JD 7800 transmission

Last year I was having trouble with the power shift on the 7800. It ended up being the wheel speed sensor. That was fun.

It's developed some random new thing. If I push in the clutch and downshift many gears (like from 15th to 6th) the transmission throws the light and won't move. You have to shut off the tractor again and it will reset. If I don't use the clutch it never messes up. It's always pulling it back quite a few gears and only when the clutch is used. Shutting it off always resets it. Baling with it was fun until I figured it out. I just pull out of the windrow as it ties and dump the bale while I loop back around to start the windrow back up. It was kind of a pain when pulling he baler through town and I accidentally pushed in the clutch and dowshifted at the stop sign. I had to shut it off!

Fluid is clean and full. Filters were just changed. I had Deere test the hydraulic presssure at the remotes and everywhere else they could. All were within spec.

Any ideas?

I have had them do that when they need re-calibrated. The PCU is loosing track of what gear it is in when you shifting it with the clutch down. IT is defaulting to neutral because it does not know what gear to take off in.

Even thought your moving the gear shift lever the tractor PCU still controls the shifts. SO you know you want to be in, lets say 12 gear, and you shift the lever to that, with the cutch down. Since you have moved several gears while setting still, the PCU is not sure what gear it should take off in. So it defaults to neutral.

Have you calibrated the transmission since you replaced the wheel speed sensor??? If not then this could carry back to then. I always re-calibrate them after replacing any of the sensors that the PCU uses to calculate the shifts.

I do not remember off the top of my head all the steps to do this on a JD 7800. It is some thing you can do yourself. No computer required on a JD 7800. You will need a short hose to jump across one of the SCVs to warm the hydraulic oil up to operating temperature. If it is cooler you may have to block off the oil cooler with some card board to get the oil to heat up. This is critical on a JD 7800 to get a good calibration. The newer tractors are not as sensitive on this.

I remember doing a re cal when I worked at Deere. The 7800 had not been out to long but I remember warming up tractor and doing something in the command center. It been to long for me to remember,but it was easy.
If you don't mind me asking what did you find out at the dr.?