Pre-2000 Ignition Control Module (ICM) - How It Works
Courtesy of Caddyshack100
This Powertrains Ignition Control System controls fuel combustion by providing a spark to ignite the compressed air/fuel mixture in each cylinder at the correct time. This ignition control system has several advantages over a mechanical distributor ignition system.
No moving parts to wear out.
No mechanical load on the engine.
Elimination of mechanical timing adjustments.
Located for easier service and improved reliability.
Improved high engine speed performance.
The Ignition Control System consists of the following components:
Two crankshaft position sensors (A and B).
Crankshaft reluctor ring.
Camshaft position sensor.
Ignition control module.
4 separate ignition coils.
Eight spark plug wires and conduit.
Eight spark plugs.
Powertrain Control Module (PCM).
The Ignition Control System does not use a conventional distributor or a single ignition coil. In this ignition system, both ends of each of the four ignition coils are connected to a spark plug. Each coil is connected with spark plugs on companion cylinders, i.e., on top dead center at the same time (1-4, 2-5, 6-7, and 3-8). One cylinder is on its compression stroke when the other one is on its exhaust stroke.
When the coil discharges, both plugs fire at the same time by using the engine block to complete the electrical circuit. The cylinder on the compression stroke is called the event cylinder and the one on the exhaust stroke is the waste cylinder. The two cylinders share the energy available from the ignition coil to fire both spark plugs. This method of ignition is called waste spark ignition.
Since the polarity of the ignition coil primary and secondary windings does not change, one spark plug always fires with a forward current (center electrode to ground electrode) and its companion plug fires with a reverse current (ground electrode to center electrode). This is different from a conventional distributor ignition system that fires all the plugs with the same forward current flow.
It is possible for one spark plug to fire even though a plug wire from the same coil may be disconnected from its companion spark plug. The disconnected plug wire acts as one plate of a capacitor and the engine block acts as the other plate. These two capacitor plates are charged as a spark first jumps across the gap of the connected spark plug. The plates are then discharged as the energy is dissipated as the spark continues. Voltage requirements are very high with an open spark plug or wire. The ignition coil may have enough reserve energy to fire the connected plug at idle, but possibly not under some engine load conditions. A more noticeable misfire may be evident under load; both spark plugs may then not fire.
Crankshaft Position Sensors and Reluctor Ring
The two crankshaft sensors are located on the front bank (BANK 2) of the engine block between cylinders 4 and 6. Crankshaft position A sensor is located in the upper crankcase and crankshaft position B sensor is located in the lower crankcase. Both sensors extend into the crankcase and are sealed to the engine block with O-rings. The crankshaft position sensors are not adjustable.
The magnetic crankshaft position sensors operate similar to the pickup coil in a distributor. When a piece of steel (called a reluctor) is repeatedly moved over the sensor, a voltage will be created by the sensor that appears to go On-Off-On-Off-On-Off. This On-Off signal is also similar to the signal that a set of breaker points in a distributor would generate as the distributor shaft turned and the points opened and closed.
The reluctor ring is cast onto the crankshaft between the #3 and #4 main bearing journals. The reluctor ring has 24 evenly spaced notches or air gaps and an additional 8 unevenly spaced notches for a total of 32.
As the crankshaft makes one complete revolution, both the A and B sensors will produce 32 On-Off pulses per revolution. In addition, the A sensor is positioned 27 degrees of crankshaft revolution before the B sensor. This creates a unique pattern of On-Off pulses sent to the ignition control module so that it can recognize crankshaft position.
The camshaft position sensor is located on the rear cylinder bank (BANK 1) in front of the exhaust camshaft. The camshaft position sensor extends into the rear cylinder head and is sealed with an O-ring. The camshaft position sensor is not adjustable.
As the rear cylinder bank exhaust camshaft turns, a steel pin on its drive sprocket passes over the magnetic camshaft position sensor. This creates an On-Off-On-Off signal sent to the ignition control module similar to the crankshaft position sensors. The camshaft position sensor produces one On-Off pulse for every one revolution of the camshaft or every two revolutions of the crankshaft. This allows the ignition control module to recognize camshaft position.
Ignition Control Module
The Ignition Control (IC) module is located on top of the rear camshaft cover. The IC module performs several functions:
It monitors the On-Off pulses produced by the two crankshaft and one camshaft position sensors.
It creates a 4X and 24X reference signal (4X REF HI and 24X Crank) sent to the PCM for ignition control.
It creates a camshaft reference signal (CAM HI) sent to the PCM for fuel injection control.
It provides a ground reference (REF LO, CAM LO) to the PCM.
It provides a means for the PCM to control spark advance (BYPASS and IGNITION CONTROL) called IGNITION CONTROL MODE.
It provides a limited means of controlling spark advance without PCM input called MODULE MODE.
The IC module is not repairable. When a module is replaced the remaining components must be transferred to the new module.
Four separate coils are mounted to the module assembly. Each coil provides the spark for two spark plugs simultaneously (wasted spark ignition). Each coil can be replaced separately.
