John Deere 755 transmission problem - very slow forwards, normal backwards

[bluebottle47]

I wonder if anyone else has come across this one?

The tractor is generally working fine despite its age (over 4500hrs) but recently started to be reluctant to go forwards - very slow whether in high or low range, as if you'ld lifted almost completely off the pedal. Transmission pump and motor seem to be ok as it goes as fast as ever backwards, with no loss of power. There's no obvious disturbance of the external control linkage, though I haven't yet tried readjusting same: I'd have thought that if the problem lay there, it would be pretty obvious! The control lever on the transmission appears to be moving normally with no obvious restriction of movement.

The problem seems to have been of insidious onset: I only really noticed it when it had got so bad I couldn't miss it.

Any bright ideas before I attack it? Any thoughts most welcome!

 

[joecdeere]

I have a PDF of the tech manual you are welcome to. Some reading might tell you what you need to know

shoot me your email if you want it

Joe

 

[bluebottle47]

Thank you for you kind offer, Joe, but I already have a PDF of the tech manual, and reading it made me decide that I'd avoid going inside the transmission like the plague!

Fortunately, I think I've found the problem, and it's external, praise be.

What I couldn't see before some initial dismantling was that there was a problem with the neutral return lever. One of the rollers which engage the speed control lever had disintegrated and there was restriction of the movement of the speed control lever in a forward direction. This loss of motion wasn't easy to spot in the control rod outside the chassis frame, but with the neutral return lever removed, it was very obvious.

Curiously, it was the inner roller that had failed: as far as I can see, it is the outer roller that acts on the speed control lever, and I can't see what the inner roller actually does without further dismantling - does anyone know? Anyway, the rollers are standard sealed 22x8x7mm deep grove ball bearings, and cheap as chips - 2 on the way by mail order.

The eccentric bush in the centre of the neutral return lever was also very stiff due to a blocked grease nipple and hence lack of lube.

I don't understand the purpose of the eccentric - maybe it's there to ensure that, when setting up, the roller sits in the bottom of the vee of the speed control lever when in neutral. The tech manual doesn't mention its function at all, though it is labelled in many diagrams. Again, does anyone know?

I'll report back when the new rollers are fitted.

Thanks again for your support.

Ian

 

[bluebottle47]

Doh! A quick search here on creep, and I know what the eccentric is for!

Sorry, guys .....

 

[johndeland]

The eccentric is used to find the null position for the swash plate. By adjusting the eccentric you will find the perfect middle for the at rest position (no pedals depressed)

JohnD
1995 JD 755, 60" mower, material handling system, cab & loader
Ottawa, ON, Canada

 

[bluebottle47]

Thanks, guys, and the second roller operates the in-gear starter inhibitor switch.

Everything working fine now!