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Can a faulty coil cause AM radio interference? Intermittently, I get AM radio static that increases and decreases with engine RPM......not all the time just occasionally comes and goes, but only on the AM band. Coil? Plug? Radio? (1998 stock Becker radio) ...........ideas?

More likely EM interference from the alternator.

Quote:

Originally Posted by Rob175 (Post 643228)

It could do. A spark gap is a simple form of transmitter, think of lightning, my lightning detecotr picks up the noise to about 4,000 miles away, mainly in the kiloherts region, so below AM band. Spark noise is amplitude in nature so causes much more problems on AM (amplitude modulation, low frequency in KHz) than FM (frequency modulation, high frequency 88-10MHz).

If the coil and plug is working correctly then the spark is inside the engine and is therefore metal shielded, it should give some suppression.

If the coil is breaking down some of the [spark] energy could be realesed across the coil body, outside the engine and it would create more electrical noise.

Years ago you used to have to fit a capacitor/suppressor across the power lead to the radio, although this might have been to attenuate noise from the alternator as suggested by JFP.

Quote:

Originally Posted by Rob175 (Post 643228)

This might be your problem.

https://www.youtube.com/watch?v=UBYUnN_n3zY

Quote:

Originally Posted by DrRobin (Post 643225)

When you measure the resistance to be less than 0.7 ohms, that is just the primary coil and my guess would be that it is the secondary which is more important. You can measure the secondary resistance, but a better measurement would be the insultation resistance, for which I haven't seen any figures.

If the insulation has cracked, unless you get all of the moisture out before it is sealed with epoxy you will still compromise the insulation resistance and therefore might end up with a weak spark. I would only do this as a short term fix until new coils can be obtained.

It took me 3 hours to change all six coils/plugs and coils were £31 each in the UK, I doubt it is really worth the effort of removing a coil, drying it out, filling with epoxy and re-fitting, well not in our current autumn climate of cold temperatures and rain.

If you were a little closer, you could have 6 Beru coils, all with at least one crack, but the postage would be more than they are worth.

PS anyone in the UK is welcome to them if they pay the cost of posting from Northumberland.

I’d agree that insulation resistance is a key characteristic that correlates more on strong (or weak) spark.

I’m routinely checking or actually replace spark plugs, like every 10K miles or so, when I also inspect coils. It takes me less than 2 hours and never bother me doing it. Never had body cracks but repaired one tube with electric tapes and it has been working fine for 2 years.

Thanks for your offer, I can afford new ones, the question is really if mine is bad or not performing as it should and what is effective evaluation method, as it seems clear that the service life is all over the map based on responses here.

Quote:

Originally Posted by blue62 (Post 643227)

Continuity and Ohm's resistance are not the same thing.
They are two different tests.
If you have a variation in reading "within the spec." your still within spec.
Any spec. has an allowable tolerance.
Primary and secondary Ohm's resistance can be checked with a Multimeter.
Spark color can be checked with a simple spark checker.
You want a fat blue spark not a weak yellow spark.
You can do a cylinder power balance test to see if a coil is possibly having a negative power balance effect on the engine.
You can have the coils scoped.

But in my opinion at less then $50.00 apiece shipped to my front door. If I have doubts about the condition of a coil I replace them.
Anything else is penny wise and pound foolish.

I know continuity and resistance are two different things, but effectively you are checking both at the same time when you are checking coils with a multimeter for breakage, short or insulation leakage.

I agree that you can waste time doing all those tests to pin point the coil issue when you can just buy new ones and forget about it, ‘if you have any doubt’…. my point is how to confirm that doubt by reasonable test so that you don’t end up just replacing perfectly good coils with zero appreciable improvement. They are not expensive, it just bothers me wasting good parts, just because I can replace them.

Quote:

Originally Posted by Boxstard (Post 643250)

I have never found a coil to be at fault where I could not detect the fault.
I have always been able to see physical damage, high Ohm's resistance, weak yellow spark. Or no spark.
That is over a period of 50+ years of playing with cars.
I have had very few coils fail.
Although in my old VW bug, heat from the engine was hard on them.

Quote:

Originally Posted by blue62 (Post 643260)

My takeaway from this threat is no need to change coils as PM, as they can last long time and you will likely notice symptoms. It is my habit to change spark plugs routinely at much shorter interval as factory suggests, so I can read them to see health of combustion and detect weak spark, if I don’t feel it from driving,

My first car was a 73 VW bug and probably the very first thing I did for ‘performance’ upgrade was Bosch blue coil….. I guess I’m spoiled now, no need to adjust valve gap every 3K miles, adjust distributor point gap, set the timing with strobe light, fiddle with carburetors, etc….