and bottom pieces that seal on the spool and move with it...so as to provide an additional .075" of movement each way of the spool inside the valve body. Don't know if it'll solve the problem, but it works with air on the bench...all the pressure goes to the outlets at the new full travel and nothing bypasses.
We'll see tomorrow.

FYI, I'm really pleased with this fix. One finger steering while moving empty on blacktop and good steering with a full bucket of dirt. Still no tire movement while stopped, but the slightest motion forward or reverse and boom, steering like a '64 Oldsmobile. I'm getting 1200psi at the pump both ways at deadhead and zero centered. Whatever the bypass was is gone and the full force of the pump is now working the cylinder.