Coil - Ignition


No-Start Diagnosis

  • Check the ignition switch.
  • Connect a 12-volt test lamp from the coil tachometer (tach) terminal to ground.
  • Turn on the ignition switch.
  • The test light normally should be on. if the test light is off, there is an open circuit in the coil primary winding or in the circuit from the ignition switch to the coil battery terminal. on many chrysler and ford systems, the test light should be off because the module primary circuit is closed. since there is primary current flow, most of the voltage is dropped across the primary coil winding. this action results in very low voltage at the tach terminal, which does not illuminate the test light. on these systems, if the test light is illuminated, there is an open circuit in the module or in the wire between the coil and the module.
  • Crank the engine and observe the test light. If the test light flutters while the engine is cranked, the pick-up coil signal and the module are okay. When the test lamp does not flutter, the pick-up and/or module are bad. A pick-up is tested with an ohmmeter. If the pick-up coil is satisfactory, the module is defective. Before testing the pick-up, check the voltage supply to the positive primary coil terminal with the ignition switch on before the diagnosis is continued.
  • If the test light flutters, connect a spark plug to the coil secondary wire, and ground the spark plug case. The test spark plug must have the correct voltage requirement for the ignition system being tested.
  • Crank the engine and observe the spark plug. If the test spark plug fires, the ignition coil is satisfactory. If the test spark plug does not fire, the coil is probably defective because the primary circuit no-start proved the primary circuit is triggering on and off.
  • Connect the test spark plug to several spark plug wires and crank the engine while observing the spark plug. If the test spark plug fired in step 4 but does not fire at some of the spark plugs, the secondary voltage and current is leaking through a defective distributor cap, rotor, or spark plug wires, or a plug wire is open. If the test spark plug fires at all the spark plugs, the ignition system is working fine.
  • If the cause of the no-start condition has not yet been found, check the ignition coil with an ohmmeter. If the winding resistance readings are not within specifications, replace the coil or coil pack. If the coils are fine, check the primary circuit for proper voltage.

Ignition Coil Resistance

  • With the key off and the battery lead to the ignition coil disconnected, use an ohmmeter to measure the primary and secondary winding resistance of the ignition coil. when checking the resistance across the windings, pay particular attention to the meter reading. if the reading is out of specifications, even if it is only slightly out, the coil or coil assembly should be replaced.
  • To check the primary windings, calibrate an ohmmeter on the X1 scale and connect the meter leads to the primary coil terminals to test the winding.
Ohmmeter connected to primary coil terminals.
  • An infinite ohmmeter reading indicates an open winding. The winding is shorted if the meter reading is below the specified resistance. Most primary windings have a resistance of 0.5 to 2 ohms, but the exact manufacturer's specifications must be compared to the meter readings.
  • To check the secondary winding, calibrate the meter on the X1,000 scale and connect it from the coil's secondary terminal to one of the primary terminals.
Ohmmeter connected from one primary terminal to the coil tower to test secondary winding.
  • A meter reading below the specified resistance indicates a shorted secondary winding. An infinite meter reading proves that the winding is open.
  • In some coils, the secondary winding is connected from the secondary terminal to the coil frame. When the secondary winding is tested in these coils, the ohmmeter must be connected from the secondary coil terminal to the coil frame or to the ground wire terminal extending from the coil frame. Many secondary windings have 8,000 to 20,000 ohms resistance, but the meter readings must be compared to the manufacturer's specifications. The ohmmeter tests do not indicate such defects as defective insulation around the coil windings, which causes high-voltage leaks. Therefore, an accurate indication of coil condition is the coil maximum voltage output test with a test spark plug connected from the coil secondary wire to ground as explained in the no-start diagnosis.

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