Misfiring / coil resistance 3.8 Ohms?

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nwxh
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Misfiring / coil resistance 3.8 Ohms?

Post by nwxh »

Yesterday I had no problems driving, including a few miles stretch on a 70mph dual carriageway, until I noticed some misfiring passing through a village and low speeds.

Given this, I considered it unlikely to be fuel related, but just in case, took off the cap: no noticeable vacuum. Except I did consider fuel vaporisation.

I did notice oil around the coil terminals but cannot tell whether this is a leaking coil or from the engine etc. This morning, I decided to check the coil resistance and to sand and clean the terminals. The coil resistance was 3.8 Ohms, higher than usually reported max 3.4 ohms. Is this acceptable or a sign that the coil is giving up?

I cleaned the terminals with emery and put electrolube on before driving to work. But only got as far as the end of the road without issues, so went back, got needle nose pliers and pinched the female lucar terminals. I then drove to work without issues.

Could it simply be that the terminals needed a good clean and tightening, or is that just coincidence as my cleaning induced a new problem which was then rectified?
oliver90owner
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Re: Misfiring / coil resistance 3.8 Ohms?

Post by oliver90owner »

Don’t know. Not many meters read resistance to 0.1 Ohms.

If the coil is leaking, it should be changed. Have you considered that the problem might be plug, plug lead, distributor cap, contact points related? Even a loose connection to the coil, or a loose plug/plug cap?

You may have fixed it. But, there again….
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geoberni
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Re: Misfiring / coil resistance 3.8 Ohms?

Post by geoberni »

I find it unusual that a Coil would degrade by a fraction of an Ohm at a time.
It's only 2 lengths of wire wound around a core and soldered at the ends; it's the joints that are usually the problem these days and if it's going to break, it's going to break.
Although a caveat that with older 'quality' coils, you might find that old age has allowed some windings to rub together with vibration and short out over years of use, thus reducing the value a little. Not going to slightly increase the reading though.
Far more likely the external connections were dirty/loose.
nwxh wrote: Thu Oct 14, 2021 12:48 pm The coil resistance was 3.8 Ohms, higher than usually reported max 3.4 ohms. Is this acceptable or a sign that the coil is giving up?
I've got a brand new Coil in the garage and if I really press hard, to get through the manufacturer's bright finish on the terminals, it reads 3.8 Ohms. Yes, you're right, I actually went out and checked it because I had a feeling it was just short of 4 Ohms, but I wanted to be sure of my facts.
If I don't ensure I've got through the protective finish, it reads 6 or 7 Ohms.

The one on the car flickers around 3.8/3.9 Ohms.

You can go online and find people/articles reporting that the winding should be all manner of readings, typically from 1.0 Ohms to 5.0 Ohms but it really depends on the manufacture's Spec.

I would say that a multimeter will show if a Coil is dead, because it will be open circuit, but it won't show if you have one that's partially shorted or is breaking down with heat during use.

Only last week I linked to the photos I had of my faulty Coil, which caused me the break down twice. Here it is again:
viewtopic.php?f=11&t=69837&hilit=coil&start=20#p636467
Basil the 1955 series II

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nwxh
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Re: Misfiring / coil resistance 3.8 Ohms?

Post by nwxh »

Many thanks. Everything seemed fine this evening as well, so perhaps it was just the lucar connectors. Does anyone replace them with ring terminals?

The distributor is an Accuspark but I'm considering making that the spare and getting a better one. Plugs/leads are unknown, but I have no idea if the new ones would be any better considering quality of new parts.
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geoberni
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Re: Misfiring / coil resistance 3.8 Ohms?

Post by geoberni »

Well my Coil on the car is a Lucas Gold and the spare is an Accuspark, and as I reported, they are essentially the same 3.8 Ohms.

Basil has an Accuspark electronic unit, so I've 3 connections on the Coil. The original 2 are ring terminals, but the supply for the electronic unit is a Spade connection.
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nwxh
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Re: Misfiring / coil resistance 3.8 Ohms?

Post by nwxh »

geoberni wrote: Fri Oct 15, 2021 1:40 pm Well my Coil on the car is a Lucas Gold and the spare is an Accuspark, and as I reported, they are essentially the same 3.8 Ohms.

Basil has an Accuspark electronic unit, so I've 3 connections on the Coil. The original 2 are ring terminals, but the supply for the electronic unit is a Spade connection.
My one coil connection which totally fell off was the positive to Accuspark while I had just passed the M11 northbound to A14 eastbound loop. I managed to make it to the recess and fixed it in 30 seconds but it was not ideal. I re-crimped it which made the terminal a tighter fit: the quality of the wire is not as good as the loom or what I've used for extra electronics. I think I'd favour a ring terminal in future having (seemingly) had issues with the other two.
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