The information below makes it sound like timing is pcm controlled.
The Direct Ignition System (DIS) or Electronic Ignition (EI) is used on the 1987-92 2.0L (VIN 1 and H), 2.2L (VIN G and 4), and all 1987 and later 2.8L (VIN W) and 3.1L (VIN T) engines.
The DIS or EI system does not use the conventional distributor and ignition coil. The system consists of two (4-cylinder) or 3 (6-cylinder) separate ignition coils, an ignition module, DIS ignition module or Ignition Control Module (ICM) as applicable, crankshaft sensor or combination sensor, along with the related connecting wires and Electronic Spark Timing (EST) or Ignition Control (IC) portion of the computer control module (ECM/PCM).
The distributorless system uses a "waste spark" method of spark distribution. Companion cylinders are paired and the spark occurs simultaneously in the cylinder with the piston coming up on the compression stroke and in the companion cylinder with the piston coming up on the exhaust stroke.
Example of firing order and companion cylinders:
Notice the companion cylinders in the V6 engine firing orders remain the same, but the cylinder firing order sequence differs.
The cylinder on the exhaust stroke requires very little of the available voltage to arc, so the remaining high voltage is used by the cylinder in the firing position (TDC compression). This same process is repeated when the companion cylinders reverse roles.
It is possible in an engine no-load condition, for one plug to fire, even though the spark plug lead from the same coil is disconnected from the other spark plug. The disconnected spark plug lead acts as one plate of a capacitor, with the engine being the other plate. These two capacitors plates are charged as a current surge (spark) jumps across the gap of the connected spark plug.
These plates are then discharged as the secondary energy is dissipated in an oscillating current across the gap of the spark plug still connected. Because of the direction of current flow in the primary windings and thus in the secondary windings, one spark plug will fire from the center electrode to the side electrode, while the other will fire from the side electrode to the center electrode.
These systems utilize the EST or IC signal from the computer control module, as do the convention distributor type ignition systems equipped with the EST system to control timing.
In the Direct Ignition or Electronic Ignition system and while under 400 rpm, the DIS ignition module (Ignition Control Module) controls the spark timing through a module timing mode. Over 400 rpm, the ECM/PCM controls the spark timing through the EST/IC mode. To properly control the ignition timing, the computer control module relies on the following information from the various sensors:
See Figure 1
A magnetic crankshaft sensor (Hall Effect switch) is used which is remotely mounted on the opposite side of the engine from the DIS/IC module. The sensor protrudes in to the engine block, within about 0.050 in. (1.27mm) of the crankshaft reluctor.
The reluctor is a special wheel cast into the crankshaft with seven slots machined into it, six of which are equally spaced 60° apart. A seventh slot is spaced 10° from one of the other slots and serves as a generator of a "sync-pulse". As the reluctor rotates as part of the crankshaft, the slots change the magnetic field of the sensor, creating an induced voltage pulse.
The CKP sensor, ICM or DIS ignition module sends reference signals to the computer control module (ECM/PCM), based on the Crankshaft Position (CKP) sensor pulses, which are used to determine crankshaft position and engine speed. Reference pulses to the computer control module occur at a rate of 1 per each 180° of crankshaft rotation for vehicles through 1993, or 7 per 360° of crankshaft rotation for 1994 vehicles. This signal is called the 2X or 7X reference because it occurs 2 or 7 times per crankshaft revolution, depending on the year of your vehicle.
For 1994 vehicles, the 7X reference signal is necessary for the PCM to determine when to activate the fuel injectors.
The computer control module (ECM/PCM) activates the fuel injectors, based on the recognition of every other reference pulse, beginning at a crankshaft position 120° after piston Top Dead Center (TDC). By comparing the time between the pulses, the DIS module or Ignition Control Module (ICM) can recognize the pulse representing the seventh slot (sync-pulse) which starts the calculation of ignition coil sequencing. The second crankshaft pulse following the sync-pulse signals the DIS module to fire the No. 2-5 ignition coil, the fourth crankshaft pulse signals the module to fire No. 3-6 ignition coil and the sixth crankshaft pulse signals the module to fire the No. 1-4 ignition coil.
There are two separate coils for the four cylinder engines and three separate coils for the V6 engines, mounted to the coil/module assembly. Spark distribution is synchronized by a signal from the crankshaft sensor which the ignition module uses to trigger each coil at the proper time. Each coil provides the spark for two spark plugs simultaneously (waste spark distribution).
Two types of ignition coil assemblies are used, Type I and Type II. During the diagnosis of the systems, the correct type of ignition coil assembly must be identified and the diagnosis directed to that system.
The 2.0L (VIN H) engine is equipped with the Type 1 module/coil assembly. The Type I assembly has two twin tower ignition coils, combined into a single coil pack unit. This unit is mounted to the DIS module. If any of the coils are faulty, the entire module must replaced. A separate current source through a fused circuit to the module terminal P is used to power the ignition coils.
All other engines are equipped with the Type II coil/module assembly. The Type II coil/module assembly has two or three (depending upon engine) separate coils that are mounted to the DIS or IC module. Each coil can be replaced separately. A fused low current source to the module terminal M, provides power for the sensors, ignition coils and internal module circuitry.
The DIS module or ICM, as it's called in later years, monitors the crankshaft sensor signal, then, based on these signals, sends a reference signal to the computer control module (ECM/PCM) so that correct spark and fuel injector control can be maintained during all driving conditions. During cranking, the module monitors the sync-pulse to begin the ignition firing sequence. Below 400 rpm, the module controls the spark advance by triggering each of the ignition coils at a predetermined interval, based on engine speed only. Above 400 rpm, the ECM controls the spark timing (EST) and compensates for all driving conditions. The module must receive a sync-pulse and then a crank signal, in that order, to enable the engine to start.
The DIS module or Ignition Control Module (ICM) is not repairable. When a module is replaced, the remaining DIS/ICM components must be transferred to the new module.