T4.75 Low Power Steering/Transmission Pressure

[1994NA]

Stop Error On T4.75, Low Power Steering Pressure / Transmission Pressure
2019 Powerstar 75
Symptoms

• Day started out with all running good, no apparent problems, then STOP error comes up along with a symbol on the display that closest resembles the low pressure, and steering wheel. However I could not find exact symbol in the service manual or owners manual or online.
• Checked hydraulic fluid, it was in the normal range.
• Note, this tractor shares the hydraulic fluid with the steering, loader, and transmission. There is no separate reservoir for the power steering as many posts I have reviewed claimed.
• Tractor was able to return to shop ok, power shuttle seemed sluggish but engaged ok, but continued to shut down in about 1 minute. Would restart and operate normally, and move but would repeat the problem.
• Front end loader and other hydraulics work smoothly and show no anomaly.
• Removed (while awaiting new one to arrive) the power steering/transmission filter on the side of the transmission case, still no solution.
• Replaced suction filter on the left hand side of the engine, prior to the duplex pump, still no solution.
• To test electrical, I disconnected the low pressure switch for this circuit, as it is normally closed and opens upon normal operating pressure. This removed the STOP error, and the tractor would continue to run. So it is not a issue with the low pressure switch, monitoring circuit.
• Now the tractor seems to exhibit true low pressure symptoms, when the power shuttle is engaged, the tractor seems to want to engage, but only slightly and may rock forward or back slightly, but has no engagement power to move. However, the power steering still operates smoothly and does not exhibit any problems with its function. Throttle up the engine, same thing, like you are only slightly engaging the clutch. Tractor is immobile.

I have the full service manual, which does not show the posted error screen. Have been also looking for how to test the pressure on the power steering system, but have not found any relevant posts, and service manual is also not helpful here. I have thoughts like a broken spring in a high-pressure bypass valve in the system, bad secondary (power steering) pump etc. but thought I would post before tearing deeper into the system. Any help is appreciated.

 

[TractorTech]

I don't have a service manual on a T4., but the pressure should be about 2-2400 psi

 

[1994NA]

Somewhat solved.
Reviewing the hydraulic schema for the transmission hydraulic circuit, I discovered a spring loaded check valve near the transmission filter (which was removed at the moment). What I discovered is that this check valve is also a differential-pressure device, that severely limits flow if there is no restriction at the 30 micron transmission filter. I put the old filter back on and the power shuttle started working again. Still have the low pressure switch disconnected, but will reconnect that once I put a new transmission filter on. Will update in a few days after the new filter arrives.

 

[DieselMonk]

Your tractor complains about low hydraulic fluid / pressure. That pic isn’t a steering wheel, it’s a gear with an oil drop in the middle. I guess you already checked the hydraulic oil level and changed all the filters. On the individual hydraulic pumps for the different systems there should be a little screen filter. I think what you have to do is to check your hydraulic pressures everywhere and see what the problem is.