Point Ignition Coil Check OHMS?

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73Dartmang

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Happy friday everyone. Having an issue after the car is fully warmed up after driving a couple miles. First lets talk coil stats. Im getting a (primary circuit OHMS reading of 1.8 OHMS) and a (secondary circuit reading of 9680 OHMS). What is spec for the old coils using a ballast resistor and points? It appears to be the original coil Part #2495531. I know the last owner did not replace. Car now has 97,000 miles. Oh yeah it's me 1968 Dart slant six.

Now for the issue leading to checking my coil. Car at a steady 50MPH seems to feel like it shifts down, then back up. It's not though as I tested this. Cruising then blaahhh, then back up. Then drive a few minutes then slow, then back up. Almost feels like it's ...dirt in the carb but..It feels so consistent that I dunno yet hmmmm....

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You simply cannot check a coil primary in this manner. The simple reason is that the vast majority of ohmeters are not accurate down below a few ohms, and this reading is affected --unless you are very very careful--by contact resistance of your leads, as well as the leads themselves and even the connections at the meter
 
Multi-meter is brand new. I was very careful as I have used this tool many times before. Do we know the specs of the coil?
 
according to info in my 64-71 chiltons.....
primary side: low scale, coils used with balast resistor about 1.0 ohm, without about 4.0 ohms resistance
secondary side: between 4k and 8k ohms. says some special (mallory, ect) may go as high as 13k.
 
Have you tried using gas dry? You may have some water in the tank. The reading you get are about right for the coil but are these reading when the coil is cold or hot/failing?
 
Cold coil. Real cold actually as it's 25 degrees in mass and about to snow. Gas dry sounds good. I was thinking about some sea foam and seeing what happens.
 
73Dartmang said:
Multi-meter is brand new. I was very careful as I have used this tool many times before. Do we know the specs of the coil?
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As 67dart273 you can acurately measure resistance in the single digit range with a two wire multi-meter. Low resistance measurements can only be accurately measured with a 4 wire meter.

That being said the fact that it happens after the car has been run for awhile would suggest you have a temperature induced break down inside the coil (if it's not something un related). You will likely not be able to check for this with a multi-meter since it is happening at temperature and high voltage.

Best thing to do is borrow a coil from someone and try it. If the issue goes away then you know its the coil.
 
dgc333 said:
As 67dart273 you can acurately measure resistance in the single digit range with a two wire multi-meter. Low resistance measurements can only be accurately measured with a 4 wire meter.

That being said the fact that it happens after the car has been run for awhile would suggest you have a temperature induced break down inside the coil (if it's not something un related). You will likely not be able to check for this with a multi-meter since it is happening at temperature and high voltage.

Best thing to do is borrow a coil from someone and try it. If the issue goes away then you know its the coil.
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two wire meter...four wire meter..:stop:..geeze youz guyz is slpittin hairs.
im sure if your dealin with solid state circuts it makes a difference if your readin .25 or .30 ohms, but by golly this is an ignition coil:roll: if hes seein 1.8 ohms thats plenty close enough.
 
powerwagonpaul said:
two wire meter...four wire meter..:stop:..geeze youz guyz is slpittin hairs.
im sure if your dealin with solid state circuts it makes a difference if your readin .25 or .30 ohms, but by golly this is an ignition coil:roll: if hes seein 1.8 ohms thats plenty close enough.
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Look, man, just every damn once in awhile, us old bastards know WTF we are talking about. I was "into" electronics at a very early age, spent 6 years in the U.S. Navy as a GCA RADAR electronics tech, and my last few years I worked for Motorola installing E911 radio/ telco gear.

IF YOU HAVE a coil WITH AN INTERMITTENT SHORT between two windings of the PRIMARY, this constitutes a DC CHANGE OF RESISTANCE down into the MILLIOHMS, and this a CHANGE that you CAN NOT READ on ANY current production affordable meter that any of you guys out there OWN

To boil this down into SIMPLE terms, if the COIL reads 1.8 ohms and is GOOD, but then if when it HEATS UP and FAILS due to a short in the primary, IT MIGHT JUST STILL READ 1.8 ohms on your silly little 300 dollar Fluke meter.
 
Could also be just a bad ballast resistor-can only test by replacing it. And yes a high end dedicated ohm meter can find low ohm problems (I use a Vacuum tube HP with the four wire system) but is not worth most peoples money for some thing you would us once in a life time. But If you have a friend that has one it is time to go over and ask him for some help.
 
