Testing coils in 1974 25 HP Evinrude

[Steve D]

Not too familiar with these, but how do you test the coils in a motor with the low tension magneto setup? I have a multimeter and always test coils the way Doug Penn shows in his video but I don’t think he covered these specifically. I tried using the same procedure but the needle didn’t budge and stayed all the way to the left for the secondary circuit. Normally it would move to the 6, 7 reading on the "K" (1,000) scale. Maybe that’s the result and they’re bad, I just don’t know. Do I use a different scale on the multimeter, or is it a different procedure altogether? And also is there a simple test for the charge coil as well. Thanks.

(I’ve actually removed these and the charge coil from the motor and installed the standard coils under the flywheel and the motor runs great. But I’d still like to know how to test this style and know for sure if these are any good).


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[cajuncook1]

I made this video to show an eBay seller that the coil he sold me was bad. He nicely sent me a good used coil. He was happy sent I him a video to show him of the bad coil testing.

https://youtu.be/TbBCaZjXY-Y

[Steve D]

Alright cajun, that’s exactly how I tested and got the same results as you. So now I know the procedure is the same for this style coil and both of mine are shot. Appreciate the help, thanks!

Any info on testing the charge coil?

[Mumbles]

quote Steve D:

Any info on testing the charge coil?

Being a ’74 model, your motor will have the low tension mag with a driver coil. To check it, undo the leads at the points and when hooked up to an ohmmeter, the reading should be 1.45 +/- 0.4 ohms.

As for the ignition coils, I like to change them when I see them arcing all over the place! 😀

[Steve D]

Ok Mumbles, thanks. That coil looks nice and toasty!

[legendre]

quote Steve D:

(…) but the needle didn’t budge and stayed all the way to the left for the secondary circuit. Normally it would move to the 6, 7 reading on the “K” (1,000) scale.

In other words, the secondary winding is ‘open’ – like a blown-out fuse. This will usually render the coil completely inoperative, though there are +some+ cases where it will continue to work, for a while, anyway.

[fleetwin]

It seems strange that BOTH coils have open secondary circuits…
I would check the plug boot/terminal spring. These things get rusted easily and lose contact with the plug lead. I would pull the plug boot off and check the secondary resistance again directly on the secondary wire.
PS- The secondary resistance of your coils is significantly higher than your universal mag coils. You will need an ohm meter that can read between 10,000-15,000 ohms clearly.

[Steve D]

Ok thanks Fleetwin, I tried that and did get a reading out of one around 14-15 on the 1,000 scale. And on the other coil something I’ve never seen when testing coils…the needle was actually fluctuating back and forth. I had the red lead poked right into the plug wire making direct contact, and the black lead steady on the ground terminal, and it fluctuated randomly between 15 and 50.

Could a bad driver coil damage the coils?

On a side note, I am leaving for a couple days on a fishing trip and wanted to use this motor but it didn’t run quite right. I replaced the points, condensers, rebuilt the carb but the coils were the only thing I didn’t have but suspected they might be bad. I had the universal coils on hand and so I converted this over.

I’ll have to pick up the conversation when I get back in a few days. Thanks for the help guys.

[cajuncook1]

quote fleetwin:

It seems strange that BOTH coils have open secondary circuits…
I would check the plug boot/terminal spring. These things get rusted easily and lose contact with the plug lead. I would pull the plug boot off and check the secondary resistance again directly on the secondary wire.
PS- The secondary resistance of your coils is significantly higher than your universal mag coils. You will need an ohm meter that can read between 10,000-15,000 ohms clearly.

Thanks Don, I forgot that the low tension coils reading had a higher resistance reading than the universal coils.

[fleetwin]

quote Steve D:

Ok thanks Fleetwin, I tried that and did get a reading out of one around 14-15 on the 1,000 scale. And on the other coil something I’ve never seen when testing coils…the needle was actually fluctuating back and forth. I had the red lead poked right into the plug wire making direct contact, and the black lead steady on the ground terminal, and it fluctuated randomly between 15 and 50.

Could a bad driver coil damage the coils?

On a side note, I am leaving for a couple days on a fishing trip and wanted to use this motor but it didn’t run quite right. I replaced the points, condensers, rebuilt the carb but the coils were the only thing I didn’t have but suspected they might be bad. I had the universal coils on hand and so I converted this over.

I’ll have to pick up the conversation when I get back in a few days. Thanks for the help guys.

OK, that reading sounds normal. I wouldn’t poke through those wires to check the resistance though. Like I say, those plug/boot/terminals are notoriously lousy connections. The spring terminals get rusty, and the little pin loses contact with the coil wire. I would go ahead and cut the leads back (just past where you poked them-unless you poked them way back towards the coil) and replace the boot terminals. I’m thinking there is nothing wrong with those coils, unless it is burned through/arcing like the one pictured! Your meter may have been fluctuating because of a weak meter ground connection on the coil ground lead. Clean up the ground terminal, have an assistant help if needed. Keep in mind that holding the two meter leads with you fingers on them might show a reading in the range of 50,000 ohms.
I don’t really think a bad driver coil could damage the coils, but surely ANYTHING is possible when it comes to electricity, so I would never say never. The driver coil has been update/improved on these engines since yours was made, so it might be worth replacing it even if it tests OK.
I’m assuming this in NOT an electric start engine. The one thing that could damage the coils is inadvertently using the stop circuit leads as grounds for 12 volt accessories on a remote electric start boat harness.
PS- what was the original problem that caused you to convert to universal mag? No spark, weak spark? Another problem area for the low tension system is a chafed/grounded primary harness under the mag plate. These leads are constantly flexing/twisting and can be damaged easily.

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