How to check coild ohms

Luke0927 · Jun 18, 2014

I had another thread about 51 Super A that after ran for a while will drop RPMs and power then have to cool and run fine. Couple of sugestions could be the coil. It was converted to 12v some time in the past. Looks like there is no external resistor, how do I know if it is internal do I just need to check the ohms? Also where do I measure? on a side note I've noticed once its bee run a long while hour or so....it seems to do it for example when you go up on the throttle....last time it did this was on a 1-2 shift (when bad weather was rolling in and I was headed for the shed). It started to idle down...I shifted to 1st and (I was going down hill) as the as the engine turned back over it rev'd it back up and it was fine. For example same as you were pulling it off to start it. Hopefully that makes sense...I have also had to pull it off to restart before when it has done that because it would not restart (like it was flooded)

Bob · Jun 18, 2014

With the primary wires disconnected, measure Ohms across the two primary terminals on the coil with and ACCURATE-at-low-Ohms Ohmmeter.

Before testing, touch the meter leads together and see that it "zeros".

A coil used for 6 Volts or on 12 Volts with a ballast resistor will be in the 1.5 to 2 Ohms range.

A true 12 Volt coil will be in the 3 to 4 Ohm range. (A HOT 12 Volt coil will be on the low side of that.)

Luke0927 · Jun 18, 2014

ok so on the coil I see where it connects to the switch then it has a round terminal that has the wire that goes off to the distributors. Are these the primary connectors...sorry my strong point. I have a fluke volt meter think that will be good enough?

Owen Aaland · Jun 18, 2014

The little wires are the primary wires. The coil and spark plug wires are secondary circuit wires.

John T · Jun 18, 2014

"ok so on the coil I see where it connects to the switch then it has a round terminal that has the wire that goes off to the distributors. Are these the primary connectors...sorry my strong point."

The two small screw terminals (may be + and -, may be BAT and DIST) are the Low Voltage primary, one wires to ignition switch, other wires to the small side terminal on distributors.

If its a 12 volt typical stock farm tractor it will be around 2.5 to 4 ohms (many around 3), if its a 6 volt it will be more like 1.25 to 2 ohms range (many around 1.5)

The big center HV Tower is the High Voltage Secondary output which uses a coil wire to feed the center of the distributor cap (its HV input). It will read several thousand ohms.

"I have a fluke volt meter think that will be good enough"

Probably assuming it has a low ohms scale.

NOTE the low voltage low energy ohm meter test will ONLY show if a coil is definitely bad such as an open Primary or Secondary. It can pass both continuity and ohms tests and still be bad as many failures happen under HV breakdown and/or after warmed up.

John T

How to check coild ohms

Luke0927

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Bob

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Luke0927

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Owen Aaland

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John T

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