How to Check a Tractor Coil With a Multimeter

Disconnect the wires from the positive and negative terminals of the coil. Remove the secondary cable, or "coil wire" from the coil tower. Clean the coil surfaces if any oil or grime between the terminals. Replace the coil if any cracking or carbon tracks are present on the coil tower.

Select the ohm scale on the multimeter that registers the lowest values. Touch the test leads together while monitoring the meter. Calibrate the meter, or replace the meter battery if the reading is above zero ohms. You can allow for slight fluctuations if the test leads are not new.

Place one lead on the positive terminal and the other on the negative terminal to check the coil primary resistance. Replace the coil if the reading exceeds 3.5 ohms. Remove the lead from the positive terminal and place it on the coil case to check for shorted windings. Return the lead to the positive terminal, and probe the coil case with the other lead. Replace the coil if any value below infinity is displayed.

Adjust the meter setting to a scale that registers values of 20,000 ohms or more to test the secondary resistance of the coil. Place one lead onto the metallic contact inside the secondary coil tower. Touch the other lead to each primary terminal, one at a time. Replace the coil if the readings are below 10,000, or above 30,000 ohms.