Ford Aerostar 1986-1997 Repair Guide

General Information


See Figure 1

In the Distributorless Ignition System (DIS), known as Electronic Ignition (EI) on 1993-96 vehicles and used on all 4.0L and 1996 3.0L engines, all engine timing and spark distribution is handled electronically with no moving parts. This system has fewer parts, which require replacement, and provides a more accurately timed spark. The system does not change from 1990 to 1996 with only two exceptions. The first exception is that on all 1996 engines, the ignition system does not use a separate Ignition Control Module (ICM) to regulate the firing of the spark system. These engines use the Powertrain Control Module (PCM) to handle this task. The second exception is only in the names of the various components of the ignition system. The following components' names were changed with the 1993 model year:

The EEC-IV Module became the Powertrain Control Module (PCM)
The EDIS module became the Ignition Control Module (ICM)
Variable Reluctance Sensor became the Crankshaft Position (CKP) Sensor
The EEC system became the Electronic Ignition (EI) system

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Fig. Fig. 1: Electronic Ignition (EI) system components-except 1996 models

Through-out the EI system procedures, the newer names for the components will be used, unless there is a difference between the earlier (pre-1993) and later (1993 and newer) components.

The EI ignition system consists of a Crankshaft Position (CKP) sensor, an Ignition Control Module (only on 1990-95 models), a Powertrain Control Module (PCM) and one 6-tower coil pack.

The EI system operates by sending crankshaft position information from the CKP sensor to the ICM (PCM in 1996 models). The ICM generates a Profile Ignition Pick-up (PIP) signal and sends it to the PCM. The PCM responds with a SPark OUT (SPOUT) signal containing advance or retard timing information that is sent back to the ICM. The ICM processes the CKP and SPOUT signals and decides which coils to fire. In addition, the ICM generates an Ignition Diagnostic Monitor (IDM) signal to the PCM, which is used to indicate a failure mode and also provide a tachometer output signal.


See Figure 2

The CKP sensor is a passive electromagnetic device which senses movement of a "36 minus One" tooth wheel (essentially a 35 tooth wheel with a space, from where the 36th tooth is missing, to provide a reference signal) located behind the crankshaft pulley. An A/C voltage signal is generated which increases with engine RPM and provides engine speed and crankshaft position information to the ICM.

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Fig. Fig. 2: The Crankshaft Position (CKP) sensor determines engine TDC for the ignition module


The ICM is a microprocessor-based device with coil drivers which make decisions about spark timing and coil firing. The ICM turns the coils on and off at the correct time and in the proper sequence based on information from the CKP sensor and a pulse width modulated signal (SPOUT) generated from the PCM. The module receives CKP sensor and SPOUT signals and produces PIP and IDM output signals, which are sent to the PCM.

On the 1996 models, the PCM performs the ICM tasks.


The PCM receives IGN, GND and PIP signals from the ICM, then generates a SPOUT signal based upon engine speed, load, temperature and other sensor information. An IDM signal is received from the ICM to determine if an ignition failure mode should be recorded.


See Figure 3

The coil is turned on (coil charging) by the ICM, then turned off, thus firing two spark plugs at once. One is for the cylinder which is to be fired (on compression stroke) and the other to the mating cylinder which is on the exhaust stroke. The next time the coil is fired the situation is reversed, The next pair of spark plugs will fire according to the engine firing order.

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Fig. Fig. 3: The ignition pack consists of 3 coils, each of which sequentially fires 2 cylinders simultaneously