Can I test the coil w/ a multi-meter? Batt. / Distr. ignition

[Alan]

Hello to all;
Is there any way I can test the coil, on my battery / distributor ignition? Without having it running or buying a special tester??

Motor was running rough, and then quit suddenly. Like hitting a switch. Wouldn’t make a single pop after that.

Testing suggested weak spark. Replaced points, condenser, and rotor. Distributor cap good, (almost new).

Testing still results in very weak, intermittent spark. Quadruple checked points; they break perfectly.

I’m wide open for testing suggestions. Wiring, etc.

Flywheel has not slipped; flywheel and distributor are timed correctly.

Very long story short; I’ve had this motor running beautifully in the past. Wiring was crispy when I got it, but has been cleaned up and worked fine. Coil is a replacement, with only about 20 or 30 hours on it.

After much work on the entire fuel system, no improvement. Yes, this is the 80 HP V4 I’ve posted on a million times. Yes, I have fumbled, and replaced parts needlessly w/out thorough diagnostics; the classic rookie approach to repairs.

All suggestions welcome.
Many thanks, and happy motoring to all.
Alan

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[amuller]

If it was mine I would test the primary and secondary resistance, and resistance to ground, with an ohmeter. This will not detect all coil failure modes, but many of them. Specs should be in manuals. Check point resistance–can look good but not be. Check there is a 12 volt supply to coil with ignition turned on. Ignition (key) switches do act up. Do you get a hot spark from the coil-to-distributor lead while cranking? You have tried more than one condenser? Battery ignition systems are generally pretty easy to diagnose.

[Alan]

Thanks amuller!
I couldn’t find specific resistance numbers for this particular coil, but the interwebs say I should be seeing something like 6 – 10 or 15,000 ohms when measuring between the positive terminal on the coil and the output tower which goes to the distributor. Repeated testing shows no connection between these two points, at all. Can’t get a single flicker out of the ohm meter, on any setting.
Between the positive and negative terminals, the numbers I found online were between .4 and 2 ohms; I get 7.
So, bad coil, right?
I get 12.7 V at the wire which goes from the positive terminal on the coil to the ignition switch.
Please let me know if I tested this wrong, or if this indeed a bad coil.
Sure would be nice to have this solved.
Appreciated;
Alan

[amuller]

Sounds like a bad coil to me.

It’s possible to use a cheap generic ignition coil. Something like this one: https://www.summitracing.com/parts/smp-uc15t/overview/. Just use a coil intended for use with points and not needing a resistor (unless your motor does use a resistor or resistance wire in the primary lead).

[Alan]

Many, many thanks!
Since I do have a resistor wire, I’ll go with an OEM replacement coil. Not cheap, but oh well.
I am so very happy to find something which is actually broken.
I headed down some wrong paths, but boy howdy my carb is clean!
Happy motoring.

[amuller]

I see OMC coil 379569 listed by Marineengine for over $225. Resistor/non resistor generic coils cost the same. But of course if you want it to look original then go for it.

[Alan]

Yeah, I see a coil on feeBay for a little over $100. I got this last one from a reputable club member, and was glad to pay like $95.
But that one on the Summit site was about $12.95. In general, I try to stick w/ OEM parts. But this last one was NOS OEM, and had no more than 30 hours before it died.
Any suggestions for a generic 12V coil, that is plenty resistant to vibration and heat, and suitable for use w/ my one ohm resistance wire?

[amuller]

Generally what you want is a 12 volt coil "for use with external resistor" or something like that. Unfortunately specs are often lacking for coil primary resistance and so on. But in general the resistor is similar in value to the coil primary, so when running the coil voltage should be in the range of 6 to 8. Cars typically had a bypass setup so the coil got full voltage while cranking. Does your motor have such a setup?

Couple of possibilities:

"Universal Tractor Replacement Ignition Coil 12 Volt" 12 volt, for use with external resistor" You can probably buy such a coil at a tractor supply store, or a NAPA auto parts, or order it on line. Here is something from Amazon: https://www.amazon.com/Parts-Player-Ign … PGK7K4RAQG Or this one: https://www.amazon.com/dp/B006EFZGUY/re … B00Q0D3JYA Or here is one that comes with a bracket and resistor:https://www.amazon.com/dp/B004AIYAZE/ref=psdc_15729031_t1_B004AJ28MK

The generic coil might have a different diameter so you might want to order one with a bracket, or order a bracket separately. Something like this:https://www.summitracing.com/parts/sum-g3862/overview/.

Really, no need to agonize over this. Measure the coil resistances before you install it so you have a baseline.

[Author]

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