MF3690

Hello everyone! New to the forum and in search of some advice/help with my 1992 Massey Ferguson 3690. Have owned the tractor for about 10 years and have never used the PTO, now I need to use it on a second baler we bought..

Here are the details: Engage the switch in the cab, PTO engages and spins for about 4-5 seconds then shuts off and the light on the dash blinks. When I was in a pinch, I swapped the wire from the diff lock solenoid to the pto solenoid and ran it like that fine, but without the use of brakes obviously.. So we know the solenoid is fine. Switch is fine. In my mind it all pointed the PTO speed sensor, so I installed a new one and still didnt fix the problem. I've looked over the wires as well as I could (they are in a loom) and tested them and they seem fine also. I thought maybe the wires to the PTO speed sensor were broke or grounding somewhere, so I rigged up a new wire from the sensor to the connector at the autotronic box and still nothing.

I will also say that, all other functions with the autotronic box work fine and I've tediously looked it over and nothing seems fried on the board..

Any ideas would be much appreciated.
Thanks!

MFRoss

This reply is probably a little late since I just registered.

I have the service manual for the 3600 series MF and it says the Autotronic will disengage PTO (with flashing light) when hydraulic pressure drops below 17 bar (246 psi) causing the PTO clutch to slip excessively. It will also disengage if the implement is blocked causing clutch slippage. That being said, the Diff Lock, 4WD, and Hare/Tortoise range changing would not function correctly if hydraulic pressure was low.

With what you have described, my guess would be that the Autotronic box is faulty. The fact that you eliminated the speed sensor being the cause and no other problems exist with the tractor, it makes me think the box is malfunctioning.

I have a 1994 3690 that had Autotronic problems with the 4WD. Power was available to the Autotronic box from the switch (which removed power from the solenoid to engage 4WD), but nothing was coming out of the box to the 4WD soleniod to disengage the 4WD when 4WD switch was off. The Autotronic box is expensive, so I did a little rewiring job and was lucky enough to fix the 4WD and retain the Autotronic safety functions. In your case, you could bypass the Autotronic box, but unfortunately, you would lose the safety net the box provides for the PTO, which is progressive engagement and clutch operation. Serious damage could occur to the tractor without these safety features.

You might double check the PTO speed sensor is adjusted correctly (I can't find the procedure if it even exists), and if so, I would buy a new Autotronic box, or take it to a MF dealer where they should be able to hook up to the box and confirm that it is faulty. I hope this helps a little.

MFRoss

Another thought occured to me. I originally replied to this post as I was searching for information on a PTO noise problem I was experiencing with my 3690. I have since been through the PTO and discovered that the PTO brake had been malfunctioning at sometime in the tractors life. The brake had worn into the gear hub that it brakes against. The brake is spring loaded on, and is released with hydraulic pressure when the PTO is switched on. In my case, I believe hydraulic pressure may have leaked by the seals of the brake assembly and prevented the brake from completely disengaging. I replaced all of the seals in the brake and the brake slowly comes on when the PTO is switched off. Before, the brake stopped the PTO shaft immediately and abrubtly, as if it was nearly on all the time.
Anyway, if the brake is stuck on, it may cause clutch slippage and automatic disengagement of the PTO. The PTO brake is part of the top link plate that is bolted to the tractor. It is easily removed. You could pull it off and have a look at the brake surface and the gear hub. You can also hook compressed air up to the hydraulic line that controls the brake and see if it moves the brake.
You might already have things figured out. I just posted to help others who might be searching for things to try. Good Luck.