JD 770 Can't engage PTO

BikeSanta · Dec 30, 2014

I'm new to this forum, and I know the JD 770 is an old topic, but I have a PTO problem and could use advice. I recently bought this tractor (1990) with a JD model 70 loader, and about 2200 h on it. It has 4WD, and is in pretty good shape, both cosmetically and operationally. I've been using it for a couple of months for general dirt moving and other duties, with no problems. I was attaching my snow blower when I discovered that, even with the clutch all the way down, all I got when pulling up either PTO lever was gear grinding. Of course I stopped pulling on the PTO lever! The first advice I got was to adjust the clutch free play. I found that the clutch linkage was already as short as it could go. I've read that the clutch plates sometimes stick, but when I push down the clutch pedal, I don't "feel" the second clutch. Other than pulling the tractor apart - which is not likely to happen this winter - I can't think of anything else to try.

One mechanic suggested I defeat the rear PTO interlock by pulling apart the connector and installing a jumper. Then I could start the tractor with the PTO engaged, if I made sure the PTO would turn freely first. He directed me to the interlock on the right side, below the PTO output shaft. I tried this, but the tractor still didn't start if the rear PTO was engaged. I see there is a second interlock switch on the left side, below the seat. Haven't tried that one yet.

Does anyone have a suggestion? Do you know which interlock switch does what?

beenthere · Dec 30, 2014

Your mechanic has the right idea with defeating the interlocks.
Somehow need to get the clutch plates unstuck, and how I've done it before is to start the tractor with the PTO in gear. The put a load on the PTO with the clutch disengaged.
That is hard to do with a snowblower. Much easier to do with a different attachment that doesn't require the tractor to be moving. I did it with a rotary brush mower that I could lower onto a small stump.
Possibly just blowing snow will eventually free it up, but there is not much momentum built up with a snow blower like there is with a rotary cutter (without an over-running clutch on the PTO).
Wish you well.
I don't think adjusting the linkage is going to free up the clutch. And I don't think there is an inspection plate to force a screwdriver in between the plates either.

beenthere · Dec 30, 2014

If it may help, here are the 770 electrical circuit drawings (old version and new version) from the tech manual showing the interlocks. Ignore extra image, can't delete it for some reason.

jd110 · Dec 30, 2014

The pto interlock is the one on the left side under the seat. Since you don't feel the second stage on your clutch, most likely the transmission clutch is worn out. That is one disadvantage to the two stage clutch. Typically, most people wear out the transmission clutch much faster than the pto. This makes the relationship between the two to get out of sync. The only adjustment you can make on the outside is free travel. If you could get to the pressure plate, you could readjust it to make it work properly even with a worn trans clutch. Unfortunately you need to split the tractor to get to the adjustment and it would be foolish to not replace the worn clutch while it is apart.

BikeSanta · Dec 30, 2014

beenthere said:
If it may help, here are the 770 electrical circuit drawings (old version and new version) from the tech manual showing the interlocks. Ignore extra image, can't delete it for some reason.

Sounds like something to try. The folks I bought the tractor from never used the rear PTO, and hadn't used the mid PTO in years. Entirely possible the clutch is simply stuck. My neighbor has a bush hog I might be able to use to try your trick.

Now I need to get the motor started with the rear PTO engaged. I tried one interlock switch to defeat the PTO interlock, which didn't work. There's only one left. Thanks.

BikeSanta · Dec 30, 2014

jd110 said:
The pto interlock is the one on the left side under the seat. Since you don't feel the second stage on your clutch, most likely the transmission clutch is worn out. That is one disadvantage to the two stage clutch. Typically, most people wear out the transmission clutch much faster than the pto. This makes the relationship between the two to get out of sync. The only adjustment you can make on the outside is free travel. If you could get to the pressure plate, you could readjust it to make it work properly even with a worn trans clutch. Unfortunately you need to split the tractor to get to the adjustment and it would be foolish to not replace the worn clutch while it is apart.

Very logical advice. The switch just below the seat was my next step to get the motor running with the PTO engaged. I too am surprised that the PTO clutch wore out first, since the tractor was used for mowing with mid PTO. No way to look inside without splitting the tractor. Thanks for your help.

diesel466 · Dec 31, 2014

On the right side of the clutch housing is a screen that you can remove to see clutch, You have to remove the plastic panel down by your feet. I would try engaging the pto with the engine off and have someone hold clutch pedal down and see if you can turn the pto shaft by hand.

BikeSanta · Dec 31, 2014

diesel466 said:
On the right side of the clutch housing is a screen that you can remove to see clutch, You have to remove the plastic panel down by your feet. I would try engaging the pto with the engine off and have someone hold clutch pedal down and see if you can turn the pto shaft by hand.

I saw that inspection plate, but didn't know what I was looking at. Thanks for pointing that out. I'll try that tomorrow. At least I should be able to see what's going on in there.

kebo · Dec 31, 2014

First, HOW MUCH grinding are we talking about?? A lot, or just a little? On my 790, when I engage the PTO it will grind for a bit, then the rotating gears eventually slow down enough to allow the PTO to engage. This is the way the design works.

I suspect you might be afraid of damaging the gears, and as soon as you feel/hear them grinding, you stop trying to engage the PTO. Have you tried to really engage the lever forcefully one time, just to see if it can actually do it?? If you have, and it was a no go, then you may need to take a serious look at the internals.

I'm guessing that it's fine and the PTO is just not broken in yet because it's never been used. Try pulling the lever up a little bit more forcefully one time and see if you can get it to jump in. Just make sure the throttle is at the lowest idle setting you can run it at, that will help. Good luck.

BikeSanta · Jan 1, 2015

kebo said:
First, HOW MUCH grinding are we talking about?? A lot, or just a little? On my 790, when I engage the PTO it will grind for a bit, then the rotating gears eventually slow down enough to allow the PTO to engage. This is the way the design works.

I suspect you might be afraid of damaging the gears, and oon as you feel/hear them grinding, you stop trying to engage the PTO. Have you tried to really engage the lever forcefully one time, just to see if it can actually do it?? If you have, and it was a no go, then you may need to take a serious look at the internals.

I'm guessing that it's fine and the PTO is just not broken in yet because it's never been used. Try pulling the lever up a little bit more forcefully one time and see if you can get it to jump in. Just make sure the throttle is at the lowest idle setting you can run it at, that will help. Good luck.

I did try being a little forceful with the PTO lever at low rpm. I'm afraid there was no indication that I was slowing the gears. At this time, I'm inclined to think that either there is a serious clutch problem, or the PTO clutch plate is frozen to the flywheel. I'll post the results of tomorrow's experiments. Thanks to you all for the helpful advice!