The Distributorless Ignition System (DIS) for all Saturn engines is an electronic system designed to provide spark for air/fuel combustion in response to timing commands from the Powertrain Control Module (PCM). System components include the PCM and the ignition module, which contains 2 coil packs. Each coil pack is made up of 2 spark towers. Spark plug wires deliver voltage from the towers to the spark plugs located in the cylinder head bores. The DIS module receives inputs from the Crankshaft Position Sensor (CPS) to monitor engine position and rotation. The module provides output signals, based on the CPS signal, which are used to drive the instrument cluster tachometer, and are also used by the PCM to determine engine timing.
When the ignition is switched to the ON (or RUN) position, battery voltage is applied to the DIS module, but no spark occurs because the CPS sensor shows no engine rotation. When the engine begins to rotate and reference signals are received, the PCM allows the DIS to fire when the first cylinder reaches Top Dead Center (TDC), the end of the compression stroke.
During normal engine operation, the PCM will control spark timing advance or retard according to various sensor inputs. This is called the Electronic Spark Timing (EST) mode. While operating in the EST mode, the PCM will vary engine timing for optimum performance. PCM control of the ignition timing will continue unless a problem occurs and the BYPASS mode is entered, during which the DIS module will determine engine timing.
If the vehicle stalls, the engine will cease rotation, thus ending CPS reference pulses. Should this occur, the DIS will discharge any charged coils and halt spark plug firing within 500 milliseconds after the last CPS reference pulse. The DIS will not fire any plugs again until engine rotation resumes.