Chart C-4 Distributor Ignition (DI) System Check

Chart C-4:

Ignition Circuit:

TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.

1. Two wires are checked, to ensure that an open is not present in a spark plug wire.

Engine RPM information is supplied to the Electronic Brake and Traction Control Module (EBTCM) from the ignition coil. Check tach circuit for a short to ground.

1a. If spark occurs with Ignition Control (IC) connector disconnected, pick-up coil output is too low for IC operation.

2. A spark indicates the problem must be the distributor cap, rotor, or coil output wire.

3. Normally, there should be battery voltage at the "C" and "+" terminals. Low voltage would indicate an open or a high resistance circuit from the distributor to the 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.

4. Checks for a shorted Distributor Ignition (DI) control module or grounded circuit from the ignition coil to the module. The DI control module should be turned "OFF," so normal voltage should be about 12 volts. If the DI control module is turned "ON," the voltage would be low, but above 1 volt. This could cause the ignition coil to fail from excessive heat.

With an open ignition coil rimary winding, a small amount of voltage will leak through the module from the "Batt +" to the "Tach" terminal.

5. Applying a voltage (1.35 to 1.5 volts) to DI control module terminal "P" should turn the module "ON" and the tach terminal voltage should drop to about 7-9 volts. This test will determine whether the DI control module or coil is faulty or if the pick-up coil is not generating the proper signal to turn the DI control module "ON." This test can be performed by using a DC test battery with a rating of 1.35 to 1.5 volts. (Such as AA, C, or D cell.) The test battery must be a known good battery with a voltage greater than 1.35 volts.

6. This should turn "OFF" the DI control module and cause a spark. If no spark occurs, the fault is most likely in the ignition coil because most DI control module problems would have been found before this point in the procedure. A module tester (J24642) could determine which is at fault.

DIAGNOSTIC AIDS
If the vehicle is not equipped with a tach lead, it will be necessary to disconnect the gray coil connector from the coil and jumper the "B" terminal of the harness connector to the "B" terminal of the coil, using a fused jumper and terminal test adapter kit J35616. This will allow access to the "coil side" tach terminal for testing.