Mustang 960: Jerky steering / intermittent right sided drive failure

[joestewart]

This problem has developed over time and now it has gotten worse and is really starting to annoy me. This is a 1992 rubber tired Mustang 960 with the "tee handle" steering. The problem is difficult to describe. The diesel engine and hydraulic lift cylinders and controls work fine.

The left sided drive motor appears to work fine and responds well to the operator's movement of the "Tee handle".

It is the right sided drive that seems to fail intermittently. When you twist the "tee handle" to make a left turn, it seems like the machine begins to make the turn, but all of sudden the power to the right sided tires is lost (it is as if hydraulic fluid delivery to that right sided motor is suddenly shut off). When you return the "tee handle" to the neutral position and then twist it again to make the left turn, the right sided hydraulic motor will suddenly "grab" and make some half hearted attempts to spin the right sided tires (and it DOES spin them a little), but then power to the right sided tires seems to be lost again. This happens over and over again. You cannot spin a left handed circle with this machine (as I could do before this problem developed).

This is making it real difficult to get jobs done. I do not think it is the hydraulic motor or the chain, although I have not examined either one in detail yet. I was thinking it is more of a problem with hydraulic fluid delivery to the right sided drive motor.

Has anyone had a similar problem? Are there any 960 gurus out there? We're getting prepared to do a detailed exam on this machine and would love to have some ideas before we start. Thanks

 

[lantraxco]

Best guess generally about a hydrostatic drive, motor fault, or relief valve weak.
Question: Will it do a spin in reverse on the right side? If it has good constant power in reverse on the right side, it's almost certainly a relief valve.

 

[joestewart]

I finally got around to test driving this skid steer. My friend has been using it, despite the steering woes.

I found that in moving forward, this skid steer will not spin to the left - this would be counterclockwise if you were a bird looking down at it (but it CAN spin to the right).

In reverse, it also will not spin to the left (this would be clockwise). But it CAN spin to the right when in reverse.

We have the repair manual and the parts manual at our disposal.

My friend is in agreement that this sounds like a "relief valve" problem.

Question: Does anyone know where on this machine the culprit relief valve would be located?

Many thanks. J. Stewart

 

[lantraxco]

So, right side weak in forward AND reverse.

Weakness on one side, in one direction, almost certainly relief valve. Weakness on one side in both directions, possibly both reliefs for that side, more likely weak pump, most likely weak motor. YMMV

As a guess, I think that model uses geroller motors, so reliefs are most likely in the pump end caps.

 

[joestewart]

Looking into it. Thanks

 

[joestewart]

Finally got around to diagnosing and fixing this Mustang, so I thought I would post a follow up. Things move slow around here.

My friend poked around and found that there are four relief valves (two on each side) on the hydrostat at the circled locations in the photos. The troubleshooting is covered on page 5-2 and 5-6 of the service manual (Note: if you need a free pdf of the service manual, or parts manual or operators manual, message me or see my separate posting). He removed the relief valve from one side and installed it into the other side and then test drove the machine and he discovered that the problem had reversed sides! The conclusion was that the relief valve was bad.

He purchased a new one - it was a challenge for him to find one - and he installed it and all is well. If you have to replace yours, get ready for some sticker shock - I think he paid $250 for that valve.

 

[apetad]

Where did you find the relief valve???