C-4D2 ~ Misfire Under Load

Diagnostic Chart





Circuit Diagram

Circuit Description
The Electronic Ignition system uses a waste spark method of distribution. For example, in this type of system the Ignition Control Module triggers the No. 1 / No. 4 coil pair resulting in both No. 1 and No. 4 spark plugs firing at the same time No. 1 cylinder is on the compression stroke at the same time No. 4 is on the exhaust stroke, resulting in a lower energy requirement to fire No. 4 spark plug. This leaves the remainder of the high voltage to be used to fire No. 1 spark plug.
On this vehicle, the Crankshaft Position Sensor is mounted to the engine block and protrudes through the block to within approximately 0.05 in. of the Crankshaft Reluctor. Since the Reluctor is a machined portion of the Crankshaft and the Crankshaft Position Sensor is mounted in a fixed position on the block, timing adjustments are not possible or necessary.

Test Description
The numbers below refer to circled numbers on the diagnostic chart.
1. If the "Misfire" complaint exists at idle only, the diagnostic chart C-4D-1 must be used. Engine speed should drop approximately equally on all plug leads. A spark test such as a ST-125 must be used because it is essential to verify adequate available secondary voltage at the spark plug. (25,000 volts).
2. If the spark jumps the test gap after grounding the opposite plug wire, it indicates excessive resistance in the plug which was bypassed. A faulty or poor connection at that plug could also result in the miss condition. Also, check for carbon deposits inside the spark plug boot.
3. If carbon tracking is evident, replace coil and be sure plug wires relating to that coil are clean and tight. Excessive wire resistance or faulty connections could have caused the coil to be damaged.
4. If the no spark condition follows the suspected coil, that coil is faulty; otherwise, the Ignition Control Module is the cause of no spark. This test could also be performed by substituting a known good coil for the one causing the no spark condition.