JD 7800 trans troubles

notjustair

Well-known Member
I've posted in the past about some odd trouble with the 7800's powershift. I have a little more info and hope I can get some input. I'm starting to panic as this is my planting tractor.

It started about a year ago every once in a while. You start it up, put it in a gear, and it will move about 10 feet and then roll to a stop. The display goes to "0" gear and the "trans" light blinks until you put it in neutral or park. If you give it a minute it would usually be fine. When it started it seemed like it was only anything above 6th gear, but now it is any gear and yesterday did it in reverse for the first time. Yesterday it did it to me six times until I shut it off and restarted it. If I do that it will often fix it. It won't do it after the tractor has been running and operating fine. If it is going to do it then it does it when you first take off.

Of course I can envision it rolling to a stop in the middle of planting with 800 acres left to plant.

It used to be infrequent but is now nearly every time I fire it up cold. There is enough fluid and filters have been changed.

A while back the suggestion was the solenoids on the front of the transmission. Is that still the suggestion now that it does it in all ranges and reverse? I bought one of the solenoids (since it was $500 and I wasn't sure it would fix it) and was going to just cross my fingers and replace one but I wasn't sure whether I would need to drain the rear ended first. Are those submerged?

Deere thought it likely would not be throwing codes I could check. I have no problem taking it to them but I refuse until it does it EVERY time because they will throw $5000 worth of parts at it until they find it. Actually it was $7000 the last time they pulled that stunt. I have a very good relationship with the local dealer but the only two experienced mechanics retired and it has suffered greatly.

What are your thoughts? I'm about to panic and buy the adapter to be able to run the vacuum on the 1760 planter on the old 4440. I know that tractor would plant all day long again without fail. I just don't want a big paper weight in the yard.
 
Notjustair... I am buy no JD mechanic, worked on dozers and other brand tractors all my 73 years. To me by it doing it when started to me it may be a hydraulic or mechanical problem rather than electrical. That thought is also reinforced by it not giving you a error code. You may have (1) a weak supply pump (2) clogged strainer (3 ) sucking air some where. The fact that it started in the higher gears then has worked its way down also indicates weak or low hydraulic pressure to close the clutch packs. Now JD seller can probably come on here and help you better but that was just my first thoughts.
 
I'm leaning towards a pressure problem as well. I thought maybe it was low on fluid but it is within the marks on the sight glass. I'm thinking it's less likely electrical. I took the transmission cover plate off underneath hoping to find a bad connection to a solenoid but all of the connections were clean and tight. I'm wondering about pump pressure. I'm not familiar enough with the system to know what pumps drive the different clutch packs. If it was IH I could tell you all that but this newer JD stuff escapes me.
 
I am going to repeat what I said when this post came up before. If you want to guess, go cheap. I would lean toward the speed sensor down on the rear left side of the diff housing. When it fails the transmission defaults to neutral. Have seen it before. Real strong feeling its not the "solenoid"
Sorry you have such a crappy dealer.
 
Sounds like tech 7 has some experience with the jd, might look at the sensor wiring. Normally when the electrical functions miss is when you will have a reported error on the computer.
 
I would think along the lines of tech7 for cheap option. If you look back back through archives here n john deere board i was having similar problems this time last yr on 7800 19speed powershift. Would drop out gear light flash. Carnt rember if it would read neutral on dash. Stared intermittent within 1st hr of start up not every day then got worse n worse. Till one day wouldnt go more than 10ft. Was throwing codes but they recon it should not show a speed reading but it always did. Ended up doing speed sensor $180 part n $800 sumthin in labour. Had to take steps n 1 fuel tank off. And up under there sumwhere. Hasnt missed a beat since touch wood
 
I can't tell you what is wrong with your tractor but I can give you some advice on how to move forward after trying to diagnose and fix a similar problem with my 6420.
First, make arrangements to get the crop in assuming the tractor is down.
Next buy the manuals. They aren't cheap but guessing and unneeded parts aren't either. CD are cheaper than paper but I don't find them as handy. The Deere Tech manuals basically tell you how to take it apart and put it back together. The ones that will really help are the Operations and Tests and Diagnostics manuals. You will have to determine the problem and fix it yourself because you don't have the money to spend on parts and time the dealer will throw at it and I mean throw at it because they don't have the technicians with the skills or time to properly diagnose the problem.
To me it sounds like the ECM is defaulting and shifting the transmission into neutral for some reason. I'm not familiar with the 7000 series tractors are there fault codes that can be read? Is the trans light a trans pressure light? The way you describe letting it sit and then it will work fine afterward doesn't sound like a hydraulic problem.
 
Have you checked for any codes like we suggested a few weeks back???? If the light is blinking it is having an electrical issue or not reading speed/pressure. SO you more than likely have a bad speed sensor or a bad sensor in the shift control circuit. I can tell you it is HIGHLY unlikely that it is a mechanical problem since it works fine most of the time.

These transmissions are controlled by a computer control. If that controller can not see pinion speed, engine speed, or clutch pack pressures it will not shift.

Also shutting it off and then restarting it is rebooting the controller. So throwing parts at it would make fixing it harder. You need to find the problem not GUESS at what it might be. Just looking at the wiring plug on the sensor tell you NOTHING about if it is working. It may even be working but at a different resistance than it should be and that will cause it to not work.

So without looking for codes or having the gauges to check all the shift point pressures you not likely to find the trouble.
 

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