Electronic Ignition (EI) System

Crankshaft Sensor Operation:

Ignition System:

EI Fault Line Circuit:

OPERATION
A description of the EI operation from crank to run will help to explain the characteristics of the EI system. When the ignition key is switched "ON," there is power to the EI fault line. The reference signal will be low, and the bypass and IC signals from the ECM will be low. There is no coil current being delivered, nor is there any crankshaft position sensor voltage. As the engine is being cranked, crankshaft position sensor voltage is produced corresponding to the slots. Before the EI recognizes the odd pulse, the ignition control module will not charge any of the coils with current. This is because the ignition control module does not know the engine position. The recognition of the odd pulse is recognition of engine position and is referred to as synchronization. This is why the odd pulse is commonly called the synchronization, or sync pulse. The reference signal does not toggle until after synchronization. When the reference signal starts to toggle, after synchronization, fuel is delivered because the ECM triggers fuel pulses from the reference high-to-low transitions.

Once the system is synchronized, the ignition control module knows which coil to charge first. The first coil to be charged after the sync pulse is 4-7. During initial cranking, the engine is rotating at speeds under 400 RPM. Below 400 RPM, the ignition control module will start current to the proper coil at 51 degrees BTDC of the proper cylinder. At speeds below 400 RPM, the firing of the coils occurs at 6 Degrees BTDC. Since the ignition control module is determining its own spark timing, the system is said to be operating in bypass mode. It is bypassing the spark advance Ignition Control (IC) signal from the ECM. The ignition control module provides accurate spark timing until stable engine operation is obtained (speeds above 400 RPM). While in bypass mode, the module pulls the IC signal to less than 1.3 volts and the ECM keeps the bypass signal low.

When it reaches 400 RPM, two modes of operation are changed by the module. First, the ignition timing follows a built in bypass advance. Secondly, the coil current is controlled by a "Closed Loop" system in the ignition control module. The bypass advance timing is present in the ignition control module. The amount of advance is selected purely by engine speed. Knowing the engine speed and engine position the ignition control module predicts when the selected advance should occur and fires the ignition coil accordingly. This is true of the bypass mode only.

ELECTRONIC IGNITION (EI) FAULT LINE
This EI fault line signal tells the ECM when the ignition control module detects a fault with the ECM inputs. This signal is normally in a high level (about 50% of battery voltage) with the ignition "ON."

The EI fault line from the ignition control module has a pull-up resistor to B+ (R1) and a series output resistor (R2). The EI fault line output is located on the 14 pin connector in "H". CKT 1310 is connected to pin "A-14" of the ECM. The ECM has a pull-down resistor (R3). The ECM will monitor 7.1 volts across R3 to ground (7.1 volt drop). This is the normal logic high state of the EI fault line. Because this is a voltage divider circuit design, the logic high is approximately 50 percent of the voltage.

By monitoring the voltage across the resistor (R3), the ECM can read a logic 0 (low) on the EI fault line under three conditions:

1. The ignition control module detects an IC fault input on CKT 423 (IC).
2. The EI fault input CKT 1310 to the ECM is open (poor connection / connector pin).
3. The EI fault line is shorted to system ground (pinched wire).

The ECM logic can sort out conditions 2 and 3 above (wiring / connections problem) by pulling the bypass CKT 424 line low and checking the EI fault line to return to a high state.

The ECM can respond to the EI fault line test through these three events:

1. Store Diagnostic Trouble Code (DTC) 16 if the EI fault line does not return to high logic with bypass commanded low (CKT 1310 problem).
2. Store Diagnostic Trouble Code (DTC) 36 if the EI fault line is low (for example: open or grounded IC signal), or too may pulses have been received at ignition control module on CKT 423 (IC).
3. Turn "ON" the Malfunction Indicator Lamp (Service Engine Soon).

RESULT OF INCORRECT OPERATION
An open or ground in the EI fault line will set DTC 16. An open or ground in the IC, or bypass circuit will set a DTC 36 or DTC 42 and cause the engine to run on the ignition control module timming. This will cause reduced performance and poor fuel economy.

The ECM uses information from the MAP and engine coolant temperature sensors in addition to RPM to calculate spark advance as follows:

Cold engine = More spark advance.
Engine under minimum load based on RPM and low amount of air flow = More spark advance.
Hot engine = Less spark advance.
Engine under heavy load based on RPM and high amount of air flow = Less spark advance.