GM S-Series Pick-ups and SUV's 1994-1999 Repair Guide

General Information


See Figure 1

The distributorless Electronic Ignition (EI) system is used only on the 2.2L (VIN 4) engine. This electronic system is designed to provide spark for air/fuel combustion in response to timing commands from the Powertrain Control Module (PCM, often referred to in the past as the ECM). System components include the PCM, the Ignition Control Module (ICM) which contains 2 coil packs and the Crankshaft Position (CKP) sensor. Each coil pack is made up of 2 spark towers. Spark plug wires deliver voltage from the towers to the spark plugs located in the cylinder head bores. The ICM receives inputs from the crankshaft position sensor in order to monitor engine position and rotation. The module provides output signals, based on the CKP signal, which are used by the PCM to determine engine timing.

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Fig. Fig. 1: Crankshaft sensor-to-crankshaft reluctor relationship

The crankshaft position sensor is mounted in the side of the engine block and protrudes within approximately 0.050 in. (1.27mm) of the crankshaft reluctor ring. The reluctor is a special wheel which is cast into the crankshaft with seven slots that are machined into it. Six of the slots are evenly spaced (60 degrees apart) while a seventh slot is spaced 10 degrees from one of the other slots. As the reluctor rotates with the crankshaft, the slots change the magnetic field of the sensor, creating an induced voltage pulse. The unevenly spaced slot is used as a reference point so the ICM can tell the PCM what cylinder is next approaching TDC.

A distributorless ignition system such as this one operates based on the "waste spark'' method of spark distribution. Each cylinder is paired with the cylinder that is opposite it (1 and 4 or 2 and 3) in the firing order. The spark occurs simultaneously in the cylinder coming up on the compression stroke and in the cylinder coming up on the exhaust stroke. Since the cylinder on the exhaust stroke requires very little of the available energy to fire the spark plug, most of the voltage will go to fire the cylinder on compression. As the process is repeated, the cylinders reverse roles.

When the ignition is switched to the ON (or RUN ) position, battery voltage is applied to the ICM but no spark occurs because the CKP sensor shows no engine rotation. When the engine begins to rotate and reference signals are received, the ICM will control spark by triggering each of the 2 ignition coils at a pre-determined interval based only on engine speed. This ignition operation during engine cranking is known as bypass timing mode.

Once engine speed rises above 400 rpm, the PCM will take over control of the ignition control circuit in order to compensate for all driving conditions. This is known as Ignition Control (IC) mode. During normal engine operation in IC mode, the PCM will control spark timing advance or retard according to various sensor inputs, in order to obtain optimum performance. In IC mode the PCM will rely on the following information:

Engine load (as determined by manifold pressure/vacuum)
Atmospheric pressure
Engine temperature
Manifold air temperature
Crankshaft position
Engine speed (rpm)

PCM control of the ignition timing will continue unless a problem occurs and the bypass timing mode is again entered, during which the ICM module will determine engine timing based on preset values. If the vehicle stalls, the engine will cease rotation thus ending CKP reference pulses. The PCM and ICM will cut the ignition. Should this occur, the ICM will not resume plug firing until engine rotation resumes.