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My 1978 C121 will not start. It will not crank over. The voltage at the battery post on the key switch is 12.3 volts. When I turn the key to the run position the voltage at the battery post on the key switch drops off.


I tested the clutch switch. good

Replaced the key switch.

Seat switch is bypassed. 


I am suspecting the PTO interlock switch but I have no idea how it works and how to test it. I am very mechanically inclined but I have never seen a switch like that before. 


Help Please.







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Voltage dropping off is an indication of bad connections. Before you start replacing anything, clean and tighten all electrical connections. If the problem remains you can take a small jumper wire from the battery positive to the small terminal on the solenoid which should actuate the solenoid and starter. 

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I replaced the wire connectors for the key switch. What other connections do you suggest I clean?

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6 minutes ago, kcuttuck said:

What other connections do you suggest I clean?

All of them! The battery cables (+ & -) at both ends and the cable from the solenoid to the starter. Any corroded or loose connection anyplace in the electrical circuit will result in voltage drop.

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The C-121 is a cradle motor and has an extra ground wire on the cradle. Make sure it is connected well.  I've seen vibration of the motor take a toll on them.

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I found a bad amp meter. It showed power though it till there was a draw. 


Thanks for the help.

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You can bypass the ammeter by placing both wires on the same terminal. Disconnect the negative battery cable before you do to prevent an accidental short.


A more economical repair is to replace the ammeter with a voltmeter and it will give you more useful information. A voltmeter needs to be powered by the ignition circuit plus a simple ground to the chassis.



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