JD 770 Can't engage PTO

[BikeSanta]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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!

 

[BikeSanta]

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.

diesel466 - Thanks to the tip from jd110, I bypassed the correct interlock and can easily start the tractor with rear PTO engaged. I removed the screen on the right side of the bell housing and can see both clutch disks. With the engine off, the clutch pedal clamped all the way down and the rear PTO engaged, I can't turn the PTO by hand. Both clutch disks appear to be about the same thickness. The main transmission clutch disk has about .030"+ of float, and can be moved back and forth. The PTO clutch has no space on either side and can't be moved.

From looking at the transmission diagram, it appears that the throw-out bearing is likely of the carbon variety, rather than the ball bearing type. If so, it might be the throw-out bearing that's shot. The operation of the 2-stage pressure plates is a mystery to me, but even a worn pressure plate shouldn't cause this problem.

So, I'm back to the premise that either the PTO clutch is frozen, or I can't get enough clutch travel to disengage it, possibly due to a worn throw-out bearing. Either way, it's likely that I'm looking at splitting the tractor in the spring.

 

[beenthere]

Bike
Looked up a schematic of the dual clutch engaged and disengaged on the tech manual and will post the two. Hope it helps your figuring. Now that you can get the tractor started with PTO engaged, maybe put some attachment on the PTO to try to break loose the PTO clutch. May not take much. I assume a tool (long screw driver?) can't be pushed through where you see the PTO clutch plate tight together?

 

[BikeSanta]

Bike
Looked up a schematic of the dual clutch engaged and disengaged on the tech manual and will post the two. Hope it helps your figuring. Now that you can get the tractor started with PTO engaged, maybe put some attachment on the PTO to try to break loose the PTO clutch. May not take much. I assume a tool (long screw driver?) can't be pushed through where you see the PTO clutch plate tight together?

beenthere - Thanks for these diagrams. They do help me understand what's going on. I did poke around with a screwdriver, but I was a concerned I might cause a biggrer problem. I tried breaking loose the clutch by clamping the clutch down, engine off and rear PTO engaged. I grabbed a hold of my snow blower second stage impeller and tried turning the PTO. I could turn the PTO shaft a little, and I could see through the access hole in the bell housing that I was turning the whole clutch/flywheel assembly. But I could not break the clutch free. I could just turn it back and forth a few degrees. I may try a longer lever on the PTO if I can think how to make one up. Thanks for your ideas.

 

[jd110]

I had forgotten there is a screen on clutch housing on the 770. On the 870, 970, and 1070 I have adjusted The pto though the holes in the clutch housing. I am attaching the picture of the adjusting screws. If you can see theses screws (3 of them) through the hole and can get wrenches on them to adjust them, you can get it fixed without splitting the tractor. Adjust these three screws to .060" under the head of the bolt. You will probably find this clearance now more than that.

 

[pmsmechanic]

If you can see that the pto clutch is releasing then I'm thinking that the clutch disc is rusted onto the flywheel. If that is the case then forcing a strong pointed knife between the flywheel and the clutch disc while holding the clutch pedal down might be enough to release it.

 

[BikeSanta]

IView attachment 405426 had forgotten there is a screen on clutch housing on the 770. On the 870, 970, and 1070 I have adjusted The pto though the holes in the clutch housing. I am attaching the picture of the adjusting screws. If you can see theses screws (3 of them) through the hole and can get wrenches on them to adjust them, you can get it fixed without splitting the tractor. Adjust these three screws to .060" under the head of the bolt. You will probably find this clearance now more than that.

jd110 - Thanks for the photos. Very helpful. I might be able to get to those bolts. I haven't figured out how to turn the assembly more than just a little using the PTO. If it were a gas tractor, I'd take out the plugs, but with the diesel, hand-turning the engine is a problem. We're going to have a few days of very cold, bad weather, and I'm working outside. When the weather improves a little, I'll give this a try. I might be able to "bump" the engine around with the starter. I saw that your photo is from the TM1470 manual. I went ahead and ordered the manual on CD from John Deere. Thanks.

 

[BikeSanta]

If you can see that the pto clutch is releasing then I'm thinking that the clutch disc is rusted onto the flywheel. If that is the case then forcing a strong pointed knife between the flywheel and the clutch disc while holding the clutch pedal down might be enough to release it.

pmsmechanic - Thanks for the suggestion. I don't have much space to work in, but I did try to put a thin tool in there - much thinner than a screwdriver. I discovered that there was no space on either side of the PTO clutch disk. So if the disk is rusted to the flywheel, it's rusted to the PTO pressure plate as well. Thanks for your advice, but I don't think I'm going to get anywhere with that. If I can turn the assembly to where I can see the bolt heads, as suggested by JD110, I can at least see if they are adjusted correctly.