How to test coils?

RichJohnson · Post by **RichJohnson** » Mon Oct 03, 2022 7:34 pm

Is there a way to test coils to determine if they are still good or going bad or failed?

I have 3 take out coils with no clue as to their viability.

'54 M38a1 all stock. I had a misfire issue and a friend suggested I replace the condenser first.
It was a recent install in the last two years probably chicom product.
I put an old US issue condenser back in that I had removed just for GP and the problem was gone.

Two weeks later Im out putting around off roading in 100 deg heat and suddenly I get misfires again. A random put here and there that got worse. Jeep was harder to start when hot and then it made a constant putting machine gun sound near the top RPM of each gear.
Someone at our meet pulled my distributor cap off and had me crank it over and he said the bright white tiny spark in the points means my condenser is still good, that it would be yellow if it were bad. Along with the fact it ran much better after it cooled for a few hours points to the coil.

So is there any test procedure for testing coils?

Kendall · Post by **Kendall** » Mon Oct 03, 2022 8:51 pm

Try searching the web by using search terms such as "testing coil willysmjeeps" and you'll get several hits such as this one:
http://www.willysmjeeps.com/v2/modules. ... pic&t=5652

RichJohnson · Post by **RichJohnson** » Mon Oct 03, 2022 10:05 pm

Thanks Kendall.

I actually did a forum search on "coil testing" before I posted my question. I read the posts it returned for about 30 min then gave up and posted my question.
Wes's answer on that post was what I was looking for, thank you.

Post by **wesk** » Tue Oct 04, 2022 12:07 am

Rich,

I updated that discussion, made a few corrections and added working photo links. You may want to re-visit the discussion.

RonD2 · Post by **RonD2** » Tue Oct 04, 2022 4:48 pm

Hi Rich,
After you have a known good one and your jeep is running great at operating temp over nice long runs --- why not remove it, stick it in the glove box with a few tools just in case, and put your test coils in one by one and run them? (on a back road)

Bad ones ought to show up pretty quick. Maybe even before you get out of the drive way.

RichJohnson · Post by **RichJohnson** » Tue Oct 04, 2022 6:36 pm

Thanks Wes, great photos and I do have that adapter kit.
Im not really learned on dwell and how to deal with it but I seem to have got this far not knowing. I will have to read that up in the TM Principals of automotive vehicles that I have.

So I Ohmed out my 4 coils that I have and not sure what it all means

#1 the problem child, a reproduction dated 6-19 install, primary 11.6 ohms and 11.5k secondary

#2 a reproduction that I think I used in the past, 11.6 ohms primary and 11.6k secondary

#3 GI oldie, 6.4 primary and looks open on secondary

#4 GI oldie 12.3 Ohms primary and 14.7k secondary

#5 GI oldie in a take out distributor. 11.6 ohms primary 10.3k secondary

Are all of these bad? I do not have a new coil to test the primary side resistance on but these are not .5-1 ohms primary as Wes stated in that other thread.
Its possible the chicom reproductions are different new but the GI ones I have are also high

Post by **wesk** » Tue Oct 04, 2022 10:08 pm

but these are not .5-1 ohms primary as Wes stated in that other thread.

That was a posting error. I corrected that to read 6.0 to 6.5 ohms primary. Try testing all of your cools heated up. You can use a heat gun just long enough for the coil to become uncomfortable to hold long in your hand and record those readings. Often times a coil that does well or fair at room temperature will show it's bad side when hot.