67Dart273 said:
Look, man, just every damn once in awhile, us old bastards know WTF we are talking about. I was "into" electronics at a very early age, spent 6 years in the U.S. Navy as a GCA RADAR electronics tech, and my last few years I worked for Motorola installing E911 radio/ telco gear.

IF YOU HAVE a coil WITH AN INTERMITTENT SHORT between two windings of the PRIMARY, this constitutes a DC CHANGE OF RESISTANCE down into the MILLIOHMS, and this a CHANGE that you CAN NOT READ on ANY current production affordable meter that any of you guys out there OWN

To boil this down into SIMPLE terms, if the COIL reads 1.8 ohms and is GOOD, but then if when it HEATS UP and FAILS due to a short in the primary, IT MIGHT JUST STILL READ 1.8 ohms on your silly little 300 dollar Fluke meter.
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exactly.....agreed...100%......and its still just an ignition coil. all im sayin is that fancy four wire meter that nobody can afford is overkill for checkin a coil. all you need is a good quality multimeter of 50,000 ohm per volt or better. if its good its good. if its bad its bad.
 
powerwagonpaul said:
exactly.....agreed...100%......and its still just an ignition coil. all im sayin is that fancy four wire meter that nobody can afford is overkill for checkin a coil. all you need is a good quality multimeter of 50,000 ohm per volt or better. if its good its good. if its bad its bad.
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Or you can just get a "Dr. Frankenstein" type coil tester and see if it breaks down. Mine will test the coil only or with the condenser. Has a heat-up mode too.
 
powerwagonpaul said:
exactly.....agreed...100%......and its still just an ignition coil. All im sayin is that fancy four wire meter that nobody can afford is overkill for checkin a coil. All you need is a good quality multimeter of 50,000 ohm per volt or better. If its good its good. If its bad its bad.
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and you still do not get it
 
powerwagonpaul said:
exactly.....agreed...100%......and its still just an ignition coil. all im sayin is that fancy four wire meter that nobody can afford is overkill for checkin a coil. all you need is a good quality multimeter of 50,000 ohm per volt or better. if its good its good. if its bad its bad.
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The only thing you can check with a run of the mill multimeter is whether it has an open you won't be able to check for internal break down of the insulation. Most coil failures a multimeter is pretty much use less for finding internal shorts. I have coil kicking around in the garage that measures in spec with at the low voltages applied when making resistance measurements. It won't run an engine at all. Took it to work and used a corona tester on it and the secondary windings started breaking down at 500 volts.
 
Geez.. Didn't mean to start such a bickering match on multi-meters. I hate the stupid things to.:cheers: Idles really smooth right now and got a few more ponies. Checked my gas cap as I should have after filling up a few times and no rubber gasket to insulate from moisture getting in. Might have gotten some moisture in there as I have been using the self serve power washer....Lets see what happens next... Coil is original and has 97,000 miles on it. ..

http://www.seafoamsales.com/
 
73Dartmang said:
Geez.. Didn't mean to start such a bickering match on multi-meters.
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This bickering has a purpose. Someone above posted an example of a coil that "tests good" using a multimeter and won't even start a car. We are trying to save those who would "do" such a test and then believe what they concluded means the coil is good.

You could spend a lot of money "around" the problem. Some of us have been there.
 
67Dart273 said:
This bickering has a purpose. Someone above posted an example of a coil that "tests good" using a multimeter and won't even start a car. We are trying to save those who would "do" such a test and then believe what they concluded means the coil is good.

You could spend a lot of money "around" the problem. Some of us have been there.
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I'm gonna post my $.02

With Coil's, In my experience, the fastest way to troubleshoot is just to swap in a test coil in your shop that you know is good. I know I have 3 or 4 laying around, that I have swapped in before to identify a bad coil. (Last one went bad in 2002 while on the highway). I've pulled one out of a working car to do ignition testing on another car before also.

The basic analysis with the two-wire meter is just a step in the troubleshooting process to see if you have a broken circuit within the coil. It isn't a step to say 'Yes' it is good, but to say 'No' it hasnt been proven bad yet.

I've had coils that are fine until they get up to operating temperature, then they quit working. As you know, neither a 2 or 4-wire meter will not find this when the engine is cold. AND The 2-wire meter WILL find it once the coil is heated up and the contacts separate. I've NEVER used a 4-wire meter for anything on a old car.

With what I see posted, I have to agree with PowerWagon Paul. Test it with the two wire. If it's still suspect, swap in a coil you know is good and go from there.

If you're concerned about the cost of having a test coil around your garage, next time you are at the salvage yard, pick up 2 or 3 for a buck each, Swap them into/out of your car to make sure good, and put on the shelf or in the drawer until you need one.
 
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