Starter Solenoid

[1970FordTK]

As the topic states, this is about the starter solenoid. Today I went to start the 100 (1970 ford f-100 360 autolite 2bbl C-6) All elelctical on the truck has been replaced with new including the solenoid. When it would sound started with the key turned to the start position, with it back to the run spot it cut off. Took the wire to the coil off of the solenoid, truck started and ran fine. Replaced the solenoid and it went back to working fine. Just curious as to if anyone would know why after 7 months the solenoid would mess up like that, I mean I could understand it failing going to the starter but I've never had one fail like this. Any ideas from the folks with vast experence?

[3406kris]

The situation you describe is usually indicative of a bad ballast resistor. On the solenoid, the small terminal on the left triggers the solenoid to send juice to the starter. The small terminal on the right is for a points car only, it supplies 12volts directly to the coil to give it a boost to start quickly when the key is in the start position. Once the key returns to the run position, the right terminal no longer supplies 12volts to the coil. When the ballast resistor is bad, no 6v power flows to the coil, so the truck won't run with the key in the run position. When you try to start the truck, the right terminal bypasses the resisted coil wire and it runs, let off the key and it dies.

I'm sure if I had time to ponder this I could figure it out, but I just got up!

[Thunderfoot]

When it would sound started with the key turned to the start position, with it back to the run spot it cut off. Took the wire to the coil off of the solenoid, truck started and ran fine. Replaced the solenoid and it went back to working fine.

Yep, that was the solenoid that caused that... They're actually two switches in one in the solenoid, the one for the starter and the one to boost the voltage to the coil for cold start as 3406kris talked about above.

The switch inside for this function shorted to ground inside the solenoid when it was off instead of being open, so it grounded the coil wire, which kills the engine. Usually when this goes bad they ground all the time and you can't even get the engine started, but it is a semi common thing to happen...

I've had bad experience with new solenoids the last few years, I got 2 new ones from the parts store in a row and right out of the box and they had different problems.

Don't think anything you did caused the problem. Hopefully the replacement will last / work better for you.

[1970FordTK]

Thanks for the quick reply guys. Good to see you back Thunderfoot, guess I'm just use to the good old Made in USA stuff that doesn't exist anymore. I take that back, my living shop manuals here online are still mostly Made in the USA. (And the Denis Carpenter stuff at the other end of the state) For under 15 bucks guess it might be a good idea to keep one in the tool box on the truck. Once again thanks for the info, I'm with you though Thunderfoot not use to one failing like that. Guess its my old truck warning me about a problem before it would strand me.