What are the symptoms of a bad or weak ignition coil?

[Rembrant]

Gentlemen,

I'm going to do some engine testing this week but was just curious what are the symptoms of a bad or a weak ignition coil? Mine is the original 1984 coil sitting on top of the intake in the stock location.

I have never had to replace one before so I don't know the symptoms of a bad one. Do they just outright fail and go from 100% to 0%, or do they weaken over time?

If I was to buy a replacement, do I just get a typical aftermarket replacement? Or is there a better one to install as an "upgrade" that will work with a stock DSII ignition system?

 

[Gary Lewis]

Coils can go from 100% to 0% in a heartbeat. Mine did on my '72 F250. Turned it off and it then wouldn't start. Had a broken wire inside. (Long story.)

But, that was on the primary where there are a few turns of large wire. If a wire breaks you stop. On the secondary where there are lots of turns of small wire it is possible for there to be bad insulation that effectively shorts out several of the turns and the voltage output drops. That gives a weak spark, but a spark nonetheless.

And the symptoms of that are a miss under heavy load. As the load goes up so does the cylinder pressure, and that increases the difficulty for the spark to jump across the plug's gap. If the voltage in the secondary of the ignition system isn't high enough, at some load it won't spark.

As for replacement, the DS-II system doesn't like a hotter coil than stock as that usually means more current for the box to sink, which means more heat. To me, spark is spark. If you have enough spark to light the mix off a bit more doesn't make much difference. I'd go back with a stock style coil. It worked and the box was happy.

[ArdWrknTrk]

Coils can go from 100% to 0% in a heartbeat. Mine did on my '72 F250. Turned it off and it then wouldn't start. Had a broken wire inside. (Long story.)

But, that was on the primary where there are a few turns of large wire. If a wire breaks you stop. On the secondary where there are lots of turns of small wire it is possible for there to be bad insulation that effectively shorts out several of the turns and the voltage output drops. That gives a weak spark, but a spark nonetheless.

And the symptoms of that are a miss under heavy load. As the load goes up so does the cylinder pressure, and that increases the difficulty for the spark to jump across the plug's gap. If the voltage in the secondary of the ignition system isn't high enough, at some load it won't spark.

As for replacement, the DS-II system doesn't like a hotter coil than stock as that usually means more current for the box to sink, which means more heat. To me, spark is spark. If you have enough spark to light the mix off a bit more doesn't make much difference. I'd go back with a stock style coil. It worked and the box was happy.

One symptom of a failing coil is extreme heat.

If the internal insulation starts to break down, shorting coils out of the loop, it will saturate much quicker and become a filament.

Missing and exhaust explosions (as opposed to backfire in the air cleaner) are another symptom.

[Rembrant]

One symptom of a failing coil is extreme heat.

If the internal insulation starts to break down, shorting coils out of the loop, it will saturate much quicker and become a filament.

Missing and exhaust explosions (as opposed to backfire in the air cleaner) are another symptom.

Thanks Gents,

On the same topic, but different question...with otherwise all stock ignition components, what do you guys recommend for plug wires?

I currently have Ford 8mm wires...nothing special.

[ArdWrknTrk]

Thanks Gents,

On the same topic, but different question...with otherwise all stock ignition components, what do you guys recommend for plug wires?

I currently have Ford 8mm wires...nothing special.

I'm rocking NAPA Belden premium wires.

You don't have anything insane like a top fuel magneto.

You don't need anything too special.

[Gary Lewis]

You don't have anything insane like a top fuel magneto.

You don't need anything too special.

Agree. The ones you have should be fine, assuming the were knew with the engine.

[grumpin]

You don't have anything insane like a top fuel magneto.

You don't need anything too special.

Agree. The ones you have should be fine, assuming the were knew with the engine.

You can check your coil with a multimeter.

https://itstillruns.com/test-ford-ignition-coil-5941137.html

[Rembrant]

You can check your coil with a multimeter.

https://itstillruns.com/test-ford-ignition-coil-5941137.html

Cool. Thank you sir, I'll check it this evening.

[Rembrant]

You can check your coil with a multimeter.

https://itstillruns.com/test-ford-ignition-coil-5941137.html

Looks like my coil is still within spec...

According to the link you posted, a good coil should read between 0.4-2 ohms on the primary side, and between 6,000-15,000 on the output side.

My existing 35 year old Ford coil reads 1.3 ohms and 8,460 ohms.

The new aftermarket one I picked up today reads 3.0 ohms and 11,870 ohms.

Maybe I'll install the new one and keep the old one as a spare. I've been meaning to put together a little behind the seat kit with the old truck essentials...lol.

[grumpin]

You can check your coil with a multimeter.

https://itstillruns.com/test-ford-ignition-coil-5941137.html

Looks like my coil is still within spec...

According to the link you posted, a good coil should read between 0.4-2 ohms on the primary side, and between 6,000-15,000 on the output side.

My existing 35 year old Ford coil reads 1.3 ohms and 8,460 ohms.

The new aftermarket one I picked up today reads 3.0 ohms and 11,870 ohms.

Maybe I'll install the new one and keep the old one as a spare. I've been meaning to put together a little behind the seat kit with the old truck essentials...lol.

Sounds good!

IIRC the one on my Bronco was like 13K, a little high I thought. Put the new one on and no more cold start problem!