Chart C-4 Ignition Misfire Check

Chart C-4 Diagnostic Chart:

CIRCUIT DESCRIPTION
The ignition system uses a waste spark method of spark distribution. In this type of ignition system the Ignition Control Module (ICM) triggers the #1 and #4 coil pair resulting in both #1 and #4 spark plugs firing at the same time. #1 cylinder is on the compression stroke at the same time #4 is on the exhaust stroke, resulting in a lower energy requirement to fire #4 spark plug. This leaves the remaining high voltage to fire #1 spark plug.

CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. "Current Weak Cylinder" on the Tech 1 scan tool will display the correct cylinder number only if a single cylinder is misfiring. If more than one cylinder is suspected of misfiring, disconnect the fuel injectors one at a time with the engine running and continue diagnosis on any cylinders which do not cause the RPM to drop when its injector is disconnected.
2. Ensures that the injectors are being actuated by the Powertrain Control Module (PCM). If not, the procedure (Part 2 of 3) should isolate the cause.
3. A spark tester such as J 26792 (ST-125) or equivalent must be used to ensure adequate secondary voltage at the spark plug. Spark should jump the tester gap at all 6 leads. This test simulates a load condition.
4. If ignition coils are carbon tracked, the coil tower spark plug wire boots may be damaged.
5. By installing a normally operating coil, a determination can be made as to the fault being the coil or ignition control module.


DIAGNOSTIC AIDS
A misfire may occur intermittently or under extreme conditions. If a misfire cannot be duplicated, try spraying the coils and plug wires with a fine water mist to check for shorts.

Note: A customer, under wide open throttle conditions, may experience fuel shut-off at high Rpm's. RPM fuel shut-off point is Programable Read Only Memory (PROM) controlled, based on axle ratio. This could be interpreted as a misfire under load.