Ignition Timing

Several types of ignition timing control systems are used on 1989 models, conventional HEI/EST system and 3 types of distributor less ignition systems. When engine speed reaches 400 RPM or more (about 5-15 seconds after starting), ECM senses this on the RPM reference wire from the ignition module. When this RPM signal is sensed, ECM transmits a constant 5-volt signal to the ignition module on the ignition by-pass wire. This changes the position of the by-pass switch in ignition module. When this occurs, ignition module no longer controls firing of ignition coil. Instead, timing is controlled by ECM on EST wire of ignition module.

The Programmable Read Only Memory (PROM) portion of ECM has a basic spark advance curve based on engine speed. Spark timing is calculated by ECM whenever an ignition pulse is present. Spark advance is controlled only when engine is running (not during cranking). Input signal values are used by ECM to modify PROM information, increasing or decreasing spark advance to achieve maximum performance with minimum emissions. To check ignition system operation, see appropriate component CHART C4 for that system.

An Electronic Spark Control (ESC) system is also used on some models. There are 4 basic components to ESC system: a detonation (knock) sensor, a high energy ignition system, an ESC controller (some models), and the ECM.

When detonation (engine knock) occurs, detonation sensor produces a low voltage AC signal. This signal goes to the ESC controller or directly to the MEM-CAL unit inside the ECM, depending upon ignition application. On models using an ESC controller, controller supplies the ECM with a 12-volt signal. When detonation occurs, controller grounds the 12-volt signal to the ECM, pulling the signal down to zero volts. The ECM interprets this as a need to retard timing.

The ECM then retards spark timing until the ESC controller returns the 12-volt signal, or until the MEM-CAL no longer receives knock sensor signals. A malfunction in the ESC circuit should set Code 43. If Code 43 is not present and ESC system is suspected as the cause of driveability problems, see appropriate component CHART C5 for that system.

• HEI-EST - Vehicles not using a C(3)I (3.3L and 3.8L), IDI (2.3L) or DIS (some 2.0L, 2.5L, 2.8L and 3.1L) EST system are equipped with a Delco-Remy High Energy Ignition system with Electronic Spark Timing (HEI-EST). The distributor contains a 7 or 8-terminal HEI-EST control module. The distributor is connected to the EST system by means of a 4-wire connector, leading to Electronic Control Module (ECM). Connectors on the 8-terminal ignition module are sealed at distributor and ignition coil.
• Computer Controlled Coil Ignition (C(3)I) - C(3)I is a distributor less system used on 3.3L and 3.8L models. It consists of ECM, ignition module, 3 ignition coils (type III uses a 3-coil single unit, type II uses 3 individual coils in one unit), crankshaft position sensor, camshaft sensor (3.3L models use a combination cam and crank sensor) and connecting wires. Ignition module uses a sealed 14-pin connector cable which goes directly to the ECM. Input signals from the Hall Effect crankshaft (RPM and crank position) and camshaft (TDC No. 1) position sensors are used by ignition module to determine engine speed and piston position. This information is passed to ECM which determines when to fire spark plugs. Using camshaft sensor signal, ignition module then selects and sequentially triggers each of 3 interconnected coils, causing spark plugs to fire at the proper time. Each cylinder is paired with the cylinder opposite it in the firing order. These pairs are 1-4, 2-5 and 3-6. Both cylinders are fired at the same time; the cylinder on compression and the cylinder on exhaust. Since the cylinder on exhaust requires little available voltage to arc (low compression=low resistance), bulk of the voltage produced is used to fire the spark plug of the cylinder on the compression stroke. A failure in the C(3)I system should set a Code 41 or 42.
• Direct Ignition System (DIS) - DIS is a distributorless system used on 2.0L (VIN 1), 2.5L, 2.8L (VIN W) & 3.1L models. The 2.3L system is referred to as the Integrated Direct Ignition (IDI) system. The operation of both systems is quite similar to that of C(3)I system. It consists of 2 or 3 ignition coils (4-cylinder or V6), spark plug wires, ignition module (located under coil pack), a crankshaft position sensor and necessary wiring. On 2.3L models, coils, module and spark plug connectors are all combined in one unit which plugs directly onto spark plugs. Rather than a crankshaft position sensor mounted at crankshaft pulley (such as C(3)I), spark is timed by a signal sent from a crankshaft sensor mounted on side of block. This signal is received by ECM (through ignition module) and is used to trigger each coil at the proper time. See CRANKSHAFT POSITION SENSOR in INPUT SIGNALS section of this article. As with the C(3)I system, each cylinder is fired consecutively with the cylinder opposite it in the firing order. On the 2.8L and 3.1L, cylinders No. 1-4, 3-6 and 2-5 are paired. On 2.0L, 2.3L and 2.5L engines, cylinders No. 1-4 and 2-3 are paired. Each pair of cylinders is fired by its own ignition coil. Idle Air Control (IAC) Valve (Fuel Injected Models) The IAC valve is mounted on throttle body and controls the amount of air by-passed around the throttle plate. The IAC motor moves its pintle in and out in steps (0-fully seated, 255-fully retracted) to control engine idle speed. If engine RPM is too low, pintle is retracted and more air is by-passed around the throttle plate to increase engine RPM. If engine RPM is too high, pintle is extended and less air is by-passed around the throttle plate to decrease engine RPM. Normal counts on an idling engine should be 4-60. When engine is idling, ECM determines proper position of IAC valve based on battery voltage, coolant temperature, engine load and engine RPM. If IAC valve is disconnected or connected with engine running, IAC loses its reference point and has to be reset. Resetting of IAC is accomplished on some models by turning ignition on and off. On other models it may be necessary to drive vehicle (at normal operating temperature) over 35 MPH with circuit properly connected. Problems in IAC circuit should set a Code 35. To check function of IAC system, see Code 35 chart and appropriate component CHART C2 for that system.