Smoked HEI coil.

[swifty]

Been getting my 72 swinger going and was trying to get a blinker light on dash to work. In the process I noticed the voltmeter was only reading 10 volts. I was checking the ground when I heard a hiss and saw smoke coming out of engine compartment. Coil was smoking. Removed battery terminal and unhooked coil. Hooked battery up and back to 12 volts.

So I started looking and found my tach wire, I have under dash not not hooked up, may have been touching ground. It is an ebay hei ready to run with an e coil. Car worked fine before this. So would this take out coil? And what about hei module? Is there a way to test before anything else gets smoked?

 

[67Dart273]

It should not affect the module IF it was a GM HEI. EXCEPT what you have is NOT a real HEI. There is only ONE HEI and that is GM HEI, so here's the deal:

YES a grounded tach wire could ruin the coil HOWEVER

Some ignition systems cause the coil to draw current when powered, you should NEVER leave the key in "run" for any length of time

Points, if points are closed, will ruin the coil AND the points

Mopar factory breakerless, can ruin both the coil and module

Pertronix, I BELIEVE draws current this way so it is also a fail risk

The system you have? I am not familiar

AND FOR THE GOLD:

A properly operating HEI module DOES NOT draw current with the key left on!!!!

 

[67Dart273]

And now, a story from the old days. . ..........My cousin used to have a '70's Jeep Wagoneer with the breakerless system they used, Autolite? Don't remember. Anyhow, he comes into the store one day and wants a replacement ignition module

Says, "you know what might be causing them to go bad, this is the third or fourth one!!!"

I thought a second, "your kids maybe listening to the radio with the key on, you know, like the engine is on?"
HE LOOKS AT ME REALLY FUNNY!!! and says "how else do you listen to the radio?"

TURNS OUT HE WAS THE GUILTY!!!! Didn't know the difference between "run" and "accessory!!!"

 

[brian6pac]

I have never seen a coil of any kind not draw current with the key in run.
I worked for GM for 15 years and the biggest coil killer was poor quality plug wires, so maybe it was on it's way out and the key on killed it.

 

[Tooljunkie]

With a temp gun, check the things that warm up with key on- coils get smoking hot. Alternators too. ..

 

[67Dart273]

I have never seen a coil of any kind not draw current with the key in run.
I worked for GM for 15 years and the biggest coil killer was poor quality plug wires, so maybe it was on it's way out and the key on killed it.

I don't know why you did not notice this. I'm telling you, a genuine GM HEI does not draw current with the engine stopped. I noticed this when doing my first conversion, a Toyota 20R with factory distributor firing an HEI 4 pin installed in a Cletrac dozer. When I wired up the 67, same thing. It may be that not all replacement aftermarket modules do this.

 

[KitCarlson]

Grounding the coil wire might damage the coil driving transistor in the HEI module. It can be checked with an Ohm meter with ignition off, take HEI wire off the coil (-) , and check continuity to ground. If it measures less than 100 Ohms, the transistor is shorted and module is bad.

Yes the coil is likely damaged. The HEI circut limits the coil current to around 5A. By shorting the tach wire to ground, and no ballast resistor in the circut, the 0.5 Ohm high energy coil will reach 24A. That is about 5 times the normal current, and the coil will burn the enamel insulation and potting material after several seconds. When the tach wire removed, the coil kickback and current release damages the transistor. Energy increases as current squared, so if the current is 5x higher than normal the energy the transistor is absorbing in the protection mode is 25x greater, hence failure. In normal operation the HEI controls peak coil current, and is very reliable.

 

[swifty]

An update. Picked up a new coil from Napa. Put it in car. Checked from negative side to ground with key off, reads open. Hooked everything up . Turned key on, voltmeter read 12 volts. Turned over and fired right up. Checked if getting hot and seemed fine, more heat from headers than anything. May need to relocate. Took around block. Ran fine.

Guy at Napa said there should be a resistor on it, only needs 9 volts when running. I thought hei wanted 12volt all the time or is that just the module?

The original coil is one of those cheap ebay ones. Looked pretty cheap compared to new one. So maybe it just was ready to go and the grounded tach helped it along

 

[KitCarlson]

Lucky the HEI works.

No ballast resistor required, however use one if you have plan to short tach wire again. The plastic poo is from the lead short, not cheap coil. The expensive coil will not last a short either.