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So I bought this 1975ish 401c a week ago Saturday. The batteries were weak. Slow to turn over and slow to start. When it runs, it runs like a champ. I put my genuine Harbor Freight multimeter across the terminals while running and it said 12.2-12.5 volts. Seems a bit low. I had my buddy at Autozone charge and test the batteries. They're good. I put them back in the tractor. I bought a volt meter gauge and put it on. I got out the test light and found a wire with keyed power coming out of the ignition switch. Spliced the hot into that. Then I crimped a banjo on the ground wire and started fasten it with a sheet metal screw into the side of the battery box. As soon as it made contact that tractor acted like it was possessed and started its self with the key off. Glad I park it in neutral or my name would be Squish. So I turned the throttle down to an idle and turned the key on and off. Kept right on going. I pulled the screw back out. It ran for another 20 second then died. So I undid everything and put it back the way it was. The electrical system is completely dead now. Before I left for work I took the positive battery terminals off just to be extra safe. I'll deal with it tomorrow. Any ideas where to start? This is my first diesel. Anyone know where to find a wiring diagram?
 

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Is fan belt tight & does fan belt back extend above sides of pulley's? If battery ground cable is still attached to rusty battery box I'll suggest to relocate to starter mounting bolt. Have you checked electrical connections on alternator? IIRC voltage regulator is hidden under the dash. Wiring diagram can be located in JD 401C tech manual.
 

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Today I only had a bit of time to look at it. The ignition switch to to solenoid looks like it's done right. I downloaded a service manual last night for $20. I found why the tractor went totally dead. Someone added a battery shut off switch to the ground cable. It vibrated apart and the ground cable was dangling out the side. Tomorrow I'll have time. It has a serious voltage drain too. I want to find that and fix it too.
 

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If you have a Alternator on the tractor you may have a bad Diode causing a weak charge and a discharge when off. You can check by disconnecting the Positive wire to the alternator and if the draw goes away there is your problem.
 
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