Mazda Trucks 1987-1993 Repair Guide

General Information


An Electronic Distributorless Ignition System (EDIS) is used on the Navajo. The system consists of a crankshaft timing sensor (VRS), EDIS ignition module, ignition coil pack, the part of the main computer (ECU) that deals with spark angle and related wiring. The crankshaft timing sensor, used on the Navajo, is a variable reluctance-type sensor, triggered by a trigger wheel machined into the rear of the front crankshaft damper. The signal generated by this sensor is called a Variable Reluctance Sensor signal (VRS). The VRS signal provides the timing information to the EDIS module. The EDIS Module receives the VRS signal from the crankshaft timing sensor. The module processes the VRS signal into Profile Ignition Pickup (PIP) information and transmits it to the ECU (Engine Control Unit). The ECU processes the PIP signal along with signals received from other engine sensors according to a spark advance map programmed into the ECU. Using this information, the ECU produces a Spark Angle Word (SAW) signal which it sends back to the EDIS module. Using the SAW and VRS information, the EDIS switches primary current to the ignition coils like the contact points in a breaker point type ignition system, except that it sequences the spark among the three coils and provides the optimum amount of spark advance and dwell. The EDIS module also sends back to the ECU and Ignition Diagnostic Monitor signal (IDM) which the ECU uses to indicate a failure mode and which is also used to provide an rpm signal to the instrument panel tachometer. The ignition coil pack contains three separate ignition coils which are controlled by the EDIS module through three coil leads. Each ignition coil fires two spark plugs simultaneously; one plug on the compression stroke and one on the exhaust stroke. The spark plug fired on the exhaust stroke uses very little of the coil's stored energy. The majority of the energy is used by the spark plug on the compression stroke. During some EDIS faults, the failure mode effects management (FMEM) portion of the EDIS ignition module will maintain vehicle operation. If the EDIS module does not receive the SAW input, it will result in a fixed spark timing of 10° BTDC (before top dead center). If the EDIS module does not receive the VRS input, synchronization cannot be achieved, and the engine will not start.