ignition short

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Nov 11, 2012 | 06:13 PM

lil badger 97

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ignition short

I have a 77 ford truck with a 351 ,the ignition system keeps shorting out, killing spark. If i replace the starter solenoid, it works for a while, then goes out again. I have replaced 4 solenoids this month. Does anyone know what could be shorting the solenoid out? Had the truck for 8 yrs. now, never had a problem with it till now. It has a mallory dist. it was in when i bought it. Thanks.

Nov 11, 2012 | 06:37 PM

fmc400

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From: Austin, TX

Can you explain what you mean by "shorting out," and what specifically is telling you this is happening? Saying something is "shorting out" with limited detail is rather vague.

Nov 11, 2012 | 07:23 PM

lil badger 97

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Sorry about that. the engine turns over fine ( new battery, starter, alt.) I checked the coil / spark plug wire, no spark. put a new solenoid on, fires right up. Something is happening to the ignition side of the solenoid. it runs fine till i shut it off. then won"t start again, till i change solenoids.

Nov 11, 2012 | 08:11 PM

Franklin2

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The power from the ignition comes from the keyswitch. Do you have 12v on the + side of the coil with the key in the run position?

If your solenoid has the two small terminals, one of them does go to the coil, but it's just the coil resistor bypass. It just sends power to the coil during cranking, but it still should have power coming from the keyswitch also.

The keyswitch does get it's power from the battery side of the solenoid, but if that wire went bad, it should kill everything in the truck. Domelight, radio, everything.

Nov 12, 2012 | 01:15 PM

fmc400

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From: Austin, TX

A couple minor and respectful corrections:

Quote:

Originally Posted by Franklin2

The power from the ignition comes from the keyswitch. Do you have 12v on the + side of the coil with the key in the run position?

For Duraspark II (factory for the OP's '77), you will see 6 to 7 volts at the coil with the engine in RUN because the ignition module keeps the primary circuit closed with the engine at a standstill.

Quote:

Originally Posted by Franklin2

If your solenoid has the two small terminals, one of them does go to the coil, but it's just the coil resistor bypass. It just sends power to the coil during cranking, but it still should have power coming from the keyswitch also.

The coil positive does not receive any power from the key switch in START, only from the 'I' post of the solenoid. The coil bypass circuit has its own hot-in-START pole because it is hard-tied to the coil positive terminal, and therefore any other loads on this special hot-in-START circuit would get back-fed through the ballast with the key in RUN. I might have misinterpreted what you said.

Agreed that I would check power at the coil with the key in RUN to first identify why you are not getting spark. If that looks fine, then make sure you have battery voltage at the coil with the engine cranking.

Nov 12, 2012 | 02:13 PM

Franklin2

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The newer systems have the coil bypass incorporated in the ignition switch, but since he has a 77, it might be incorporated into the starter relay on the fender. I am not sure when they switched over and started using the bypass in the ignition switch.

Nov 12, 2012 | 08:42 PM

lil badger 97

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Thanks guys, Just bought a new ignition switch, I"ll check the + side of the coil with the key in the on position first , then while i"m cranking to make sure it has power.