Understanding My Ignition Coils

[Larry M.]

I don't know much and I'm sure my questions will show that but a little new info can help a lot.
My car is a 64 L84 F.I. The specifications in the shop manual only show an 091 coil but I believe the 087 is correct for base and L75. The chart shows (Primary)/(Secondary) coil resistance ranges as (1.02-1.13)/(8000-10500)

The chart shows a ballast resistor of 1.4-1.62 ohms

1. My 091 coil(old and potentially original to the car) measures 1.3/9170. I quit using this coil because by process of elimination I thought it was the source of an engine falter under minor pressure. I put another coil on and the issue went away. Up until this point I've assumed that the coil breaks down with current and heat...what are the common failures of a coil? I'd love to find out this expensive coil is good. Do the OHM numbers indicate an issue on this coil? I was using a 0.2-0.3 ohm ballast resistor(a 1385) with this coil, I think because that is what an expert and this forum have advised me. Would higher ohms on the BR per the shop manual have been better?

2. I put in an NOS 087 measures 1.6/7940 and a 7154 BR at 1.8 ohms. Car runs fine.

3. I have another NOS 087 at 1.6/8150. Can anyone tell me what the specs or ranges should be...maybe from the 65 shop manual.

4. Lastly, I have a repro 091 which measures 2.2/12200. I had this in the car with the 1931385 BR and it ran fine. I've been told these are poor coils and a primary reason is low ohms, But this one is over the top on ohms. Does this indicate I have one of the better units or what?

I guess my overarching question is whether these coil ohm readings tell me much for normal street driving?

[Duke W.]

Re: Understanding My Ignition Coils

The 091 coil/0.3 ohm were used on all '63 engine until mid-production when the 087/1.8 ballast was substituted on 250/300 HP engines. The SHP/FI engines continued with the 091/0.3 ballast through the end of '64. For '65 all engines other than those with TI were equipped with the 202 coil/1.8 ohm ballast.

The 091/1.8 combination offered higher primary current for more ignition energy, but there were a lot of customer complaints about burned points. and it happened to my 340 HP SWC circa 1965. It was more of a problem in cold weather.

Chevrolet wrote a TSB on the subject, and I believe it's posted somewhere on this site or the CF. I recall it suggested swapping the 0.3 ohm ballast for the 1.8 in the winter. That gave me a chuckle. I wonder how many people actually did that.

Using the 1.8 ohm ballast will help prevent burned points, and I don't think it would be a problem to use with either the 091 or 087 coil.

The coil is basically a transformer. We can measure primary and secondary resistance, but it takes specialized equipment to measure inductance. There is anecdotal evidence that some coils, both OE and aftermarket misbehave when hot, so it's a good idea to measure resistance when hot on the car or heat it in an oven to about 150 degrees and test resistance.

You can buy an inexpensive functional replacement at any auto parts store, so it might not be a bad idea to buy one and test and then keep it in the car as a spare in case you get getting stranded.

Duke

[Larry M.]

Re: Understanding My Ignition Coils

Duke...thanks. Sorry to get you to write this again. I searched "coil specs" and learned a lot answering most of my questions but your suggestion to test hot was not one of them so thanks again. I think in the second paragraph you meant to type 091/0.3 but I understood because I read your other stuff last night.

[Duke W.]

Re: Understanding My Ignition Coils

Right you are... thanks for pointing that out. The "edit" function is no longer available a day later, but as you say, it should be "091/0.3 combination".

Duke