Topic: Shorted Wire at Starter Solenoid: the Real Story

[midlife]

  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

 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?
 

[jwc66k]

My 66 A Code Hardtop required a lot of cranking too. The engine would turn over but would not start, until you stopped cranking, then it would "catch" and run. I eventually replace the solenoid and took the old one apart (a nasty habit I've had for decades). There is a copper "disc" inside the solenoid that connects the battery to the starter when the ignition key is turned to start (aka - a relay). There is also a bare, braided (18ga?) wire from the disc to the terminal that provides 12 volts to the coil only when starting. That wire was broken. There is no resistance test that will tell you that there is a lack of continuity (an open circuit) for the start terminal on the solenoid. A voltage test shows "some" voltage there, so you are stuck. Replacing the solenoid is the only option. That was 33 years ago.
Jim 

[midlife]

Interesting.  I had a similar problem when I first got Midlife, but traced the problem to stripped bolt at the starter itself, so while it did crank, it pulled the voltage down so low that when I released the key, the normal ignition voltage at the coil was too low to keep the car running.

[Bob Gaines]

  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

 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?

Very interesting,I hadn't heard that explanation before. With appreciation I will file that in my data bank in case something like that comes up in the future. I will also use caution and recommend caution to others about excessive cranking for the reasons you described.

[gjz30075]

Randy, you have me nervous now.    Our cars usually sit for lengths of time so excessive cranking usually is  needed, so, what do
you think 'excessive' means here?

[carlite65]

after sitting for a 'long time' if you prime the carb you will not have to crank so much.

[midlife]

Randy, you have me nervous now.    Our cars usually sit for lengths of time so excessive cranking usually is  needed, so, what do
you think 'excessive' means here?

Excessive: more than 10 seconds at a time.  After 10 seconds, rest 15 seconds and try again would be my recommendation. 

[Bossbill]

Here are the guts of a 2701966.
It has a spiral, concentric ring of copper that connects on the "I" side that flexes as the disc is pulled into contact with the battery and starter terminals.
There is no date, but it is a Ford original.

[ruppstang]

Here is the results of a different solenoid failure. I pulled this car in the shop to change the alternator and adjust the choke. It was a 289 3spd manual. I had disconnected the battery then changed the alternator and then adjusted the choke. I then started to reconnected the battery cable when it touched the battery terminal there was a pop and a arc that welded the connection. I was stunned that the engine was cranking and moving the car backward. Just when I thought it could not get worse it did the engine started and the fast idle that I had fixed was working well. The car now was dragging me across the shop so I tried to pull the coil wire off but only got a plug wire. I fell off and the car and it went through the shop door. Luckily it got hung up so it did not get far before I did get to the coil wire. I first thought I had left the key on but found them on the floor mat. I thought how could this happen? I then discovered that there is power to the coil any time the engine is cranking. The solenoid must have welded on my last start or when I reconnected the battery. This was in 2009 and it was a hard lesson on solenoids and not chalking the wheels on your car when working on it. I was really lucky it was in reverse or it may have got me against the wall. I post this to remind everyone to be safe and not over crank the solenoids.           

[midlife]

Ouch!

[Bossbill]

Ruppstang -- It could have been worse. Like going over that driver's side drop-off.
Still. Wow.

[jwc66k]

The lesson here is: Use the parking brake and NOT THE MANUAL TRANSMISSION IN A GEAR to hold the car when working on it.
(It's called "experience".)
Jim

[ruppstang]

Ruppstang -- It could have been worse. Like going over that driver's side drop-off.
Still. Wow.

Yes and my work van was at the bottom of the ramp.

[midlife]

Ruppstang: I'll bet you checked YOUR shorts after that!

[J_Speegle]

.................... I was really lucky it was in reverse or it may have got me against the wall. I post this to remind everyone to be safe and not over crank the solenoids.           

Went on a years ago were a guy was trying to jump the solenoid while standing in front of the car (garage behind him) sitting in his driveway. Hit the switch - car lunged forward pinning him between the car and door. lucky for him that the car didn't start or the switch didn't malfunction  and was left with bumps and bruises but no broken bones that time. If I recall correctly it was a 67 coupe

Seen a lot worst - stupid and unlucky incidents. Kept things interesting and why I found truck work more interesting than engine work (fire department)