Hi everyone any cat need some advice from the cat guru's. A cat 966h wheelloader , has some slow steering, i checked the orbitrol unit already, all seems to be fine, put it back together and went for testing, what i discovered is that it turns good while no other operation is performed. But as long as it is shifted in forward and reverse or brake or lifting or bucket movement then the steering wheel gets stiff. So i'm wondering if there is some checkvalves or reliefvalves that needs to be checked. when it it steers smooth, the steeing pressure is 100 bar and when it touches the limit valves it is 200 bar relief pressure. if somebode had a diagram where te valves are located it would be a great help.
For a start you could help people to help you by posting a Serial Number. I see a dozen different flavours of 966H. It would be nie to know which one you have, because you can't assume they are all exactly identical when it comes to the steering system. I can't believe that you pulled an Orbitrol unit apart without doing at least some basic pressure testing on the steering system first.
S/n :TAL02065
Sounds like priority valve, if it has one
Is 3565129 applies to this one? Nige?
Looks like 2009
Don’t know which type of steering is yours HMU or Command Control. But steering on that thing have separate, pump. And nothing in drive or implement should affect it. Besides drive group. If all the pumps drives through torque converter - that might be your issue. RPM on pump drive get dropped and all of the sudden steering gets reduced flow. Maybe I’m wrong
If you talking orbitrol - than you have HMU type of steering
I was looking at RENR8828, maybe it’s the wrong one. Nige can let me know
What i also discovered is, with the secondary electrical steering, it also gets stiff, thats why i was doubting mthe orbitrol.
Hmmm! One thing - suction! All of the pumps not sucking almost anything until getting a command. So if your suction is inadequate something will be getting starved. Not sure how transmission comes in here though. Unless every time you put that thing in drive - cooling fan comes on.
DC pumps are pretty puny. It is for emergency only. And your steering might be getting tired. But everything affecting steering - that’s is kinda weird
Can you hear high pitched whine, that wasn’t there before?
Is your hydraulic fluid level low?
Try to open fill lid in reservoir, and then try again, see if it’ll take a while before steering get stiff again. Your breather or “brake vacuum” might not be working.
Attached the T&A procedures for the three steering pump pressure tests. I would suggest to do the the pressure measurements first without attempting to make any adjustments. Simply report your results. Also check the pump backup relief valve setting. Again don't try to adjust anything, just come back with your results.
The fact that they state Stand By pressure 515 psi, and margin pressure around 350 psi, on the single circuit feed - they already know that pump is undersized for full speed of steering use. In other words - system constantly on the brink of saturation. So anything that will impact pumps volumetric efficiency - will hinder operation of the steering system. The pump on those pictures looks like Danfoss Series 45.
Actually I’ve messed up. They said “should not exceed 525 psi” as first step of adjusting LS margin pressure. Not sure where OP should find differential pressure gauge ( I have several mechanical and digital Sensocontrol) The “quick test” for that is what they call “stand by” test. That’s what described first. With single circuit feed, if pump is not undersized stand by should be the same as margin. Margin Pressure Drop or “system saturation” test is important when running multiple consumers on single valve bank simultaneously, and expecting good flow sharing. Still don’t see how operating any other circuits in the machine should affect steering operation. Especially drive. All those components have only one thing incoming - torque converter ( and even that if there is no straight pump drive shaft, not connected to the transmission input shaft). And reservoir.
Ive got an “wild thought”! Does your engine RPM drop when you engaging anything together with steering? Might as well be engine performance issue.
And with drive, it might not be drive per se - your brakes are tied to cooling fan pump. And in order to shift transmission in to drive, you need to get off of parking brake while holding service brakes. Your fan/brakes pump is not on the same pump drive as steering and implement pump. So only thing are common is engine and reservoir.
Sounds like your CAT 966H has a classic case of steering priority issues , where other hydraulic functions are taking up flow, making the steering stiff when you're shifting, braking, or using the bucket. A few things you might want to check: Priority Valve – This ensures the steering gets oil before anything else. If it's sticking or worn, other functions can steal the flow, making steering sluggish. Pump Flow & Load Sensing – If the pump isn’t delivering enough flow, steering can weaken when multiple functions are in use. Relief Valve & Pressure Settings – Your steering pressure looks fine at 100 bar, but if something is off in the system, it could be affecting flow distribution. Fluid & Filters – Contaminated oil or clogged filters can restrict flow to the steering system. I’ve broken this down in detail, along with troubleshooting steps, in this guide: Hydraulic Steering: How It Works & Common Issues . Hope it helps! Let me know what you find.
Really?! I think yo need to do what I did - “hit the books”!
Hi Nige, this is a great help. I will check as soon as i get to the machine.
Filters and fluids are in its best, owner doesnt joke around with the service intervals. The priority valve could be it.
No drop
The symptom pattern on your 966H points to steering pump supply/margin problems under combined load, not an orbitrol fault, especially since both the main HMU and the electric‑backup steering go stiff whenever you add drive, brakes or implement functions. When steering is fine with no other function active and then stiffens as soon as other hydraulics or drive are engaged, it usually means the steering pump is losing available flow or margin pressure under load, or the common supply (suction/breather/engine) is being pulled down. Follow the Cat test procedure @Nige shared: measure steering standby/margin pressures at the steering pump and backup relief with no other functions active, then repeat while engaging drive, brakes and implements one by one, without changing any settings yet. Any big drop in margin under combined load points at pump or drive issues (loss of volumetric efficiency, suction restriction, breather/vacuum problem, or engine bogging) rather than orbitrol or cylinders. Watch engine rpm closely and crack the tank cap as @TVA suggested; if steering improves briefly with the cap open or if rpm sags whenever you combine steering with other functions, you have either a reservoir breathing issue or an engine/drive load problem starving the steering pump. If you or others reading the thread want a more “systems” view of what’s going on (how steering pumps, margin pressure, orbitrols and cylinders all tie together), there is a 7‑part hydraulic steering series that walks through this step by step on off‑highway equipment very similar to your loader; the examples are mobile machines, but the same logic applies directly to a 966H: https://epichydraulic.com/steering/ There is also a steering calculator where you can plug in pump flow, steering‑unit size and cylinder dimensions to see what steering performance you should get and how sensitive it is to available flow/margin loss: https://calculator.epichydraulic.com/calculators/steering/hydraulic-steering-calculator