GM Grand Am/Achieva/Calais/Skylark/Somerset 1985-98

General Information


The Computer Controlled Coil Ignition (C3I) or Electronic Ignition (EI) system features a distributorless ignition engine. The C3I/EI system consists of 3 ignition coils, a C3I ignition module/Ignition Control Module, a dual crank sensor (also referred to as the combination sensor) or two crankshaft position sensors (depending upon vehicle application), camshaft sensor, related connecting wires, and the Electronic Spark Timing (EST)/Ignition Control (IC) portion of the computer control module (ECM or PCM). The C3I/EI system uses the same Electronic Spark Timing (EST) or Ignition Control (IC) circuits as the distributor-type ignition. The computer control module (ECM/PCM) uses the EST/IC circuit to control spark advance and ignition dwell, when the ignition system is operating in the EST/IC mode. There are 2 modes of ignition system operation. These modes are as follows:

Module mode-the ignition system operates independently of the computer control module (ECM/PCM), with module mode spark advance always at 10° BTDC. The ECM/PCM has no control of the ignition system when in this mode.
EST/IC mode-the ignition spark timing and ignition dwell time is fully controlled by the computer control module (ECM/PCM). EST/IC spark advance and ignition dwell is calculated by the ECM/PCM.

To control spark knock, and enable maximum spark advance to improve driveability and fuel economy, an Electronic Spark Control (ESC) system is used. This system consists of a knock sensor and an ESC module (part of Mem-Cal). The computer control module monitors the ESC signal to determine when engine detonation occurs.


The C3I/EI system uses a waste spark distribution method. Each cylinder is paired with the cylinder opposite it (1-4, 2-5, 3-6). The ends of each coil secondary is attached to a spark plug. These 2 plugs, being on companion cylinders, are at top dead center at the same time. The one that is on compression is said to be the event cylinder and the one on the exhaust stroke, the waste cylinder. When the coil discharges, both plugs fire at the same time to complete the series circuit. Therefore, each pair of cylinders is fired for each crankshaft revolution.

Since the polarity of the primary and the secondary windings are fixed, one plug always fires in a forward direction and the other in reverse. This differs from a conventional system in which all plugs fire in the same direction each time. Because of the demand for additional energy; the coil design, saturation time and primary current flow are also different. This redesign of the system allows higher energy to be available from the distributorless coils, greater than 40 kilovolts at all rpm ranges.

During cranking, when the engine speed is beneath 400 rpm, the C3I module/Ignition Control Module (ICM) monitors the dual crank/CKP sensor sync signal. The sync signal is used to determine the correct pair of cylinders to be sparked first. Once the sync signal has been processed by the ignition module, it sends a fuel control reference pulse to the computer control module (ECM/PCM).

During the cranking period, the computer control module will also receive a cam pulse signal and will operate the injectors sequentially, based on true camshaft position only.

The sync signal, or pulse, is used only by the ignition module. It is used for spark synchronization at start-up only.

When the engine speed is beneath 400 rpm (during cranking), the C3I module/ICM controls the spark timing. Once the engine speed exceeds 400 rpm (engine running) spark timing is controlled by the EST/IC signal from the ECM. To control EST/IC the ECM uses the following inputs:

Crankshaft position
Engine speed (rpm)
Engine Coolant Temperature Sensor (ECT)
Intake air (Mass Air Flow-MAF)
Throttle valve position (Throttle Position Sensor-TPS)
Gear shift lever position (Park/Neutral Switch-P/N)
Vehicle speed (Vehicle Speed Sensor-VSS)
ESC signal (Knock Sensor)

The C3I ignition module/ICM provides proper ignition coil sequencing during both the module and the EST/IC modes.

The ESC system is designed to retard spark timing up to 10° to reduce spark knock in the engine. When the knock sensor detects spark knocking in the engine, it sends an AC voltage signal to the computer control module, which increases with the severity of the knock. The ECM or PCM then adjusts the EST/IC to reduce spark knock.


C3I Module/Ignition Control Module (ICM)

The C3I module/ICM monitors the sync-pulse and the crank signals. This information is passed on to the computer control module so that correct spark and fuel injector timing can be maintained during all driving conditions. During cranking, the module monitors the sync-pulse to begin the ignition firing sequence. Below 400 rpm, the module control spark advance by triggering each of the three coils which are fired at a pre-determined interval based on engine speed only. Above 400 rpm, the C3I module/Ignition Control Module (ICM) relays the crank signal to the computer control module as a reference signal. The ECM or PCM then control the spark timing and compensates for all driving conditions. The module must receive a sync-pulse and then a crank signal, in that order, for the engine to be able to start.

Ignition Coil

The three twin tower ignition coil assemblies are mounted on the C3I module/Ignition Control Module (ICM). Each coil distributes the spark for two plugs simultaneously (waste spark distribution).

Electronic Spark Control (ESC)

The ESC system incorporates a knock sensor and the computer control module (ECM/PCM). The knock sensor detects engine detonation. When engine detonation occurs, the computer control module (ECM/PCM) receives the ESC signal and retards EST/IC to reduce detonation.

Electronic Spark Timing (EST)/Ignition Control (IC)

The EST/IC system is basically the same EST/IC-to-computer control module (ECM/PCM) circuit used on the distributor type ignition systems with Electronic Spark Timing or Ignition Control. For vehicles equipped with either the 3.0L or 3.3L engine, this system includes the following circuits:

Reference circuit (CKT 430)-provides the ECM with rpm and crankshaft position information from the C3I module. The C3I module receives this signal from the crank sensor's Hall effect switch.
Bypass signal (CKT 424)-at about 400 rpm, the computer control module (ECM/PCM) applies 5 volts to this circuit to switch spark timing control from the C3I module/ICM to the ECM or PCM.
EST/IC signal (CKT 423)-reference signal is sent to the computer control module via the C3I module/ICM during engine cranking. Under 400 rpm, the module controls the ignition timing. Above 400 rpm, the computer control module applies 5 volts to the bypass line to switch the timing to the ECM/PCM control.

