|MARK’S CORNER – AUTO REPAIR HELP|
|Ignition Control Module
by Mark Davidson
Diagnosing and troubleshooting engine starting problems typically involves inspecting the ignition control module. The ignition control module is used to switch primary current flow for the ignition coil. The control module switches these transistors on and off based on information from a magnetic pulse generator in the distributor. Some conventional electronic ignition systems and all distributorless ignition systems, use a crankshaft position sensor in place of a distributor mounted pulse generator. Crankshaft position sensors and magnetic pulse generators both output an AC voltage signal that corresponds with crankshaft position and engine speed. This signal is also known as an analog signal. Since logic circuits used in ignition modules and engine control modules cannot process analog signals, the signal is changed into a digital signal, by a signal converter inside the ignition control module. A digital signal is basically read as an on/off signal by the control module.
There are two types of timing control used in automotive electronic ignition systems, module timing and computed timing. Module timing is a condition in which ignition timing is controlled solely by the ignition control module. Computed timing is ignition timing that is managed by the engine control module based on inputs from engine information sensors.
When an engine is first started, the ignition system is operated using module timing. When certain operating criteria has been met, the engine control module signals the ignition control module to switch to computed timing. The signal is usually in the form of a voltage output from the engine control module, that trips an internal switch inside the ignition control module. When this switch is thrown, the primary circuit transistor operates on signals from the engine control module. The engine control module calculates ignition timing based on engine information sensors, such as coolant temperature, engine load and engine speed. Troubleshooting, diagnosing, and auto repair jobs involving ignition issues are all aided by an appropriate OBD scanner.
The ignition control module can be found mounted on the engine, placed in or on the distributor or mounted to the underhood sheet metal. Since the control module contains heat sensitive electrical components, steps are taken to insulate the module from engine heat. Modules that are mounted on the underhood sheet metal or on the engine, usually contain a thick film of insulating material around the internal circuitry. In addition, engine mounted control modules may also use heat shields between the module and the engine. Distributor mounted modules are insulated using a silicone paste between the module and the mounting surface. When replacing a distributor mounted control module, the silicone paste should always be applied to the mounting surface of the module.
(Mark gave up on sports when the Browns left Cleveland and now spends his Sundays working under a shade tree in the back yard tuning his son’s soap box derby car.)