Shorted Wire at Starter Solenoid: the real story

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Shorts checker
Jan 24, 2012
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Tucson, AZ
My Car
No Mustangs at the moment.
I had a customer just 2 weeks ago that fried a wire at the starter solenoid.  What was unusual was that it was the I (Ignition) line, typically brown or red/green.  First time I heard of this problem in 13 years of working on harnesses.  Scratched my head a bit, and came up with a reason, but I wasn't too happy with the explanation.  This line burned half-way through the harness.

Yesterday, another customer called me with the identical problem!  After some back and forth, he confirmed that it was the I line that got fried.  This one got fried all the way to the connector to the underdash harness.  What is further unusual, is that both were 1967's (Yes, this is a 7123 forum but hold that thought...)

Both customers had starting problems: #1 ran out of gas, and cranked the car excessively trying to get it to start.  #2 cranked the car, but the starter solenoid stuck (welded itself inside the solenoid).  Hmmm...a clue here!  Both cases had the wire insulation burn as though it was a short.  But...that line goes to the coil, so there shouldn't be a short when one is sending 12V to the coil.  But there is...the coil is only 1.5 to 0.6 ohms (depending upon model) and from there it goes to the points.  Now then, the points are closed (grounded) 7/8ths of the time, and 1/8th of the time they open allowing high voltage to leak from the coil to the spark plugs.  The majority of the time, the points act as a ground.  Aha!  Sending 12V through a 1.5 ohm system yields 8 amps of current through an 18 gauge wire, enough to cause the wire to get hot and insulation to burn off. 

So there you have it...the reason why one is told not to excessively crank the starter is not related to the starter or the battery, but to prevent excessive current from passing through a thin wire for longer than a couple of seconds.  This is also the reason why aftermarket ignition systems want to use 14 or lower gauge wires from the I post to their system.  And this has nothing to do with the year Mustang: they all will exhibit this problem if the starter sticks or you crank the car excessively at one time.

I bet you didn't know this, eh?


Well-known member
May 6, 2019
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My Car
1973 Mach1
Just had this happen 3 days ago. Was trying to check a voltage regulator with the key on , and the Black/orange stripe wire at the solinoid started smoking.  Asked the alternator rebuilder about it and he said fileing/cleaning the ring terminal on the wire takes care of this 90 percent of the time. Found on mine the wire was broken inside the insulation and was probably working on 2 or 3 strands before I took it off for cleaning. Was broken about an inch down from the ring terminal but insulation was still intact. If it hadnt of broke completly while cleaning I wouldnt have known.


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