Shorted Wire at Starter Solenoid: the real story

[Midlife 814]

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 69/70 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?

[RPM 1,190]

How did you know I didn't know that?

[Midlife 814]

My middle name is Carnack...

[danno 128]

I understand exactly what you are describbling. The wire continues to the underdash harness, ( at least it does in a 69/70) so the section in that also probably overheated?  If my car was not in winter storage, I would measure what the current actually is in this wire.  Perhaps a fusable link would be a good idea?  

The purpose of the wire is to provide a higher voltage during the starting process, but I really wonder if it is necessary any more?  Maybe it helps to start the car when it is zero degrees and the battery is weak, but those situations never occur to a classic car owner. 

[69RavenConv 286]

Ahhh, so you're saying running out of gas can burn up your harness!! It's good to learn new things at my age. Now what are these "points" things you're talking about?

 

J/K, I really did learn something new. Nice explanation, and I'm kinda surprised it doesn't happen more often. 

[Midlife 814]

I think it does happen more often, but I think folks won't say so on the Internet due to embarrassment.  This also is a big problem with the AMK Concours Starter Solenoid, which has the sticking problem and should be avoided at all costs.

[Ridge Runner 1,112]

I used to go to the wrecking yards a pull original coils ,starter solenoids ,and voltage regulators ,if the regulators were not burned out those old ones were adjustable  

[Mach1 Driver 560]

On 3/23/2021 at 9:29 PM, Midlife said:

I bet you didn't know this, eh?

Yeah as mentioned above, a few of us know the ins-and-outs of Ohms Law. A Pertronix Flame Thrower coil can have a primary resistance of 1.5 ohms, but something like a MSD Blaster could have 0.70 ohms primary resistance. The former giving 8 amps of current flow, and the latter over 17 amps, with no way for the harness to handle it. But they sell this stuff and unsuspecting customers have problems, not realizing what they are doing. It should be noted that the coil will draw it's most amperage at idle (or starting) and as the engine speeds up, the points are closed less for a given period of time, actually lowering the coulombs (amps over a period of time). With those low resistance coils, the "run" circuit through the ignition switch and resistor wire should also be considered.

[Midlife 814]

A poster on the concours Mustang site said he had a bad starter solenoid and opened it up to see how it worked.  He found an 18 gauge wire hooked to the I post internally.  Yikes!  It's a wonder that starter solenoids don't have reliability issues like turn signal switches, horns, or points.

[danno 128]

On 3/25/2021 at 8:41 PM, Midlife said:

A poster on the concours Mustang site said he had a bad starter solenoid and opened it up to see how it worked.  He found an 18 gauge wire hooked to the I post internally.  Yikes!  It's a wonder that starter solenoids don't have reliability issues like turn signal switches, horns, or points.

There is probably a reason why they have an 18 gauge wire internally.  That small section of small wire is the fusable link. It is supposed to blow before the wire burns up. Maybe newer solenoids have a larger connection to the I terminal, and that is why we are now burning up the wires instead of the fusable link inside the solenoid>>> 

[danno 128]

On 3/25/2021 at 1:32 PM, Mach1 Driver said:

, actually lowering the coulombs (amps over a period of time). With those low resistance coils, the "run" circuit through the ignition switch and resistor wire should also be considered.

Come on Coulombs?  Although what you said is true, I only knew of coulombs in college courses on capacitance.  The 3 of us on this post are probably the only 3 people in the forum who know what you are referring to. 

[Mach1 Driver 560]

6 hours ago, danno said:

The 3 of us on this post are probably the only 3 people in the forum who know what you are referring to. 

Yeah but we're having fun! It is a solid reference though- how long you have the amps makes a big difference... besides, I went to school with Charles. 

[Midlife 814]

Coulombs: definition: an electrical term which is a contraction for Cool Bombs, which happens when you rub your cat with a rubber balloon and get sparkies.  It's the bomb , for sure!

[Mach1 Driver 560]

12 hours ago, Midlife said:

Coulombs: definition: an electrical term which is a contraction for Cool Bombs, which happens when you rub your cat with a rubber balloon and get sparkies.  It's the bomb , for sure!

...the triboelectric effect, when a tribe of electrons band together to produce Cool Bombs. No cats are harmed in this process. A glass rod works really well too, instead of a balloon, but it looks silly to rub your cat with a glass rod (which looks like a wand), unless you're someone like Samantha Stevens. And lets face it- she was the bomb! But then she usually just used a nose wiggle. Don't know how that one worked.

[RPM 1,190]

Who are you guys? One time when doing electrical work on my 69 I used a horizontal screwinator where a Hensley was needed. 

[danno 128]

On 3/29/2021 at 12:57 PM, RPM said:

Who are you guys? One time when doing electrical work on my 69 I used a horizontal screwinator where a Hensley was needed. 

That is ironic, I used a screwinator to generate static on the alley cat.  Everyone knows a Hensley will not work for that, you can harm the pussycat. 

[jjstang 51]

I can’t believe I read the whole thing.... 

after the first couple posts, it went downhill fast.

[skidmarky 50]

12 hours ago, jjstang said:

I can’t believe I read the whole thing.... 

after the first couple posts, it went downhill fast.

Me either.....when I saw Randy was involved I should have scrolled by....lol

[Midlife 814]

Oh man...you guys are killing me!

Since when was it not custom to take a thread off the rails?

[danno 128]

20 hours ago, jjstang said:

I can’t believe I read the whole thing.... 

after the first couple posts, it went downhill fast.

"life is about going downhill fast" ... as stated by Billy Kidd

[Mach1 Driver 560]

In case you weren't aware, this is the effect that Mid and I have been jokingly eluding to: