Ignition Coil Test Procedures?

[Wendell Allen]

I'm working on a stock '65 6 cylinder. The car's not mine, and I've rebuilt the motor. Now that I'm done, the motor turns over just fine, but won't fire. I don't have any spark to the plugs. The distributor and wires are fine. I've verified all other wiring according to the schematic.

My first inclination is to say that the coil is bad. But I'm hesitant to jump to that conslusion because 80% of the wiring under the dash has been rigged by a previous owner. I know this because I had to get into the dash to replace a bad ignition switch.

So I'd like to test the coil before I just start replacing parts.

Anyone know how, and what the correct resistance readings should be?

Thanks,
Wendell

[Jeff65]

Conventional coils read about 1.2 ohms. A better check is to ground the distributor terminal (coil minus) and then do a voltage check at the battery terminal of the coil. With ignition key on

If there is no voltage, the ballast resistor is open or the ignition switch is bad.

If the voltage is 12v the coil is open (dead).

If the voltage is between 7-11 volts its in the right range.

If the voltage is below 7, the coil is shorted (dead).

Do not leave the coil minus terminal grounded more than 10 minutes or the coil may be damaged. Basically do the check quickly after turning the ignition switch to "ON" then test the voltage and turn the key off. Think about it with the key off!

Okay, if you've found it in the right range, use an ohmmeter to measure the condenser. It should momentarily read low on the ohmmeter then read very high resistance, greater than 10 megohms. If it reads less steadily, replace the condenser.

If you still have problems, temporarily run a jumper wire from the starter solenoid battery terminal to the coil (+) BATT terminal after removing the ground from the (-) distributor side of the coil. Reconnect the distributor wire if previously disconnected. Try starting the car with this battery "hot wire" jumper installed. If it starts and runs, then for sure there's a break in the harness between the ignition switch and the coil.

I had a Falcon that had a broken wire in the harness between the coil and the firewall. Whenever I checked it, it read fine. After running a block or two it would stall and not run. As soon as it stopped, it would start and run fine. Drove me nuts and almost got me into an accident. I finally worked down the problem. It was a break in the wire, inside the insulation about six inches from the coil. Engine vibration simply wiggled the wire enough over the years that the wire broke inside the insulation. A $0.05 splice fixed it. Look for the obvious!