coil confusion

Pa Joe

Pa Joe

Senior Member
I got a bran new coil from FSI to go with a 12V negative ground Electronic distributor. In FSIs instructions it says the coil should read 3.0 to 3.5 ohms. Tested with a very expensive digital tester it tests at 3.7 ohms. First question, is that a problem that it's over 3.5ohms? Might add the coil was a little damp on top right out of the box. This contradicts the info in Les Andrews Vol. 2. It says 3.5 will run but with weak spark. Anyones thoughts on this would be appreciated.
 
What you have is close enough, especially with electronic ignition. The 3 to 3.5 ohm coil specification is to differentiate it from a 6 volt coil (which reads about 1.5 ohms). The difference between 3.5 and 3.7 is not what they are looking for. The low resistance 1.5 ohm coil will allow too much current to flow into the coil and burn up the electronics.
 
An 'expensive' (Fluke?) digital multimeter does not guarantee accuracy at very low resistance resolutions- when you want accuracy below + or - a few tenths of an Ohm. That 3.7 reading could be 3.0 or 4.4.

The best way to measure coil resistance with that meter is to set it to read DC amps and connect it in series with a 12V car battery and the coil. Then measure the DC voltage of the battery itself, to the nearest 1/10 volt, and use Ohm's law. E=IxR. Solve for R.

To get really accurate low Ohm readings, use a Wheatstone bridge setup.
Wheatstone bridge link
 
Yep, Mike it was a Fluke Digital. Thanks for the explanation. It was a little over my head. I just didn't know if I was splitting hairs being 2 tents over or not. And I was confused with two different statements, one by FSI and the other by Les Andrews book stating I would end up with a weak spark. Thanks Mke and Jim
 
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