Bad PTO relay reason for Blades Won't Engage

On July 19, 2017 my Gravely ZT mower blades stopped suddenly and refused to engage. The red knob you pull out to engage blades had no effect.

This happened mysteriously when my grandson was mowing and confessed he had hit a big rock. I panicked and ranted awhile but my anger abated after examining the blades which were not visibly damaged. :Mad: It turns out he apparently hit and moved the rock only with the deck. But mysteriously, the blades would not engage anymore. http://www.lawnmowerforum.com/images/smilies/confused2.gif :Confused2:

My first diagnostic check was very helpful to me: I pulled the lead going into the PTO Clutch and had my wife sit on the mower and pull-up the PTO switch (red knob) while the mower was running and the brake lever set to "park". I took voltage readings at this moment as she repeatedly switched the PTO knob. There were only sporadic "spikes" of perhaps 0.5 or 1.0 volts but not 12 to 14 volts which is the expected value. I correctly concluded: no voltage is getting to the PTO clutch.

After about a couple hours of pulling off the fender that contains all the wiring harnesses and switches and cutting my hands and fingers wrenching the PTO switch out of the harness plug (connector) I checked the PTO switch for continuity and it was OK. The wiring harness is so short that when you pull the fender off, the plug or connector to the starter key slips off (why couldn't the others?) and you have about 3 inches between the mower and fender in which to work, most connections to PTO switch and starter key must be made blind.

I studied the wiring circuit diagram of my Parts Manual and concluded that apart from shorts or breaks in wiring, the culprit must be one of the (2) relay switches. They look like about 1 inch black cubes and both connect to separate wiring bundles from the wiring harness inside the plastic fender. I removed them entirely from their plugs or connections to make the following check:

To eliminate obvious malfunctions, I checked the resistance in the coils: you just place an ohm-meter across terminals numbered 85 and 86 and measure the resistance. Both measured 89 ohms and the rated specs say 90 ohms - good enough for "government work". As far as I know, the only way to check the actual switches in these puppies is to connect a 12 volt battery to terminals 85 and 86 which energizes the coil. When I did this, I could hear the solenoid "click" and feel the relay "cube" rock a little. Ironically, both relay cubes acted the same, however the "good" relay measured continuity across poles (terminals) 30 and 87. The "bad" relay (which happened to be the PTO relay) had no continuity between poles 30 and 87.

Problem solved! :Laughing: http://www.lawnmowerforum.com/images/smilies/laughing.gif

I am now waiting for the relays to come in the mail. I ordered (2) because they are identical and both have seen the same service life.

BTW, me and my grandson are reconciled after the event. :Ashamed: http://www.lawnmowerforum.com/images/smilies/ashamed.gif

Glad to hear you got it fix, your method is a great example of how to slowly and carefully diagnosis an electrical problem. Don't you just LOVE electrical troubleshooting.

nice work there don't be too hard on the grandson. At least he is cutting some grass most grandkids here just want to play electronic games.