Chart C-4C: Ignition System Check (Impulse)
NOTE:
Test numbers refer to test numbers on diagnostic chart.
- Two spark plug wires are checked to ensure that an open is not present in a wire.
- A spark indicates problem must be distributor cap or rotor.
- There should be battery voltage at terminals "A" and "B". Low voltage on both terminals would indicate an open or high resistance circuit from distributor to coil or ignition switch. If terminal "B" voltage was low, but terminal "A" voltage is 10 volts or more, circuit from terminal "B" to ignition coil or ignition coil primary winding is open.
- Checks for shorted module or grounded circuit from ignition coil to module. Distributor module should be turned off, normal voltage should be about 12 volts. If module is turned on, voltage would be low, but more than 1 volt. This could cause ignition coil to fail from excessive heat. With an open ignition coil primary winding, a small amount of voltage will leak through module from battery to TACH terminal.
- Applying voltage (1.5-8 volts) to module terminal "P" should turn module ON. This will cause TACH terminal (Brown/Red wire) voltage to drop to approximately 7-9 volts. This test determines if module or coil is faulty or if pick-up coil is not generating proper signal to turn module on. This test can be performed by using a DC battery with a rating of 1.5-8 volts. Use of test light is mainly to allow terminal "P" to be probed more easily. Some DVOM's can also be used to trigger module by selecting ohms, usually diode position. In this position the DVOM may have a voltage which can be used to trigger module. Amount of voltage produced by DVOM in ohms position can be checked by another DVOM.
- This should turn off 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. Module Tester (J-24642) could determine which is at fault.