Coil Testing With Multi-Meter

[Cape Codder]

I had posted this question on another thread but no one ever made comment to it so I'm hoping this one will get some responses:

I just read an article that showed a coil can be tested with an ohm meter by checking across the two small posts and then checking from one small post to the spark plug connector.

This was for the Model A but did not give any spec's as it said "Determine the resistance using your multimeter and check to see if it's within the acceptable range".

What is the acceptable range on a 6V coil on the Model A?

[PalAl]

This might be what your looking for........

https://fordbarn.com/forum/showthread.php?t=148526

[PalAl]

If you didn't see this it may also be helpful.....

https://www.fordbarn.com/forum/showthread.php?t=276467

[Cape Codder]

PalAl Thanks for responding to my thread. The first one has the information I was looking for. To be honest the second one has my comments asking the above question which no one answered.
Again, thanks for your interest!

[PalAl]

Originally Posted by Cape Codder

PalAl Thanks for responding to my thread. The first one has the information I was looking for. To be honest the second one has my comments asking the above question which no one answered.
Again, thanks for your interest!

Sorry about the second one, I didn't know how to remove my mistake.

[Cape Codder]

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Thanks again.

[katy]

"Determine the resistance using your multimeter and check to see if it's within the acceptable range". What is the acceptable range on a 6V coil on the Model A?

Primary should be about 1.2 to 1.6 ohms, Secondary should be anywhere from about 3K ohms to about 9K ohms.

[rotorwrench]

The secondary is fine wire and that's the one that can open up with excessive heat for the most part. The modern coils for 12-volt specifically are around 3 ohms. The old stuff used with 6-volts and ballast resistors for 12-volt are generally pretty close to 1.5 Ohm.

The old coil testers were turned on to preheat the coil to get a test that is more in line with an operational coil since many times they break down after they are warmed up a while.

[Kurt in NJ]

With resistance measurements you can condemn a totally bad (open winding), but you can't confirm it's good--- but you need a more dynamic test-- and a hot coil , many will check ok by resistance, and make ok spark cold,

[Jerry in Shasta]

I had new coil that would not fire the plug under compression. How ever it worked fine

on the bench

[rotorwrench]

There may have been other reasons it wouldn't get spark at the plug. I bomb test spark plugs for the aircraft on a regular basis and the usually spark much better under compression that they do in open air. Bench testing is a marginal test. Just about any test without an oscilloscope and preheating is a marginal test.

[katy]

I bomb test spark plugs for the aircraft on a regular basis and the usually spark much better under compression that they do in open air.

I would suspect that is because the humidity of the compressed air is higher that open air.

[rotorwrench]

Compressed air, say 120 psi, ionizes better than ambient air pressure. If you ever use a spark plug tester, you can see it.