For vehicles equipped with the 3.1L engine, the IC system contains the following circuits:

3X reference high (CKT 430)-The CKP sensor sends a signal to the electronic ignition control module which generates a reference pulse which is sent to the PCM. The PCM uses this signal to calculate crankshaft position and engine speed (and also to trigger the injector).
3X reference low (CKT 453)-This wire is grounded through the module and makes sure the ground circuit has no voltage drop between the ignition module and the PCM, which if open, could affect performance.
Ignition control bypass (CKT 424)-During initial cranking, the PCM will look for synchronizing pulses from the camshaft and 3X crankshaft position sensor indicating the position of the no. 1 piston and intake valve. 5 volts are applied to the bypass circuit the instant these signals are received by the PCM.
Ignition Control (IC) (CKT 423)-The PCM uses this circuit to trigger the ICM. The PCM uses the crankshaft reference signal to base its calculation of the amount of spark advance needed under present engine conditions.
24X reference signal - Additional to the electronic ignition system is the 24X crankshaft position sensor. Its function is to increase idle quality and provide good low speed driveability.

Computer Control Module (ECM/PCM)

The computer control module (ECM or PCM depending upon vehicle application) is responsible for maintaining proper spark and fuel injection timing for all driving conditions.

Dual Crank Sensor/Combination Sensor/Crankshaft Position (CKP) Sensor

Vehicles equipped with the 3.0L and 3.3L engines utilize a crankshaft position sensor. The crank sensor is mounted in a pedestal on the front of the engine near the harmonic balancer. The sensor consists of two Hall effect switches, which depend on two metal interrupter rings mounted on the harmonic balancer (crankshaft pulley) to activate them. Windows in the interrupters activate the Hall effect switches as they provide a path for the magnetic field between the switches transducers and magnets. When one of the Hall effect switches is activated, it grounds the signal line to the ignition control module, pulling that signal line's (sync-pulse or crank) applied voltage low, which is interpreted as a signal. Because of the way the signal is created, the signal circuit is always either at a high or low voltage. Three crank signal pulses and one "Sync-Pulse" are created during each crankshaft revolution. Both the CKP sensor and sync-pulse signals must be received by the ignition control module for the engine to start. A bent interrupter ring could cause rubbing of the sensor, resulting in potential driveability problems, such as rough idle, poor performance or a no-start condition.

The crankshaft position sensor is not adjustable for ignition timing, but positioning of the interrupter ring is very important. A clearance of 0.025 inch (0.64mm) is required on either side of the interrupter ring. Failure to maintain adequate clearance will damage the sensor.

24X and 3X or 7X Crankshaft Position (CKP) Sensors

The 3X and 7X crankshaft position sensors are basically the same component. The 3X sensor is a three wire unit and the 7X sensor is a 2 wire unit. In 1996 the 7X type sensor was used on most models.

Vehicles equipped with the 3.1L engine use two separately mounted crankshaft position sensors: The 24X and the 3X or 7X. The 24X crankshaft position sensor is secured in an aluminum mounting bracket and bolted to the front side of the engine timing chain cover and is located partially behind the crankshaft balancer. A three wire harness connector plugs into the 3X crankshaft position sensor, connecting it to the ignition control module. And the two wire harness 7X crankshaft position sensor is attached to the right front side of the engine.

The 24X sensor contains a Hall effect switch. The magnet and Hall effect switch are separated by an air gap. A Hall effect switch reacts like a solid-state switch, grounding a low-current signal voltage when a magnetic field is present. When the magnetic field is shielded from the switch by a piece of steel placed in the air gap between the magnet and the switch, the signal voltage is not grounded. If the piece of steel (called an interrupter) is repeatedly moved in and out of the air gap, the signal voltage will appear to go "ON-OFF-ON-OFF-ON-OFF". Compared to a conventional distributor, this "ON-OFF" signal is similar to the signal that a set of breaker points in the distributor would generate as the distributor shaft turned and the points opened and closed.

A concentric interrupter ring mounted to the rear of the crankshaft balancer has blades and windows that, with crankshaft rotation, either block the magnetic field or allow it to reach the Hall effect switch. The Hall effect switch is called a 24X CKP sensor because the interrupter ring has 24 evenly-spaced, same-width blades and windows. The 24X sensor makes 24 "ON-OFF" pulses per crankshaft revolution. The Hall effect switch closest to the crankshaft, then 3X sensor, is called that because the interrupter ring has a special wheel cast on the crankshaft that has seven machined slots, six of which are equally spaced 60° apart. The seventh slot is spaced 10° from one of the other slots. As the interrupter ring rotates with the crankshaft, slots change the magnetic field. This will cause the 3X Hall effect switch to ground the 3X signal voltage supplied from the ignition control module. the ignition control module interprets the 3X "ON-OFF" signals as an indication of crankshaft position, and must have the 3X signal to fire the correct ignition coil.

The 24X interrupter ring and Hall effect switch react similarly. The 24X signal is used for better resolution at a calibrated rpm.

Camshaft Position (CMP) Sensor

The camshaft position sensor is located on the timing cover behind the water pump, near the camshaft sprocket.

As the camshaft sprocket turns, a magnet in it activates the Hall effect switch in the cam sensor. When the Hall effect switch is activated, it grounds the signal line to the computer control module (ECM/PCM), pulling the cam signal line's applied voltage low. This is interpreted as a cam signal.