GM Grand Am/Achieva/Calais/Skylark/Somerset 1985-98

General Information



See Figure 1

The DIS ignition system features a distributorless ignition engine. The DIS system consists of two separate ignition coils on the 1987-91 2.5L (VIN U) engine, a DIS ignition module, a crankshaft sensor, crankshaft reluctor ring, connecting wires and the Electronic Spark Timing (EST) portion of the Electronic Control Module (ECM).

The DIS ignition system uses a magnetic crankshaft sensor and a reluctor to determine crankshaft position and engine speed. The reluctor is a special wheel cast into the crankshaft with several machined slots. A specific slot, on the reluctor wheel, is used to generate a sync-pulse.

The DIS system uses the same Electronic Spark Timing (EST) circuits as the distributor-type ignition. The ECM uses the EST circuit to control spark advance and ignition dwell, when the ignition system is operating in the EST mode.

The DIS ignition system uses a waste spark distribution method. Each cylinder is paired with its companion cylinder (1-4, 2-3 on a 4-cylinder engine). The end of each coil secondary is attached to a spark plug. These two plugs, being companion cylinders, are at Top Dead Center (TDC) at the same time. The one that is on compression is said to be the event cylinder and the one on the exhaust stroke, the waste cylinder. When the coil discharges, both plugs fire at the same time to complete the series circuit.

Since the polarity of the primary and the secondary windings are fixed, one plug always fires in a forward direction and the other in reverse. This is differs from a conventional system in which all plugs fire in the same direction each time. Because of the demand for additional energy, the coil design, saturation time and primary current flow are also different. This redesign of the system allows higher energy to be available from the distributorless coils, greater than 40 kilovolts at all rpm ranges.

The DIS ignition system uses a magnetic crankshaft sensor which protrudes into the engine block to within approximately 0.050 inch (1.3mm) of the crankshaft reluctor. The reluctor is a special wheel cast into the crankshaft with seven slots machined into it. Six of the slots are evenly spaced (60° apart) and the seventh slot is spaced 10° from one of the other slots and serves to generate a "sync-pulse". As the crankshaft rotates, the slots of the reluctor cause a changing magnetic field at the crankshaft sensor, creating an induced voltage pulse. By counting the time between pulses, the ignition module can recognize the specified slot (sync pulse). Based on this sync pulse, the module sends reference signals to the ECM to calculate crankshaft position and engine speed.

Click image to see an enlarged view

Fig. Fig. 1: View of the crankshaft sensor-to-crankshaft reluctor relationship

To control EST the ECM uses the following inputs:

Crankshaft position
Engine Speed (rpm)
Engine temperature
Manifold air temperature
Atmospheric (barometric) pressure
Engine load (manifold pressure or vacuum)


Crankshaft Sensor

The crankshaft sensor is mounted to the bottom of the DIS module. It is used determine crankshaft position and engine speed.

Ignition Coils

The two ignition coil assemblies are mounted on the DIS module. Each coil distributes the spark for two plugs simultaneously (waste spark distribution). Each coil can be replaced separately.

Electronic Spark Timing (EST)

The EST system is basically the same EST to ECM circuit use on the distributor type ignition systems with EST. This system includes the following circuits:

DIS reference circuit (CKT 430)-provides the ECM with rpm and crankshaft position information from the DIS module. The DIS module receives this signal from the crank sensor.
Bypass signal (CKT 424)-above 400 rpm, the ECM applies 5 volts to this circuit to switch spark timing control from the DIS module to the ECM.
EST signal (CKT 423)-reference signal is sent to the ECM via the DIS module during cranking. Under 400 rpm, the DIS module controls the ignition timing. Above 400 rpm, the ECM applies 5 volts to the bypass line to switch the timing to the ECM control.
Reference ground circuit (CKT 453)-this wire is grounded through the module and insures that the ground circuit has no voltage drop between the ignition module and the ECM which could affect performance.