Testing a coil

[700rpm]

Is there a do-it-yourself, shade tree way to test a coil?

[nickair]

If it's bad, you'll get a weak spark or no spark at all. Replace it.

On a coil that tests OK, it still may need replacement. I had a coil that worked flawless when cold. When warm it partially grounded out due to a nearly invisible hairline crack at the tower which the rubber coil wire boot covered.

The primary winding of your ignition coil is the first to receive voltage from the battery. Find the resistance specifications for your car's primary coil winding in your coil specs. Then using a multimeter, place the leads on the smaller, outside poles if you have a traditional round coil. If the reading is within the range indicated in your coil specs, your primary winding is ok and you can go on to the secondary test. If it is even a little out of spec, the coil should be replaced.

The secondary winding of your ignition coil delivers the spark to the distributor to be sent to the spark plugs. If it's bad, you'll get a weak spark or no spark at all. To test the coil's secondary winding, attach the test probes to the outer 6V or 12V pole and the center pole (where the main wire goes to the distributor). Determine the resistance and check to see if it's within the range indicated in your coil specs. If it is, your coil is up to the task. If it's even slightly out of range, your coil should be replaced.


Of course it needs good connections.

[Tom Wesenberg]

The Model A coil wire didn't come with rubber boots on the ends, and I don't like to use them. I've seen where a boot on the coil has trapped moisture and shorted the coil to the primary terminal.

If you connect a battery charger to the primary terminals the meter should show about 4 amps draw. However, while running on the car the coil only draws about 1 to 2 amps.

To test for spark just connect a good condenser across the two primary terminals, then connect the battery to one coil primary and touch the other battery wire to the other coil primary, off and on to act like points opening and closing. While doing this hold the coil wire near one of the coil primary terminals and you should have a hot blue spark at least a good 1/4" long. My slant pole coil has at least a 1/2" spark.

As Nick said though, the coil may fail after it heats up, so the final test should be mounted to the car and drive it.

[fiddlybits]

I have one test that most do not... I shake the coil. If I hear "sloshing" of liquid I won't use the coil. Liquid filled coils have given me different issues over the years and also tend to be "average" coils. Solid, usually epoxy filled, coils have never failed me and usually have a hotter spark.

[larrys40]

Ray,
If you put a volt/ohm meter across the Primary ( low tension/voltage side), nothing connected to it to skew your readings, you should have about 1.2-1.4 ohms resistance. If not, the coil is bad. An open primary will produce no spark.
The secondary is the high tension ( voltage side).

If it shows correct resistance at least you have a chance to produce a high voltage spark. A good straight poll coil will produce about 10-14 KV , the slant pole coils are slightly hotter with a 12-18 KV output if they are really good.

Like Tom said, the best way for a novice to test one is to do as I prescribed with the volt/ohm meter, it good put on the car and turn on the ignition and see if you get a good hot spark opening and closing the points, it so, give it a test run. It is true that coils break down with voltage stress/heat produced by them. There have been many coils unnecessarily changed over the years due to people shotgunning problems...

On the flip side there's many bad coils at swap meets. I use a Ford Heyer test set to load test coils and check for proper spark output. I picked up 3 heyer machines back in the '80's and feel lucky to have them. They have been indispensable for testing coils/condensers/ strobing V8 and other distributors. I remember one year Doc Hampton was through with the great race one year, running a Mallory distributor on his '29 speedster, he was having ignition trouble and he brought it over on the overnight stayover and we identified the problem on the machine and dialed it in.... He went on to win the race that year... and I got a nice thank you in the mail later.

Kind of a cool occurrence...
I also like Sun's old slogan.... "Test, don't guess". Good luck in testing it out. It's better than not doing it and just wondering.

Hope that helped.
Larry Shepard

[Kevin in NJ]

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From experience, you need a coil tester. Try to keep a 60HP engine running with original coils!

Here is one you can build: Coil Tester

There are also the ones that you can buy. I usually get them for $10 or less at garage sales and on ebay they go up to like $100.

The reason why you need a tester is that you need to heat the coil. Many coils will test fine when they are cold. Let them heat up and they go real bad quick.

[Ian Curtis]

Related question... The local club and I have spent the last few weeks chasing gremlins in my truck. If you crank the car over and hold the distributor wire over the engine block you get a nice long blue spark. If you hold a plug wire off the distributor you get also get a nice blue spark. But, the car will not start. We switched it out with the ancient coil the car came with... and it runs like a top. Obviously the coil is bad, but if I'm getting a nice spark what else is causing that spark to not make the fuel ignite?

