Hey everyone, I need help trouble shooting an electrical gremlin. The truck is a 79 F100, 351M, C6, 2wd, reg cab. I have changed nothing electronic in over a year. The truck did sit for little over a month without being started. Two or three drives after a month off, the ignition system will suddenly and randomly shut off. Everything else remains on, dash lights, radio, headlights, driving lights, EVERYTHING. The engine cuts out so suddenly it's as though I turned the key off. At first it would cut out for a second or two, then fire back up without me doing anything. It would only happen once or twice, then the rest of the drive it would not happen again. Today on the way home from work it did it a few times and the last time it was off for about 30 seconds and then a huge backfire occurred. Maybe I shouldn't call it a backfire. A very loud explosion sound went through the exhaust. It sounded like a gun shot and blew my exhaust tip off. It was just clamped on, not welded. When I got home I inspected under the hood. While the truck was still idling I felt the ignition control box and it was not hot to the touch. I touched the ignition coil and it was hot enough I could not leave my fingers on it for more than a few seconds. Coincidence or not, while I was touching the coil, it cut off again. I removed the rotor cap and everything there seemed fine but I noticed a crack in the distributor. Not the cap, but the plastic "collar" that goes inbetween the metal body and cap. After reading all of that(sorry), my question is what do I test first, and how do I test it? The ignition in the cab, the ignition control box, the broken distributor part, replace the entire distributor, the ignition coil? Any help pointing me in the right direction is greatly appreciated.



-Matt

 

The backfire that you speak of could be replicated by starting engine, revving it up, kill the engine, immediately put key back in on position, the engine sparks and boom backfire. I've seen it blow apart mufflers before. In other words the backfire is probably incidental to you losing spark intermittently.

The coil will be warm, but yours sounds like it is more than warm. They are oil filled for cooling, does it look like coil has leaked? If that's the case you should replace it. You can test the coil too.

Another common cause of intermittent loss of spark is the ignition control module. It will show symptoms as you're describing before it totally fails. The ICM can be tested, but it is also an item that I like to keep spares of so it wouldn't hurt to get a spare regardless of how yours tests. just to have it on hand.

I would certainly replace the cracked part on your distributor. I don't think you need to replace the entire distributor, but take a look at the cap and rotor and check for pitting on the metal parts. If you have any it's not a bad idea to replace them.

While you're at it, check your spark plugs and maybe replace those too if you're replacing other components.

 

I really feel bad for you guys who have to deal with the Dura Spark ignitions. Those things were developed to deal with the smog regulations of the 70's. If you live in an area where you can get rid of them, go to a more simplistic ignition. Like the GM HEI or Portronics. My 2 cents.

 

See how many of the 70's HEI dont have problems with age

First I got to say any parts you buy DO NOT go for the cheapest ones spend a little more for a better part.
It does sound like what the ICM dose before it fails so I would give that a try first. You dont even have to bolt it down just unplug the old and plug in the new and go for a test run.
I am running a top of the line NAPA ICM and only changed it because the potting was dripping out of the old one and running down the inner fender onto my garage floor.
If you find the ICM did nto fix the issue you now have a spare to keep in the truck.

I also dont like that hot coil as I dont think I have ever seen one get that hot before. I would also pick up one to swap in but only swap 1 part at a time.
If you swap both and you have problems what part caused it?

