The distributorless ignition system used by the 4.0L and 1989-90 2.3L engines is referred to as the Electronic Ignition (EI) system. It eliminates the conventional distributor by utilizing multiple ignition coils instead. The EI system consists of the following components:
The function of the ICM was incorporated into the PCM beginning with the EEC-V system; otherwise the newer system operates in the same manner.
See Figure 1
The CKP sensor is a variable reluctance sensor, mounted near the crankshaft damper and pulley.
The crankshaft damper has a 36 minus 1 tooth wheel (data wheel) mounted on it. When this wheel rotates the magnetic field (reluctance) of the CKP sensor changes in relationship with the passing of the teeth on the data wheel. This change in the magnetic field is called the CKP signal.
The base ignition timing is set at 10 (plus or minus 2 degrees) degrees Before Top Dead Center (BTDC) and is not adjustable.
The CKP signal is sent to the ICM, where it is used to create the Profile Ignition Pick-up (PIP) signal.
The one missing tooth on the data wheel creates one large space between two of the teeth. The ICM utilizes this large space as a reference to help determine base ignition timing and engine speed (rpm), and to synchronize the ignition coils for the proper spark timing sequence.
The PIP signal is sent from the ICM to the PCM, which will use the PIP signal to determine base ignition timing and rpm calculations.
The ICM also receives the Spark Angle Word (SAW) signal from the PCM, which is used by the ICM to calculate the proper spark timing advance. Once all of the signals are calculated, the ICM determines the proper ON and OFF timing for the ignition coils.
The 4.0L engine utilizes one ignition coil pack, which contains three separate ignition coils. Each ignition coil fires two spark plugs simultaneously. One of the two plugs being fired is on the compression stroke (this plug uses most of the voltage) and the other plug is on the exhaust stroke (this plug uses very little of the voltage). Since these two plugs are connected in series, the firing voltage of one plug is negative (with respect to ground) and the other plug is positive.
If, for some reason, a fault arises in the EI system, the Failure Mode Effects Management (FMEM) portion of the ICM maintains vehicle operation. If the ICM stops receiving the SAW input signal, it will directly fire the ignition coils based on the CKP signal. This condition results in a fixed timing of 10 degrees BTDC.