GM Lumina/Silhouette/Transport APVs 1990-1999 Repair Guide

General Information


The electronic ignition system controls fuel combustion by providing a spark to ignite the compressed air/fuel mixture at the correct time. To provide optimum engine performance, fuel economy, and control of exhaust emissions, the PCM controls the spark advance of the ignition system. Electronic ignition has the following advantages over a mechanical distributor system:

No moving parts.
Less maintenance.
Remote mounting capability.
No mechanical load on the engine.
More coil cool down time between firing events.
Elimination of mechanical timing adjustments.
Increased available ignition coil saturation time.

The electronic ignition system does not use the conventional distributor and coil. The ignition system consists of three ignition coils, an ignition control module, a dual Hall-effect crankshaft position sensor, an engine crankshaft balancer with interrupter rings attached to the rear, related connecting wires, and the Ignition control (IC) and fuel metering portion of the PCM.

There are a number of circuits that affect ignition control. The PCM reads the information supplied by these circuits to properly control ignition timing. The PCM receives the following information:

Engine load (manifold pressure or vacuum)
Atmospheric (barometric) pressure
Engine temperature
Intake air temperature
Crankshaft position
Engine speed (RPM)

Conventional coils have one end of the secondary winding connected to the engine ground. In this ignition system, neither end of the secondary winding is grounded. Instead, each end of a coils secondary winding is attached to a spark plug. Each cylinder is paired with its opposing cylinder in the firing order (1/4,2/5,3/6).These plugs are on companion cylinders. That means that while one cylinder is at TDC on the compression stroke, the companion cylinder is at TDC on the exhaust stroke. When the coil discharges, both plugs, on the same circuit, fire at the same time. The cylinder on compression is the event cylinder; the cylinder on exhaust is the waste cylinder. The cylinder on the compression stroke requires more voltage to fire than does the cylinder on the exhaust stroke. This method is called a "waste" spark ignition system.

The ignition coil design is improved, with saturation time and primary current flow increased. This redesign of the system allows higher secondary voltage to be available from the ignition coils-greater than 40kv. (40,000 volts) at any engine RPM. Secondary voltage requirements are very high with an open spark plug or spark plug wire. The ignition coil has enough reserve energy to fire the plug that is still connected, at idle, but the coil may not fire the spark plug under high engine load. A more noticeable misfire may be evident under load, both spark plugs may then be misfiring. Running for an extended period like this, will burn out the coil.

The system on the 3400 engine uses a coil pack with one ignition coil for each two cylinders in the engine. Mounted under the ignition coils on each system is an ignition control module (ICM) that performs ignition coil switching functions and interacts with the Powertrain Control module (PCM) to optimize ignition system operation.

Three twin-tower ignition coils are individually mounted to the ignition control module. Each coil provides spark for two plugs simultaneously (waste spark distribution). Each coil is serviced separately. Two terminals connect each coil pack to the module. Each coil is provided a fused ignition feed. The other terminal at each coil is individually connected to the module, which will energize one coil at a time by completing and interrupting the primary circuit ground path to each coil at the proper time.

The ignition control module determines the correct ignition coil firing sequence, based on 7X pulses. This coil sequencing occurs at start-up. After the engine is running, the module determines the sequence, and continues triggering the ignition coils in proper sequence. It sends the 3X-crankshaft reference (fuel control) signal to the PCM. The PCM determines engine RPM from this signal. This signal is also used by the PCM to determine crankshaft speed for Ignition Control (IC) spark advance calculations. The 3X-reference signal sent to the PCM by the ignition control module is an on-off pulse occurring 3 times per crankshaft revolution.