Fried Coil issues

[blue 74]

ELECTRICAL BUMMER, The story begins a year ago when for an unknown reason my coil exploded. Thinking it was a bad coil I purchased a new one and within an hour of driving that one also cooked. Frustration set in and I decided to buy a Malory unilite distributor, new coil and just to be safe I also replaced my alternator. All was well for a year until today when the bronco stopped running. After diagnosing that there was no spark from the coil wire to the distributor I went and purchased another coil and blast resistor. After install it fired right up and I started driving home. 10 miles down the road it died. Again the coil was cooked.

I have a 74 302 running a Holley EFI system and AOD trans. The alternator is a 130 amp G3 (I believe) and as per the Mallory distributor installation instructions I am running a blast resistor between the coil and the distributor. Painless wiring harness. No electric fans. Its a very basic system

All the lights and electrical seem ok, no hot or melted wires. After the bronco was towed home and sat for a couple of hours, I put yet another new coil in to see if it would start and I reinstalled the original blast. It did but only ran for 10 seconds or so and then went back to no spark. I am reading 3 OMS from terminal to terminal on the blast so I don't think its bad. Could this issue be the voltage regulator on my alternator?

Is the coil burning up due to a spike in volts and if so where could it be coming from? Alternator? Ignition? Gremlin? HELP!

 

[Jason72Bronco]

Did I read correctly that you are running a ballast resistor between the coil and the distributor? That is not correct.

The ballast resistor should be inline with the wire from your ignition switch to your coil's (+) post. (Factory wiring had a resistance wire from the ignition to coil that served the same purpose.)
If you are not running any kind of resistance from the ignition switch to the coil (either a ballast or resistance wire), you need to. The coil should will work fine without it for a while, but Mallory is very specific in their materials that without it the ignition unit will fry. Here are the Mallory instructions. See page 3 for ballast location. https://documents.holley.com/frm34163_29440.pdf

I know the diagram is a bit confusing because they show the 12v from the ignition tied into the 12v going to the distributor, but the resistance (ballast) is to be provided to the 12v before it gets to the coil, not before it gets to the distributor. Put another way, it is the coil (+) post that needs the "resisted" 12v to it.

Honestly, I'm not sure what placing the ballast between the coil and the distributor does to the whole setup.

 

[blue 74]

Sorry about that, I must have typed it wrong. The ballast is before the coil.

 

[Viperwolf1]

You might unplug the alternator to get it running and possibly take it to a parts store to get tested. Sounds like too much voltage.

 

[blubuckaroo]

I wonder if someone has mis-wired the ignition circuits. Typically the coil gets a full 12 volts while cranking, and a reduced voltage while running. I believe the switching for this is done by the starter solenoid.
Check the voltage at the coil while the engine is running.

I have had a number of fried coils caused by my sons sitting and listening to tunes with the ignition switch in the run position, rather than switching it to the accessory position. Coils don't like to be energized without the engine running.

 

[Rustytruck]

Make sure there is no voltage at the coil with the key off.

 

[Jason72Bronco]

You might unplug the alternator to get it running and possibly take it to a parts store to get tested. Sounds like too much voltage.

This is an interesting theory...it actually could be a hyper-alternator killing the coils. Test voltage at battery posts while running and the alternator has switched on (may need to rev it). Ideally it should be around 13.8v. If not, supsect alternator, or possibly voltage regulator.

Also test the voltage at your coil (+) post while it is running. It should be 12v to start out, but when running it should drop to about 8-9v....that is the purpose of the ballast resistor or resistance wire! (The starter solenoid does not control this.) If you are getting more than 8-9v volts at the coil (+) post while it is running, you certainly may be blowing out the coil, whether that be a bad alternator or voltage regulator or something else.

Bench test your coil with the car off. First note where wires are on your coil posts and disconnect them, and your coil to distributor lead (spark plug end-type wire). You can leave the coil mounted in the truck for this "bench" test, but the readings will be affected if anything else is connected other than your tester. The Ohm reading across the (+) and (-) posts should be between 0.4 and 2 ohms. This is the primary wire reading. Next, test across the (+) coil post and where the coil-to-distributor wire plugs in. (Secondary wire) Should be between 6,000 - 10,000 ohms. Otherwise coil is bad.

And I concur with two prior posters...be certain that your coil is not getting 12v with the key out and cooking overnight. If you did a lot of new wiring, anything is possible.

 

[Viperwolf1]

You won't always see (probably rarely see) battery voltage at the coil before starting. It depends on what state the distributor is in. Points open and you'll see it, points closed and you wont.

 

[Skytrooper15]

I have a stock ignition on a 1970 302 engine.To check I take a volt meter and hook the positive lead to + on coil and ground the black lead from the volt meter.Usually I get battery voltage about 12.7 or so with the ignition switch on.Then I crank the engine and it may take 30-45 seconds and coil voltage starts dropping down to about 6 or 7 volts where it stays while running.The ballast resister is built into the wire and is between run on ignition switch and + on the coil.I still have the original yellow top coil so it must be right.Hope this helps