Ignition System: Description and Operation
Distributor With Sealed Module Connectors And Separate Coil:
PURPOSE
The distributor ignition (DI) system controls fuel combustion by providing a spark to ignite the compressed air/fuel mixture at the correct time. To provide improved engine performance, fuel economy, and control of exhaust emissions, the control module controls distributor spark advance (timing) with the Ignition Control (IC) system.
OPERATION
The ignition system uses a primary and secondary sub system to accomplish timed spark distribution. The primary system consists of a low voltage trigger device which determines base timing. This signal is modified by the ignition control module and sent to the engine and/or transmission processor (control module) for base timing reference. Another signal is sent back to the ignition control module, which has been adjusted by the control module (advanced or retarded) to trigger the coil, according to the requirements of the engine.
The secondary system consists of the ignition coil which has primary (low voltage) windings and secondary (high voltage) windings. The high voltage the secondary side of the coil generates is conducted to the spark plugs by high tension plug wires.
CONSTRUCTION
The DI-IC system consists of an electronic distributor, an externally mounted coil, two Knock Sensors (KS) and a Control Module.
To properly control ignition/combustion timing the control module needs to know:
- Crankshaft position.
- Engine speed (RPM).
- Engine load (manifold pressure or vacuum.
- Atmospheric (barometric) pressure.
- Engine coolant temperature.
The distributor has four wires that connect the IC module to the control module. These circuits perform the following functions:
- Circuit 430 "DISTRIBUTOR REFERENCE HIGH". Provides the control module with RPM and crankshaft position information.
- Circuit 453 "REFERENCE GROUND LOW". This circuit may be grounded in the distributor and makes sure the ground circuit between the IC module and control module has no voltage drop which could affect performance. If it is open, it may cause poor performance.
- Circuit 424 "BYPASS". At about 400 rpm the control module applies 5.0 volts to this circuit to switch spark timing control from the IC module to the control module. An open or grounded bypass circuit will set a Code 42 and the engine will run at base timing, plus a small amount of advance controlled by the IC module.
- Circuit 423 "IC". This circuit triggers the IC module. The control module does not know what the actual timing is, but it does know when it gets the reference signal. It then advances or retards the spark form that point. Therefore, if the base timing is set incorrectly, the entire spark curve will be incorrect. An open or ground, in the IC circuit, will set a Code 42 and cause the engine to run on the DI module timing. This will cause poor performance and poor fuel economy. A ground may set a Code 42, and the engine may not run.
Since this is a full 12 volt system, no resistance wire is used. Also, a diagnostic connector is used on some models. This connector is located in the engine compartment on the left side front fender skirt. On vehicles equipped with this connector, a tachometer may be connected between terminals 6 and G.
A tachometer connection is incorporated in the wiring connector on the side of the distributor.
NOTE
Never connect a wire directly between the Tach terminal of the distributor connector and ground as this will damage the electronic circuitry of the module. When using a timing light to adjust ignition timing, the connection should be made at the No. 1 spark plug. Forcing foreign objects through the boot at the No. 1 spark plug terminal will damage the boot and cause engine misfire.
KNOCK SENSOR (KS) SYSTEM
PURPOSE
Varying octane levels in today's gasoline can cause detonation (also known as spark knock) in an engine.
The Knock Sensor (KS) system has two knock sensors that are used on 4.3L engines. The KS system reduces spark knock (detonation) in the engine. This allows the engine to have a maximum spark advance for improved driveability and fuel economy.
OPERATION
A control module (ECM, PCM or VCM) is used in conjunction with two knock sensors to control detonation. No KS module will be found on these applications as it is internal to the control module.
A 5 volt reference is applied to the knock sensors which have an internal resistance of about 8200 ohms. This resistance will lower the applied voltage to about half or 2.5 volts. When a knock is present, a small AC voltage is produced by the knock sensors and transmitted to the control module riding on top of the already existing 2.5 volts. An AC voltage monitor inside the control module to start retarding the spark incrementally.