Spark Plug Wires
The spark plug wires connect the ignition control module to the spark plugs. It incorporates several plastic channels and conduits to keep it properly positioned and to protect it. The spark plug wires are 7 mm in diameter and the outer jacket is made of silicone to withstand high temperatures. The silicone jacket is also an excellent insulator for the high voltages used in the ignition system. The silicone spark plug boots provide a tight seal on the spark plug. Care should be exercised when connecting a timing light or other equipment. Do not force anything between the boot and wiring or through the silicone jacket. Connections should be made using an appropriate adapter.
Eight spark plugs are centrally located in each cylinder combustion chamber and can be accessed through holes at the top of both cylinder bank camshaft covers. The spark plugs have platinum pads welded to the electrodes. These pads extend the spark plug life to 160,000 kilometers (100,000 miles).
Worn, cracked or dirty plugs may give satisfactory operation at idling speed, but under operating conditions they frequently fail. Faulty plugs are indicated in a number of ways: poor fuel economy, power loss, loss of speed, hesitation, shudder, medium throttle intake manifold backfire, hard starting and general poor engine performance.
Fouled plugs may be indicated by black carbon deposits. The black deposits are usually the result of slow-speed driving and short runs where sufficient engine operating temperature is seldom reached. Worn pistons, rings, faulty ignition, over-rich fuel mixture or low heat range spark plugs may result in carbon deposits.
Excessive gap wear on plugs of low mileage, usually indicates the engine is operating at high speeds or loads that are consistently greater than normal or that a plug which is too hot of a heat range is being used. Electrode wear may also be the result of plug overheating, caused by combustion gases leaking past the threads, due to insufficient torque of the spark plug. Excessively lean fuel mixture will also result in excessive electrode wear.
Broken insulators are usually the result of improper installation or carelessness when gapping the plug. Broken upper insulators usually result from a poor fitting wrench or an outside blow. The cracked insulator may not show up right away, but will as soon as oil or moisture penetrates the crack. The crack is usually just below the crimped part of shell and may not be visible.
Broken lower insulators usually result from carelessness when gapping and generally are visible. This type of break may result from the plug operating too Hot, which may happen in periods of high-speed operation or under heavy loads. When gapping a spark plug, always make the gap adjustment by bending the ground (side) electrode. Spark plugs with broken insulators should always be replaced.
Each spark plug boot covers the spark plug terminal and a portion of the plug insulator. These boots prevent flash-over which causes engine misfiring. Do not mistake corona discharge for flash-over or a shorted insulator. Corona is a steady blue light appearing around the insulator, just above the shell crimp. It is the visible evidence of high-tension field and has no effect on ignition performance. Usually it can be dust particles leaving a clear ring on the insulator just above the shell. This ring is sometimes mistakenly regarded as evidence that combustion gases have blown out between shell and insulator.
Base Ignition Timing
The base ignition timing is determined by the relationship of the crankshaft position sensors to the reluctor ring. This relationship is not adjustable and results in a base ignition timing of 10° BTDC.
IC Module Mode
There are two modes of ignition system operation: PCM mode and Ignition Control Module (IC Module) mode. In IC Module mode, the ignition system operates independently from the PCM. The ignition control module maintains a base ignition timing of 10° BTDC and is able to change this ignition timing slightly with increased engine speed. IC Module mode is in effect whenever an ignition control fault is detected while the engine is running and it will have a noticeable effect on driveability. In PCM mode, the PCM controls the ignition timing. The PCM calculates the desired ignition timing based on information it receives from the input sensors.
PCM Timing Mode
The Powertrain Control Module (PCM) controls spark advance and fuel injection for all driving conditions. The PCM monitors input signals from the following components as part of its ignition control function to determine the required ignition timing:
Ignition Control Module (IC Module).
Engine Coolant Temperature (ECT) sensor.
Manifold Absolute Pressure (MAP) sensor.
Transaxle Range (TR) switch.
Throttle Position (TP) sensor.
Vehicle Speed Sensor (VSS).
Knock Sensor (KS).
The crankshaft reluctor ring has 24 evenly spaced notches plus 8 additional notches (shaded) used for synchronization.
As the crankshaft rotates, the notches pass the position sensors and create a voltage pulse signal in the sensor that is an input for the ignition control module (ICM).
Because of the physical location of the 2 crankshaft position sensors, the signal of B lags the signal of A by 27 degrees of crankshaft revolution.
To synchronize the ignition, the ICM first counts the number of B pulses between every 2 A pulses. There can be 0, 1, or 2 B pulses between A pulses.
When the ICM sees 0 B pulses between A pulses , it starts counting B pulses between A pulses. When the ICM counts exactly 4, it synchronizes the ignition on the very next A pulse. If the ICM counts over 4 (jumps from 3 to 5), it waits for another B pulse between A pulse to start counting again.
This process allows the ignition to synchronize and fire the first spark plug within 180 degrees (1/2 engine revolution).
The camshaft position (CMP) sensor provides the ICM with cylinder #1 firing order information, which the PCM uses for sequential fuel injection.
Using 3 sensors allows the ICM to maintain ignition synchronization even if one of the 3 sensors fails.