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 Cougar 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 or Cougar: they all will exhibit this problem if the starter sticks or you crank the car excessively at one time.
That’s good info! And the very worst case scenario is if you are holding the ignition switch in the start position, but the engine is not turning over. If the points happen to be closed, they will stay closed since the distributor shaft is not turning. Now you are getting the full 8A through this wire.
The stock coil for all 1967 - 73 Cougars is rated at 1.5 ohms. The “I” wire from the solenoid is capable of handling that amount of current (E=IXR) without melting for a good long while. If everything is stock.
If someone installs a lower resistance coil in this circuit then trouble like you are describing is to be expected. Examples include Pertronix and Mallory and Accel - there are / were many other brands that have equally disastrous results.
Of course - that makes perfect sense. If the coil resistance drops from the factory 1.5 ohms (mine measures 1.3 ohms) down to 0.66 ohms, that 8A max current through the I wire (wired direct from starter solenoid on ammeter cars) becomes 18A max which would overheat an 18 awg wire real quick. And the damage would likely occur where the wire is embedded deep inside a cable harness without air to cool it.
I know right? Everyone seems to forgot the cause and effect that happens when switching parts and often only do part of the equation rather then the whole package and open themselves to other issues. Nothing wrong with upgrading stuff but it does need to be well thought out and done right. I know plenty of people that just start tossing go fast parts on their stuff only to cause more issues and no go fast results.
What I did want to say however is this is a good PSA for carrying a fire extinguisher !
Yes! I was just thinking the same thing about carrying a fire extinguisher. I have one in our 2019 Highlander, but none in my Cougar. That wasn’t very well thought out.
