Chart C4B - Ignition System Check
NOTE:
Test numbers refer to test numbers on diagnostic chart.
- Two wires are checked to ensure that an open is not present in a spark plug wire. If spark occurs with EST connector disconnected, pickup coil output is too low for EST operation.
- A spark indicates the problem must be the distributor cap or rotor.
- Normally, there should be battery voltage at "C" and "+" terminals. Low voltage would indicate an open or a high resistance circuit from distributor to coil or ignition switch. If "C" terminal voltage was low, but "+" terminal voltage is 10 volts or more, circuit from "C" terminal to ignition coil or ignition coil primary winding is open.
- Checks for a shorted module or grounded circuit from the ignition coil to the module. The distributor module should be turned off, so normal voltage should be about 12 volts. If the module is turned on, voltage will be low, but greater than one volt. This could cause the ignition coil to fail from excessive heat. With an open ignition coil primary winding, a small amount of voltage will leak through the module (from the battery to the tachometer terminal).
- Applying 1.5-8 volts to module terminal "P" should turn the module on. The tachometer terminal voltage should drop to about 7-9 volts. This test will determine whether the module or coil is faulty, or if the pickup coil is not generating the proper signal to turn the module on. This test can be performed by using a DC battery with a rating of 1.5-8 volts. The use of the test light is mainly to allow the "P" terminal to be probed more easily. Some digital multimeters can also be used to trigger the module by selecting "OHMS" position on the meter, usually in the diode position. In this position the meter may have voltage across its terminals which can be used to trigger the module. The voltage in the ohm position can be checked by using a second meter or by checking the manufacturer's specification of the "Scan" tester being used.
- This should turn off the module and cause a spark. If no spark occurs, fault is most likely in the ignition coil because most module problems would have been found before this point in the procedure. A module tester could determine which is at fault.