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2017/10/10 20:40:54

bmac47

48" Gravley PTO problem

I have a 10 year old Gravely zero radius, Promaster 148Z. The PTO suddenly quit working when I was mowing; didn't hit anything, light grass, no loud noise or thump, it just quit. Sounded as if I had shut off the PTO/mower blade switch but everything else (engine, hydraulics, steering, transmission, etc.) kept working just fine. All belts are good and blades are not frozen. I have checked everything I can think of with no "ah hah" moments. I suspect electrical issues but I'm reluctant to just start blindly replacing switches, etc. if I can't find anything that seems dead. Any suggestions would be appreciated.

6 comments Leave a comment

AVB

This is why you need either a DVMM or Analog multimeter. Using the ohms portion of the meter you can check the continuity of the switches, wiring, and electric coil. You should never just throw parts at it as can get expensive fast and still have the problem.

Looking over the circuit of most of the 148Z and there are several versions which is why the exact model number is needed to lookup things, if the mower will run then there is only the PTO switch, wiring, and the PTO clutch coil that would be involved. Simple a circuit setup but finding a bad connection can be a little hard. However most failures would be either the switch or the PTO clutch.

Generally the PTO clutch would be between 1.7 and 3 ohms. Less than 1.7 ohms indicates either a partial short or complete short. But with clutch it probably would read as an open circuit if the PTO has failed. With PTO switch you looking for zero ohms or low as you meter reads between pins 1 and 4 with switch in the on position. Note this is with the ignition switch off; otherwise, there can live voltage in the circuit which can damage your meter. If the switch is melted then suspect a shorted clutch coil or wiring touching ground.

Also if doing a live voltage test on the PTO clutch don't connect it backwards as some have a diode to prevent the clutch hanging in an engaged position due a self energization. This backwards connection would fry the PTO and they are expensive.

2017/10/11 08:47:26

bmac47

Thanks! Very helpful to confirm my gut and give me some concrete direction. Full model # is 992078 with serial# above 4000. I removed the "switch, clutch" (not sure I understand the mfg's parts manual label here) and using my analog VO meter I checked for continuity between pins 1 and 4 with the switch in the "On" mode and got zero continuity. Bad switch? Or am I doing something wrong here?
You mentioned a possible coil but I can't locate one on either the mower or the schematic that seem to tie into the PTO switch. There are 2 coils ("coil 1" and "coil 2") on the schematic but they look to me as though they only relate to the starter function. ??

2017/10/13 15:56:16

AVB

Zero Ohms is good for a closed contact.

On your particular mower you do have one reply that controls the PTO clutch. Now the coil that I was referring to is in the PTO clutch. It is what generates the magnetism that pull in the engagement plate.

The following the wiring diagram show how the PTO is wired.

2017/10/13 16:19:33

bmac47

We are quickly outstripping my knowledge level here, but lets see if I can ask an intelligent question. I now see the correct wiring diagram and I've located the three relays on the mower; I even think I know which one is the PTO relay based on color code of the wire. If the PTO clutch is the problem, how would I know? Moreover, if that is the issue, is that as simple to replace as the parts diagram suggests or are the tricks one has to know? Alternatively, if the PTO relay is the problem, how would I know that? There are no evident scorch or burn marks and no smell of fried electronics.
By the way, I re-tested the PTO switch and now that it has warmed up inside the house, there is continuity between pins 1 and 4 in the engaged (up) position but not in the disengaged (down) position, which is what I would expect, I think? (I don't know for sure that temp was the issue, could have been simple operator error.)

2017/10/13 18:22:38

AVB

Sorry for the delay I had to my head on the rewiring I was doing here straightening out some others goof ups.

Some layman tests that you can do.

According to wiring wiring there is a purple wire with a white stripe that feeds +12v to the pto clutch you can temporary jumper this with +12v from the battery and if the pto clutch is good you heard it engage with clunk noise. Now these pto can pull up to 7 amps when but can pull over 15 amps and still work even when are partially shorted. This this extra current can burn up contact of relays and switches. In this setup I doubt the pto switch is the problem since it only take a small amount current to operate the relay. For layman test you can use an in line fuse with a 7-1/2 rating. IF it blows then the pto is pulling too much current.; otherwise, if the pto clutch engages it should be fine and I would suspect the relay contacts provided there is no broken wires or loose connections.

BTW I have seen partially shorted clutches still be operating when the customer comes in with battery dying complaints where they pulling more than the alternator can supply draining the battery.

2017/10/14 14:55:16

bmac47

well I finally was able to get back to this and jump the +12v to the purple/white wire but there was no "thump." Tried it several times with no change. Guess the PTO clutch is bad (?) but after looking at closely it a few times and noticing how long it takes me to get up off the floor each time, I guess its time to call in the pros, bite the bullet and haul it to the shop. Thanks for all your help.

2017/10/18 00:34:13

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48" Gravley PTO problem

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