Compression Sense Ignition

Both the SOHC and DOHC engines utilize Compression Sense Ignition, which eliminates the need for a camshaft position sensor. The EI module has sensing circuitry that detects when cylinder #4 has fired on its compression stroke and relays this information to the PCM. The PCM can then correctly synchronize the fuel injectors for sequential fuel injection.

The EI module uses capacitive pickup plates located under the 1/4 coil to determine when cylinder #4 has fired on compression. These plates are used to differentiate the polarity and voltage amplitude difference between the 1/4 secondary ignition circuits. Since each coil tower is of opposite polarity and the waste spark (2-4 kV) generally fires before the compression spark (10-25 kV), the module can determine cylinder #4 compression. When the EI module detects a positive to negative polarity sequence and a high negative voltage spike, it will pull the PCM 5 volt cam signal circuit to ground. The PCM knows that cylinder #4 had just fired on its compression stroke when this transition occurs.

The EI module, however, cannot always detect when cylinder #4 has fired on compression. These occurrences include:
^ During deceleration
^ Very low engine load conditions when engine is running
^ If a secondary ignition problem occurs on cylinder #1 or #4

Too few cam pulses (cam signal circuit not being pulled to ground) are a result of decreased cylinder #4 secondary resistance or increased cylinder #1 secondary resistance. Too many cam pulses (cam signal circuit being pulled to ground too often) are a result of decreased cylinder #1 secondary resistance or increased cylinder #4 secondary resistance.