Mysterious shutdown

[Scrapguy 12]

Have a 1976 c-120 hydrostatic that was bought new. I started it without problems and moved into position to begin grass mowing. Got off to close a protective gate. It was idling smoothly. I activated the PTO and left it spool up. Working fine. Moved the shifter forward and began cutting. I moved about 15 feet and it began to sputter and die. Had plenty of gas and battery was at 12.60 V. I turned the key to restart but nothing. Relieved the trans but opening the valve and pushed back into the garage. Checked the coil, ignition switch, solenoid, but found nothing out of the ordinary. In the past I removed the brake interlock and seat interlock. All that’s left seems to be the PTO interlock. Any suggestions on how to test that item or point me in a direction I missed. 

[lynnmor 7,904]

 

Connections or switches may be bad.  Try bypassing the PTO and then refer to the wiring diagram.

 

It appears we may not have the exact wiring diagram, but these may be close: Wiring

[ZXT 2,401]

Welcome to the forum! 

 

So you bought the machine new? You've had it a while then! Will it not crank over or will it just not start? If it died while mowing, and now will do nothing with the key, I'm going to bet that you either have a bad ammeter, a bad connection somewhere, or a bad ignition switch. I'd start by cleaning any connections you find, and check for voltage on both sides of the ammeter. I would say your issue could likely be the PTO switch, but that shouldn't cause it to die while running. It would only prevent it from cranking. 

 

If you can, check to see if you have power to the ignition switch. That'll get you started. 

Edited May 17, 2020 by ZXT

[953 nut 60,710]

Chances are it is just a matter of an electrical connection that has some corrosion or isn't tight. If you clean and tighten all electrical connections including grounds you will probably clear up the problem. Be sure to check the ground connection from the frame to the engine if your engine is mounted to a plate. The only components that could cause the symptoms you have told us about would be the battery cables, amp meter, ignition switch or the wires that connect these items. 

Take a look at this drawing @BOB ELLISON did and follow the path checking for voltage at each point from the battery with your volt meter or a test light.

[Scrapguy 12]

Thanks to all for the recommendations. Yes, I bought this tractor new in 1976. This is the first time I’ve been stuck on a fix. Reading your replies I realize two things I did miss. While I checked the ignition switch and the wiring to the solenoid I never actually checked for power from the switch. Second, the ammeter going bad just never clicked. Will check the chassis ground today as well. The “tank” as I call it has been a work horse over all these years. Used winter and summer for all 44 years. Can’t retire it now. 

[squonk 44,036]

Ammeter connections. I'm not sure how many times I found this issue but if I had a nickle for every time  I could trade them in for a quarter! 

Edited May 17, 2020 by squonk

[Scrapguy 12]

Well folks I must have flunked multimeter 101! No joy with starting problem. Did as was suggested. Ammeter is not the problem and have voltage from ignition to coil but no turn over on the starter. Had every connection sanded bright and main ground clean and contact. Had the PTO interlock out but not sure what to make of the readings. One hot wire to the grounds  showed connectivity but the other did not. I pushed in the switch and that made no difference. Any suggestions on how to check that switch?The “tank” is winning this battle.

[BOB ELLISON 2,981]

I would bypass the interlock switches by connecting a wire from the solenoid to the (I) on the switch. It's the red wire in the diagram. 

This will bypass both switches. That will tell you if either one is bad.

The other thing is the switch is bad.

Edited May 18, 2020 by BOB ELLISON

[Scrapguy 12]

Thanks for the bypass info. The switch was brand new and the checks I did showed it to be good so hopefully your method shows the PTO relay to be the culprit. Are these switches available.

[Scrapguy 12]

Gentlemen thanks all for listening to my tale of woe and your thoughtful suggestions. The “tank” rides again. Bobs suggestion on how to bypass the safety switches got me back to the investigation and showed how sometimes you see what you want, not what’s there. I was looking at my old wiring diagram in the original parts book and realized I mistakenly missed the red wire from the positive side of the amp meter. It’s green to the negative side but red back to the solenoid. In other words I shoulda taken a picture instead of just undoing the wiring in the beginning. Again thanks all for keeping my mind in the game and especially to Bob and any others of you for your service.

[BOB ELLISON 2,981]

We're here to help you with anything on these great tractors. Welcome to Redsquare.