A month back my truck also cut in and out like yours and it never did that before so was like?
Being it would not restart I pulled the 2 plugs apart to the ICM and found them wet, I washed the engine bay a week before, and some green stuff.
I got the truck home and cleaned the green stuff out and been good since so check the plugs too.
Dave ----

 

Thank you all so much for the replies and advice. In my area a motorcraft ICM is $125! I've read before that motorcraft is really the only brand we should be using, and I agree with that. Also, I can not find the cracked distributor part ANYWHERE. Only the cap, internal components and metal housing. With that in mind, should I get a new ICM and new distributor of the same style/type or go with an HEI distributor? The price difference would be minimal(if not cheaper for the HEI), but how do I go about wiring up the HEI version? Does the HEI distributor eliminate the ICM and canister style coil? I am pretty sure the backfire damaged my O2 sensor, so there's more cost to this repair. I say that because when the truck is running, it drives/feels the same as before but the gauge is reading 3 points higher under light acceleration BUT reading the same as before under moderate and heavy acceleration. Thanks again for all the help. WHEN I get this all figured out I'll post a video to show y'all what ya helped with. I know how much everyone loves pictures and video.



-Matt

 

To find the replacement for the cracked part, look for "duraspark cap adapter"

Regarding motorcraft ICM I would order it online, the last one I ordered was from bronco graveyard if I recall correctly but summit racing and rock auto probably carry the motorcraft part too

If you're going to go HEI do not cheap out, the cheaper HEI conversions out there do not modify the mechanical advance, it's left factory GM, the problem with that is Ford distributors turn the opposite direction to the GM distributors and the mechanical advance won't work properly. You can also run into clearance issues when you try to adjust timing and fit the air cleaner. In my own experience I've had more issues with HEI distributors and their modules as well as advance weight bushings on GM vehicles than I ever have with a Ford duraspark set up. Due to that, I'd never run one on my Ford, instead I'll keep a spare ICM handy and stick with duraspark. If you do go the HEI route you'll want to keep a spare module for that too as they can and will fail like any other ignition module out there. I think GM are more prone to failure due to heat being inside the distributor but just my opinion.

What's the O2 sensor for? Did you convert to a bolt on fuel injection setup?

The canister coil is null and void with HEI as the coil for those are bolted to the distributor cap. So there's still a coil is just all in one with the distributor. The module is built into dizzy too so you still have all the same components, GM decided to combine them all into one unit, Ford uses separate components as you've seen.

 

Thank you so much for that distributor part name! I will absolutely look for those parts online. I put in the O2 sensor and vacuum gauge to help me tune the carburetor, I swapped to an edelbrock 4 barrel intake and a holley 650. It has helped tremendously! Gentlemen, I greatly appreciate it. I now have something to look forward to this weekend. Hopefully I report back Monday with good news. Be safe and God bless!

 

I'm glad we could help.

I'm in the middle of an engine rebuild on mine and when the engine goes back in I'm installing an A/F ratio and vacuum gauge too for the same reasons you did.

Before you grab a new O2 sensor though I would get the ignition problems fixed. Any A/F ratio reading will not be accurate with the ignition not working a 100% correct. You're going to have misfires and plugs could be getting fouled. Once you have your problems sorted and go through basic tuneup steps such as sitting ignition timing and carb adjustments you may see it go back to what you saw before. It might just need to get nice and hot to clean the O2 sensor. There's obviously raw fuel getting in the exhaust right now otherwise you wouldn't have the backfire issue.

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I have ran duraspark for majority of my life and have enough problems that I can count them on one hand. I have experienced more problems with every other ignition system. I think the duraspark to hei is generally caused by the operator and/or mechanic. just my input. They have been working fine for 50 years for my vehicles. Find me an hei that outlasts that. as for the spark interruption My personal experience has yielded 2 things most common to this issue. pickup coil inside distributor or ignition switch contacts. Moisture is always the first thing to check for,

 

"After reading all of that(sorry), my question is what do I test first, and how do I test it?"

Here's how you can test the ignition system. Not mentioned is looking for damaged or loose wires. Also, test the pickup coil / stator cold, then test it again after heating it with a hairdryer. Often they will pass when cold, fail when hot.

In my youth, my friends & I would do muffler explosions on our winter beaters for our amusement. Roll down the road at 45, turn the key off to kill the ignition & fill the muffler with a perfect air fuel mix. Turn the key back on & be rewarded with a horrendous explosion. These were great for curing dogs of chasing cars, or for getting tailgaters to back off. Your ignition shut off completely, the muffler filled with air fuel, then the ignition came back on, BOOM!

