See Figure 1
Only the 1991-92 2.9L and the 1991-94 3.0L engines are equipped with the distributor ignition system. The distributor ignition system consists of the following components:
The distributor ignition system designed by Ford has two distinct configurations. The first configuration is known as the distributor mounted system, because the ICM is mounted directly on the distributor housing. The second configuration is known as a remote mount system, since the ICM is mounted on the front fender apron.
The distributor used by this system is sealed and houses the CMP sensor. The distributor does not utilize vacuum or centrifugal advance mechanisms; the ignition timing is automatically controlled by the Powertrain Control Module (PCM) and the ICM.
Ford calls this electronic ignition the Thick Film Integrated-IV (TFI-IV) ignition system. The TFI module is also known as the Ignition Control Module (ICM) which reports engine position and rpm to the PCM. The PCM then determines the proper spark timing and advance, and returns a reference signal to tell the TFI module to switch the coil, thereby by creating a spark. The PCM used on these vehicles is referred to by Ford as the Electronic Engine Control-IV (EEC-IV) module.
See Figures 2 and 3
The CMP sensor, housed inside the distributor, responds to a rotating metallic shutter mounted on the distributor shaft. This rotating shutter produces a digital Profile Ignition Pick-up (PIP) signal, which is used by the PCM and ICM to provide base timing information, determine engine speed (rpm) and crankshaft position. The distributor shaft rotates at one-half crankshaft speed, therefore the shutter rotates once for every two crankshaft revolutions.
The ICM functions in either one of two modes: push start or Computer Controlled Dwell (CCD). The push start mode allows for increased dwell, or coil on time, when starting the engine. During this mode, the ICM determines when to turn on the ignition coil based on engine speed information. The coil is turned off, thereby firing, whenever a rising edge of a SPark OUTput (SPOUT) signal is received. The SPOUT signal is generated by the PCM, and provides spark timing information to the ICM. During the push start mode, the SPOUT signal only indicates the timing for coil firing; the falling edge of the SPOUT signal is ignored. Despite the name, the push start mode is also enabled during engine starting with the ignition key.
Do not attempt to push start a vehicle equipped with an automatic transmission.
During the CCD mode, both edges of the SPOUT signal are utilized. The leading edge of the SPOUT signal is used by the ICM in the same manner as during the push start mode. The falling edge of the signal is generated to control the timing for turning the ignition coil on (the ICM no longer controls this function as during the push start mode). During the CCD mode, the coil on time, or dwell, is entirely controlled by the PCM through the SPOUT signal.
In the event that the SPOUT signal from the PCM is disrupted, the ICM will use the PIP signal from the CMP to fire the ignition coil, which results in a fixed spark angle and dwell.