Coil going out, how do you tell?
Thread Starter
Drifting
Joined: Apr 2002
Posts: 1,720
Likes: 19
From: Manhattan Beach Ca
Coil going out, how do you tell?
So I had a clear miss issue and thought it was wires touching each other. I checked the tuning and separated the wires and it seemed to run better but could feel a miss somewhere. The only thing left is the coil since everything else is new. What is the first symptom of a bad coil if there is such a thing? Thx
Race Director
Joined: May 2006
Posts: 18,771
Likes: 2,596
you failed to mention what kind of ignition system you have, points, factory HEI, etc. but every manufacturer lists resistance tests to run to determine if the coil is failing. If you cant find that information list the specifics of your system so others can try to assist.
Racer
Joined: Feb 2012
Posts: 421
Likes: 27
From: Harrisburg PA
Can't always trust the "cold" resistance tests.
I have an MSD ready to run dizzy in my '77...had an MSD Blaster 2 coil (got it used w/ the dizzy).
The coil tested perfectly within the specs of MSD (on bench testing before installation). Car started easily and ran fine until it got hot/warmed up. Once warm, the car would miss badly and stall repeatedly. Let it sit for 15 minutes, start it up, and it would run great for a few seconds until it got warm again...then same result.
Replaced the coil with a new Blaster 2, and runs flawlessly.
I have an MSD ready to run dizzy in my '77...had an MSD Blaster 2 coil (got it used w/ the dizzy).
The coil tested perfectly within the specs of MSD (on bench testing before installation). Car started easily and ran fine until it got hot/warmed up. Once warm, the car would miss badly and stall repeatedly. Let it sit for 15 minutes, start it up, and it would run great for a few seconds until it got warm again...then same result.
Replaced the coil with a new Blaster 2, and runs flawlessly.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Ignition coils can fail internally in many ways....but almost all of them have to do with wire separation. Usually, that means that the coil goes from "working" to "non-working" at the instant of failure. But, some wires can fail, but the ends remain in contact to allow function....UNTIL heat is added. Often increasing temperature can cause materials to expand enough that contacting wire ends will then separate and no longer conduct electrical energy. The best TEST would be to swap the coil out with a known 'good' (and appropriate to the application) coil. Intermittent failures are the most difficult to diagnose. But when a coil is found [and proved] to be 'somewhat' defective, it should always be tossed and replaced.
Le Mans Master
Joined: Jan 2001
Posts: 6,748
Likes: 1,390
Buy a generic replacement coil. They're cheap. Either swap it out, or to experiment, have the tools ready to swap out when you suspect the other has failed. You could carry a timing light for a while also--- just to check and see if you get spark immediately after suspected coil failure.
Mine used to fail when hot.... and as soon as I opened the hood it would start cooling off (it was a big old suupercoil mounted near the wiper motor) so it took me forever to troubleshoot.
Mine used to fail when hot.... and as soon as I opened the hood it would start cooling off (it was a big old suupercoil mounted near the wiper motor) so it took me forever to troubleshoot.
So I had a clear miss issue and thought it was wires touching each other. I checked the tuning and separated the wires and it seemed to run better but could feel a miss somewhere. The only thing left is the coil since everything else is new. What is the first symptom of a bad coil if there is such a thing? Thx
Last edited by carriljc; Nov 21, 2018 at 09:38 PM.
Race Director
Joined: Mar 2016
Posts: 14,446
Likes: 8,243
From: Napa Valley California
As stated above, a coil will operate normal when cool and will start to break down as it warms up.
They could also just fail all at once with no warning.
They are inexpensive so just purchase a good quality one and replace the one you have.
Be sure to read the manufactures instructions because some coils are required to be mounted in an upright position while others can be mounted sideways.
They could also just fail all at once with no warning.
They are inexpensive so just purchase a good quality one and replace the one you have.
Be sure to read the manufactures instructions because some coils are required to be mounted in an upright position while others can be mounted sideways.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
Points distributor? Pull cap. Make sure points are closed. Open points with thumbnail and look for spark at points. Now pull coil wire from dist cap and hold about 1/4 inch from manifold bolt or other bare metal. Open points and look for spark.if you see spark at the points, and have no spark at the coil wire then you know the coil is not producing high voltage.
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Le Mans Master
Joined: Aug 2005
Posts: 6,546
Likes: 1,978
From: Temple Terrace Florida
A quick way to check a hot coil failure is to cool it down. I use a can of compressed air to cool down an electrical component. You can buy a can at Staples or Office Depot. Once the engine starts to break up, pull over and spray the coil down with a stream of air from the can. Start it up and if the miss disappears, you have found the problem.Don't forget to give the condenser a look as well. Heat related problems can be difficult to diagnose. Jerry
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
Like others posted unless a coil completely opens it difficult to measure a problem. Best test is check with a cheap new coil.
I like the simple spark testers like: https://www.summitracing.com/parts/lil-19380/
Next step is to examine the spark plugs after some cruising and maybe WOT runs (if you dare). Follow that with compression and leak down tests. Yes a lot of work but it eliminates fundimental mechanical problems.
Now you can do some more complicated meter measurements if your meter has a tachometer and dwell function. Best you research those your self.
Let us know if we can help.
I like the simple spark testers like: https://www.summitracing.com/parts/lil-19380/
Next step is to examine the spark plugs after some cruising and maybe WOT runs (if you dare). Follow that with compression and leak down tests. Yes a lot of work but it eliminates fundimental mechanical problems.
Now you can do some more complicated meter measurements if your meter has a tachometer and dwell function. Best you research those your self.
Let us know if we can help.
Melting Slicks
Joined: Sep 2016
Posts: 2,238
Likes: 989
From: Wauconda IL