[700rpm]

I should have specified bench test a coil out of the car. I know about the primary lead/blue spark method in the car, but I have three vintage coils I wanted to test on the bench.

[Purdy Swoft]

I agree about the oil filled coils. Another thought on coils is that what may appear to be a crack on the tower could be a carbon track. This type problem was mentioned in the service bulletins. If you see what appears to be a crack, see if you can scrape it off with a knife and try it again.

[Purdy Swoft]

I've got one of the jump spark coil testers that is sold by one of the venders. The problem with this type device is that any problem with the distributor can reduce the spark that the coil will jump and can render the test useless.

[Clem Clement]

If I see oil weeping out, I dump them. I love to pickup several at local meets for $1 or lees apiece, test them and have the good ones onboard for spare.

[Purdy Swoft]

fiddlybits hits the nail on the head in post #4 about oil filled coils. They don't necessarily have to be leaking. Model A's came from the factory with a solid tar filled coil. An epoxy filled coil is also a solid coil and the only type that I would want to use on the model A. An oil filled coil should never be mounted in the normal way that the coil was mounted on the model A and I have never had anything but problems with oil filled coils on my model A's .

[jkeesey]

This has been helping me out a good bit lately

[holdover]

I have had great sucess with one of these for the past many years;

http://www.ebay.com/itm/Adjustable-H..._Tools&vxp=mtr

you need to be sure that the points/ condenser are working correctly and you need to test the coil after it is been heated up, not cold

[Y-Blockhead]

Originally Posted by Tom Wesenberg

... I've seen where a boot on the coil has trapped moisture and shorted the coil to the primary terminal ...

If you apply a small amount of di-electric grease http://www.crcindustries.com/ei/prod....aspx?id=02085 to the boot you will not have that problem. I use it on all my electrical connections to waterproof and prevent corrosion problems.

[Bruce of MN]

My car stopped running yesterday, so I am trying to find out why. There is 6V at the points and I put in a new condenser. The coil is 1.8V or so on the primary side, but 18k ohms on the secondary. This is about double what most references call for. Is that a sign the coil is the culprit?

I don't have a spare to substitute and my charger is not the old fashioned kind that could help me look for 4A current. I may try a 6v coil from Tractor Supply to see if I can get running for a club tour tomorrow.

[Pinesdune]

Run a couple test wires, right from your battery to the terminals on your coil ( leave your battery and coil installed on the car but remove car wiring from coil ) run a plug wire from the coil to a spare spark plug. Try to get the plug grounded to the head (hold it down to a head bolt with some insulated pliers) Connect one wire to coil (polarity shouldn't matter for this test) then touch the other lead to the other coil terminal (you should see sparking at the coil terminal as you are completing the circuit, no spark =bad coil) When you remove the wire that's when you are simulating the points opening and should see spark at the plug. Repeat touching the wire to the terminal to make sure your sparks good. And no spark =bad coil .


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[ModAfan]

The PDF in Post #14 (Coil test.pdf) says spark coil primary resistance should be 1.0 ohms to 1.4 ohms and secondary resistance should be 5.8 kilohms to 9.2 kilohms. The primary winding on the coil in my car is 1.4 ohms, which is fine, but the secondary resistance is 3.49 kilohms, well under the specs. I just got a new coil and it measures 1.7 ohms on the primary and 11.53 kilohms on the secondary. Both readings on the new coil are high. My question is: How far out of spec can a coil be and still be useable?

[Tom Wesenberg]

Originally Posted by ModAfan

The PDF in Post #14 (Coil test.pdf) says spark coil primary resistance should be 1.0 ohms to 1.4 ohms and secondary resistance should be 5.8 kilohms to 9.2 kilohms. The primary winding on the coil in my car is 1.4 ohms, which is fine, but the secondary resistance is 3.49 kilohms, well under the specs. I just got a new coil and it measures 1.7 ohms on the primary and 11.53 kilohms on the secondary. Both readings on the new coil are high. My question is: How far out of spec can a coil be and still be useable?

I'd say your new coil has extra wire on both the primary and secondary and will work fine.

To check for spark I would stop the engine with the points open, then turn the key on and use a screwdriver to short across the open points contacts while holding the coil wire 1/4" from a head nut. You should get a hot blue spark each time the screwdriver is removed from the contacts. If you get a heavy spark at the points contacts and a weak spark at the coil wire, the first thing I'd suspect is the condenser. If that doesn't fix it then try the coil. So far every coil I bought at a swap meet has been good.

[JackA]

What is a "slant pole" coil?