General System Description
Ignition Diagnostic Connector:
PURPOSE
The Electronic Ignition (EI) 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 ECM controls spark advance (timing) with the Ignition Control (IC) system.
OPERATION
The Electronic Ignition (EI) system is a "waste spark" system in which 2 plugs are fired simultaneously from one coil. The spark plugs are paired to a coil, so that when both fire, one cylinder is on the compression stroke and the other is on exhaust, There is little resistance across the plug gap on the exhaust stroke, so the plug requires very little voltage to fire, thereby providing the other plug (compression) with maximum available voltage.
The system uses a magnetic crankshaft position sensor which protrudes into the block, within less than 0.050" of the crankshaft reluctor. The reluctor is a special wheel cast on the crankshaft that has nine machined slots, eight of which are equally spaced (45 degrees apart). The ninth slot is spaced 8 degrees from one of the other slots. As the reluctor rotates with the crankshaft, the slots change the magnetic field of the sensor, causing an induced voltage pulse.
By counting the time between pulses, the ignition control module can recognize the pulse of the ninth slot (sync pulse). Based on this sync pulse, the module sends the reference signal to the ECM to calculate crankshaft position (used to fire coils in correct sequence) and engine speed.
To properly control timing, the ECM relies on the following information:
- Engine load (manifold pressure or vacuum).
- Atmospheric (barometric) pressure.
- Engine temperature.
- Intake air temperature.
- Crankshaft position.
The IC system consists of the ignition control module, ECM, and connecting wires. The connector terminals are lettered as shown in Chart C-4 Ignition System Check. Testing and Inspection
These circuits perform the following functions:
Reference (CKT 430)
This provides the ECM with RPM and crankshaft position information. If the wire becomes open or grounded, the engine will not run, because the ECM doesn't know that the engine is running and will not operate the injectors. If the engine cranks but will not start, refer to Chart A-3 Engine Cranks But Won't Run. Related Tests, Information and Procedures
Reference ground (CKT 453)
This wire is grounded in the ignition control module and provides a reference ground from the ignition control module to the ECM. The ECM compares reference ground with reference high voltage. If this circuit is open, or grounded at any other location than through the module, it may cause poor performance.
Bypass (CKT 424)
At about 400 RPM, the ECM applies 5 volts to this circuit to switch spark timing control from the ignition control module to the ECM. An open, or grounded bypass circuit, will set a DTC 42 and the engine will run at base timing, plus a small amount of advance built into the ignition control module.
IC (CKT 423)
This circuit triggers the ignition control module, after the engine is started and no DTC 36 or DTC 42 have been detected. An open or grounded IC circuit will set DTC 36 or DTC 42. The ECM can determine the actual timming from the reference signal. It then advances or retards the spark from that point.
ELECTRONIC IGNITION DIAGNOSTIC CONNECTOR
The electronic ignition diagnostic connector is located near the brake booster on the ECM harness. The purpose of the test connector is to allow the checking of six circuits that are at the 14 pin ignition control module connector. The ignition control module being mounted underneath the intake plenum takes some time to gain access to, so the test connector is spliced into the ECM harness between the ignition control module and the ECM.