HST problem in reverse?

[JKeane]

Hello all,
I own a MF1758 HST tractor that has begun to act strangely when I back up. Seems just fine in forward.
I feel a surging like motion, like I'm backing up over cobbles. Sort of like I feel resistance and then no resistance, if that makes sense. The engine rpm does not fluctuate.

Only about 300 hours on this unit (still a pup), HST fluid is full, running the original R4 rear tires - loaded and inflated to 15 psi. This happens in all three HST ranges.

Any thoughts on this would be appreciated.

Thanks.
John

 

[jeff9366]

Are you sure the parking brake is released?

 

[Thomas]

Under foot plate for build debris build up.

 

[JKeane]

Are you sure the parking brake is released?

Thanks. Yes, the parking brake is released. However you got me thinking in that direction. I checked and lubed the brake linkage. Didn't see any binding but a little oil can't hurt. I haven't put any time on the tractor since. Plenty of field work coming up soon; I'll let you know if it worked.
Thank you.

 

[JKeane]

Under foot plate for build debris build up.

Thanks for your response. I crawled under and check for debris. Couldn't see anything. I lubed the brake linkage (as per previous post).
Thank you

 

[BeezFun]

What kind of tires do you have? I can imagine an aggressive ag tread feeling different going backward.

 

[Hoss710]

sounds like something is binding in reverse.

 

[oldnslo]

I have no idea on what type of control is on that HST but possible there is something in the reverse logic causing the surging. Does it do this full speed reverse vs just trying to move slow like inching back to hook something up?

Any unusual sounds or does engine seem to load up a little then unload during the surging?

 

[JKeane]

What kind of tires do you have? I can imagine an aggressive ag tread feeling different going backward.

Tractor has R4 loaded rear tires. Original and in good condition.

 

[JKeane]

I have no idea on what type of control is on that HST but possible there is something in the reverse logic causing the surging. Does it do this full speed reverse vs just trying to move slow like inching back to hook something up?

Any unusual sounds or does engine seem to load up a little then unload during the surging?

I imagine that its possible that something in the reverse logic is causing this issue. Hoping for something more simple. It will surge in reverse at any speed and all ranges. The engine sound and rpm never changes.
Thanks for the input.

 

[oldnslo]

John,
at 300 hrs is this still under warranty?

Not sure of your mechanical and or electrical skills but a good service manual should show the diagnostic steps or procedures for trouble shooting the HST drive - control. (I have no clue how good or bad MF manuals are) There are two or three main styles of control. Smaller tractors typically direct mechanical control. Very simple to trouble shoot since linkage directly moves the swash plate for directional control. Usually seen on SCUT and CUT.

Another control type: hydro-mechanical where mechanical linkage from treadle pedal operates a control valve assembly that directs pilot pressure to the control piston for the swash plate. Pilot pressure comes from charge circuit. On these there are typically test ports where you take pressure readings to diagnose what is going on. Usually fairly straight forward diagnostic procedure.

Other control type: electro-hydraulic where a variable amperage is applied to a proportional coil on the HST control module for directional control. These have some form of variable signal from the treadle peddle that is input to some form of electronic controller which then outputs the milliamp signal to the coil. On these there could be test ports for monitoring control pressure but will usually also require multi-meter for checking various input and out put signals. I usually find this type of system more difficult to trouble shoot since there are more components to work your way through.

I know this all sound intimidating but with some mechanical knowledge and skill most shade tree mechanics can work there way through them.

 

[JKeane]

John,
at 300 hrs is this still under warranty?

Not sure of your mechanical and or electrical skills but a good service manual should show the diagnostic steps or procedures for trouble shooting the HST drive - control. (I have no clue how good or bad MF manuals are) There are two or three main styles of control. Smaller tractors typically direct mechanical control. Very simple to trouble shoot since linkage directly moves the swash plate for directional control. Usually seen on SCUT and CUT.

Another control type: hydro-mechanical where mechanical linkage from treadle pedal operates a control valve assembly that directs pilot pressure to the control piston for the swash plate. Pilot pressure comes from charge circuit. On these there are typically test ports where you take pressure readings to diagnose what is going on. Usually fairly straight forward diagnostic procedure.

Other control type: electro-hydraulic where a variable amperage is applied to a proportional coil on the HST control module for directional control. These have some form of variable signal from the treadle peddle that is input to some form of electronic controller which then outputs the milliamp signal to the coil. On these there could be test ports for monitoring control pressure but will usually also require multi-meter for checking various input and out put signals. I usually find this type of system more difficult to trouble shoot since there are more components to work your way through.

I know this all sound intimidating but with some mechanical knowledge and skill most shade tree mechanics can work there way through them.

Wow. Thank you for the detailed response. I'm going to have to order a service manual so I can dig in to this more. I have a lot to learn but as with most things, I usually prefer to do the work myself - so your comments are very helpful.
John

 

[JKeane]

What kind of tires do you have? I can imagine an aggressive ag tread feeling different going backward.

The tractor has the original R4 tires, loaded. This feels like something has changed.
Thanks.

 

[JKeane]

Well its been awhile since I posted about this and I have had time to use the tractor in several different types of weather conditions. Very cold to very hot. I thought that I would share my findings in the hope it would help out someone else with an HST.

Here is what I discovered. The surging in reverse only happens when the HST is set to rabbit or turtle and I'm trying to move very slowly. So I'm barely touching the pedal. When I have the HST in SNAIL mode and use the pedal the same way; I have no problems with surging. My best guess is that the oil does not move efficiently in the HST when I'm trying to move very slowly in a higher range - in reverse.