 

OH MAN! These pictures are amazingly helpful! I ordered my parts(ICM and distributor collar) to late on Friday so they arrived today 🙄. BEFORE I replace anything I’m gonna follow those directions and see what I get. I’m gonna keep the parts I ordered, a backup ICM is never a bad idea and I know the collar is cracked. I’ll update when I can. Thanks again guys.

 

Update time.

AFTER all my testing I realized it says up to ‘78, I have a ‘79. I followed the steps in this pictures and I have good news and bad news. Step 1: I got no ohm reading. I changed the meter to continuity and nothing. The key is not in the truck, the directions say turn the key to “off” … that’s the same thing right? Step 2: good results according to the manual. Step 3: nothing, no ohm reading or continuity. Step 4: good/passing results. Step 5: nothing. Could not get an ohm or continuity from either post to the tower. Step 6: good/passing results. So *exhales nervously* does my year being different cause the difference in my results or is my stator bad and need to be replaced? What about the coil? Does it need to have power applied to get a reading from pos or neg post to the tower?

SIDE NOTE: I noticed the reluctor wheel was a little rusty so I used 2000 grit sandpaper to clean it up.

 

There shouldn't be any difference between 78 & 79 model years. I think they were pretty much the same '75 through 82ish?

If I understand correctly your coil is testing okay and your getting 1.13 to 1.23 ohm resistance on the primary side (smaller terminals) and 7700-9300 ohms on the secondary side (terminal that connects to distributor) correct? I would also double check that you have high resistance between primary and secondary side. If you have any resistance at or close to 0 ohms between primary and secondary on the coil you could have an internal short and that would explain excess heat if the oil hasn't leaked out.

Regarding the pickup coil and stator, key out is the same as key off. If you're not getting 400-800 ohms as tested at the distributor pig tail, it's a bad pickup coil based on what I'm reading. If you have good readings there, reconnect the distributor, unplug the wires at the ICM that go to the pickup coil and test there. If the pickup coil tested good, but you're showing very high or very low resistance (outside the range that is spec) at the ICM connection point, that indicates you have an issue in the wiring between the ICM and distributor.

Does that help?

 

I want to clarify. Between the orange and purple wire on the distributor, I get no ohm reading on the meter. Between the black wire and ground I get an ohm reading within spec. On the coil between the positive and negative I get a reading within spec. Between the positive and the secondary I got nothing, not zero ohm, I mean nothing as if there is no connectivity. Same between the negative and secondary. I just went and started it up and it ran, maybe not got, but it seemed to run fine. By that I mean I didn’t run it long enough to warm up so I had to play with the throttle to keep it running.

 

Regarding ignition coil, infinite resistance between primary and secondary is good, if you show 0 or low resistance or really any resistance between the two your primary and secondary windings inside the coil are shorted to each other and the coil is bad.

​​​​​​On distributor, touch orange wire with red probe and touch a good engine ground with black probe, you should have infinite resistance, anything less, pickup coil bad.

Next touch purple wire with red probe and black probe to engine ground, should be infinite resistance, if not, bad pickup coil.

If either orange or purple to a good ground show any continuity, which means zero ohms or anything less than infinite resistance the pickup coil in distributor is bad.

You should see a resistance reading close to zero resistance from black wire to good engine ground. If you have high resistance here there's an issue with the ground and you may have to clean up some rust so the pickup coil has good contact with distributor body and is properly grounded and check that the ground wire doesn't have a break in it.

From purple to orange with your meter should show 400-800 ohms, if it doesn't, pickup coil is bad

If the pickup coil tests good when cold, get it warm and test again, if you have a heat gun or hair dryer as scottscott stated, you can use that to warm up the pickup coil to see if any of the readings change.
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