855 PTO will not operate

855guy · Jul 7, 2008

for past 2 yrs PTO has been intermittent. Last yr I replaced the saftey switch under the seat, helped for a while. Several times this year it has acted up again. I would fiddle with the switch and/or wiggle the wiring and it would eventually work. Last Fri. while running the brush hog, a limb bumped the on/off lever and disengaged the PTO. I got off the tractor, picked up my hat and got back on and PTO would NOT engage. I got off, lifted the seat, pulled the safety switch up and PTO started, put the seat down and nothing. Did this again, same thing - worked with set up, would not work with seat down (me on it). Hasn't worked since. I have tested the switch - meters o.k.. Have checked the wire best I can all the way from seat switch to the dash PTO lever - all seems o.k.. All connections seem good. I don't find and fuses.
Unit is a 1988 and has 1077 hrs on it.

ANY IDEAS?

unimog_jason · Jul 7, 2008

855guy,

When I first received my 755 back in January the PTO would only engage intermittently and was a cause for great concern. After a little digging I found the electro-magnet that connects the PTO level to the rod that engages the PTO wasn't getting full power. In my case I just cleaned all the terminals up and bench tested the magnet with a battery charger to ensure everything was up to spec and put it back together and it's been working great.

Another thing I did do was lube up all of the rod pivots, the previous owner didn't lube anything up so things were binding a little bit.

I would jumper the seat switch, open the hood and you should be able to look in through on the side under the dash and when you push the level forward, you should see the two parts travel together, remove the jumper and they should separate. If this isn't happening you may want to check your connections and make sure the magnet is functioning correctly.

Hopefully this helps you.

Jason

855guy · Jul 8, 2008

Jason,
Thanks for prompt post.
Great idea - unfortunately mine isn't that simple.
However you got me past several of my unknowns. I understand now what the magnet does and the function of the rod connected to it. I have NO power to the magnet, regardless of where the safety switch is. This leads me to believe I have a wiring issue someplace.

Anyone have a wiring diagram for a 6/7/855 or where to find one?

855Idiot · Jul 8, 2008

My 855 will "loose" power to the PTO too. I have found it's the magnetic switch disengaging it's self.

I was brush hogging last weekend and noticed that when the brush hog would strain, the PTO switch would disengage.

I just reset the dash lever and it would be happy again until the next time...

It seems as though I have to lift the lever slowly to make it work right.

Good luck,

Why is the switch magnetic

855guy · Jul 8, 2008

Not having the schematic of the PTO functions - I would speculate the electro magnet is energized all the time the tractor is running WHEN the safety switch(s) are made (far as I know there is only one under the seat - that works all the way out or all the way down in to make the circuit complete to the magnet).

With the safety switch 'made' the circuit is complete and the magnet will hold onto the handle and pull up the arm that really engages the PTO.

When mine worked - I never had a problem with it staying 'engaged' regardless of what bumps I hit - lawn mowing or mowing with the 5' brush hog. Maybe your magnet is either dirty (as was mentioned earlier) or getting week, time for a replacement (easy to get to on my 88', just pull the side cover and one bolt to remove the arm and a cotter pin in the rod, disconnect a plug and it is in your hand).

unimog_jason · Jul 8, 2008

You can try applying power directly to the magnet to help eliminate it, outside of that it could be the relay that acts as a delay so that if you bounce in your seat the PTO won't disengage.

I have a diagram around here somewhere, I'll scan it and send it over to you in the morning.

Jason

855guy said:
However you got me past several of my unknowns. I understand now what the magnet does and the function of the rod connected to it. I have NO power to the magnet, regardless of where the safety switch is. This leads me to believe I have a wiring issue someplace.

Anyone have a wiring diagram for a 6/7/855 or where to find one?

k4vb · May 5, 2012

Hey Jason, can you tell me exactly what the safety switch under the seat does? I lost power to my rear PTO. I lifted the seat and pulled the safety switch and it ran fine but I could not keep it pulled and sit on the seat. I unbolted it and laid it down and when I sat on the seat, I busted the switch. It has 3 wires coming to it and the best I can tell had 2 switch functions but for the life of me I can't figure out what this switch does. I realize that sitting on the seat engages power to the magnet switch behind the instrument panel and pulling it is intended to allow the PTO to run when off the tractor. If I knew exactly electrically what the switch does I could hot wire it until I can get a new switch?

A wiring diagram of the entire PTO control circuit would be great but just knowing exactly what the seat switch is doing would get me back in the field.
Thanks,
Bob

Mechanos · May 6, 2012

To diagnos: jumper the yellow wire to the pink/white wire at the seat switch. If the PTO now works, then problem was the seat switch. If the PTO still does not work, then remove the left side panel to access the time delay control module located just under the key switch. Remove the harness connector to the module and jumper the yellow wire to the blue wire on the tractor side of the harness. If the PTO works, then the TDCM is bad. If the PTO still does not work, then the magent is bad. If the magnet is working but the PTO still does not work, then the problem is not electrical... it's mechancial.