Symptom Diagnosis
The PCM uses information from the MAP and engine coolant temperature (ECT) sensors in addition to the RPM in order to calculate the spark advance as follows:
| • | A low MAP output voltage = More spark advance. |
| • | A cold engine = More spark advance. |
| • | A high MAP output voltage = Less spark advance. |
| • | A hot engine = Less spark advance. |
Therefore, a low MAP output or a high resistance in the coolant sensor circuit could cause the detonation.
A high MAP output or a low resistance in the coolant sensor circuit could cause poor performance.
Ignition Control (IC)
This system uses similar circuits between the ICM and the PCM that a distributor type system uses. Following is a brief description of the IC circuit.
The PCM sends the ignition control (IC) pulses to the ICM on the ignition control A & B. The signals are similar to the 7x reference pulses square wave except that the PCM uses the sensors inputs to determine the pulses timing to control spark advance.
When the PCM receives the 7x reference signal, the PCM determines which pair of cylinders to fire 1 and 4 or 2 and 3.
Timing
Because the reluctor wheel is an integral part of the crankshaft and the crankshaft position (CKP) sensor is mounted in a fixed position, timing adjustment is not possible.
