Ignition System: Description and Operation
AUTOMATIC SHUT DOWN RELAYPCM Output
The 5-pin, 12 volt, Automatic Shutdown (ASD) relay is located in the Power Distribution Center (PDC). Refer to label on PDC cover for relay location.
The ASD relay supplies battery voltage (12+ volts) to the fuel injectors and ignition coil(s). With certain emissions packages it also supplies 12 volts to the oxygen sensor heating elements.
The ground circuit for the coil within the ASD relay is controlled by the Powertrain Control Module (PCM). The PCM operates the ASD relay by switching its ground circuit ON and OFF.
The ASD relay will be shut-down, meaning the 12 volt power supply to the ASD relay will be de-activated by the PCM if:
- the ignition key is left in the ON position. This is if the engine has not been running for approximately 1.8 seconds.
- there is a crankshaft position sensor signal to the PCM that is lower than pre-determined values,
ASD SENSE - PCM Input
A 12 volt signal at this input indicates to the PCM that the ASD has been activated. The relay is used to connect the oxygen sensor heater element, ignition coil and fuel injectors to 12 volt + power supply.
This input is used only to sense that the ASD relay is energized. If the Powertrain Control Module(PCM) does not see 12 volts at this input when the ASD should be activated, it will set a Diagnostic Trouble Code (DTC).
CAMSHAFT POSITION SENSOR
Fig.4 Camshaft Position Sensor (CMP) Location:
The three-wire Camshaft Position Sensor (CMP) is located below the fuel injection pump. It is attached to the back of the timing gear cover housing.
The Camshaft Position Sensor (CMP) performs multiple functions. One function is to detect engine speed (rpm). Another function is to relate crankshaft position and Top Dead Center (TDC) of the number 1 cylinder. Because the CMP is now used to relate crankshaft position, the Crankshaft Position Sensor (CKP) is no longer used.
Fig.6 Camshaft Position Sensor(CMP):
The CMP contains a hall effect device called a sync signal generator to generate a sync signal.
The CMP uses three wires (circuits) for operation. One wire supplies a 5-volt signal from the Engine Control Module (ECM). Another wire supplies a sensor ground. The third wire supplies a signal back to the ECM relating engine speed and crankshaft position.
Fig.7 Notches At Rear Of Camshaft Drive Gear:
The sensor detects machined notches on the rear face of the camshaft drive gear to sense engine speed.
The CMP also detects an area on the camshaft drive gear that has no notch. When the sensor passes this area, it tells the Engine Control Module (ECM) that Top Dead Center (TDC) of the number 1 cylinder is occurring. The ECM will then adjust fuel timing accordingly.
As the tip of the sensor passes the notches, the interruption of magnetic field causes voltage changes from 5 volts to 0 volts.
The Camshaft Position (CMP) sensor is located in the distributor.
The sensor contains a hall effect device called a sync signal generator to generate a fuel sync signal. This sync signal generator detects a rotating pulse ring (shutter) on the distributor shaft. The pulse ring rotates 180 degrees through the sync signal generator. Its signal is used in conjunction with the Crankshaft Position (CKP) sensor to differentiate between fuel injection and spark events. It is also used to synchronize the fuel injectors with their respective cylinders.
When the leading edge of the pulse ring (shutter) enters the sync signal generator, the following occurs: The interruption of magnetic field causes the voltage to switch high resulting in a sync signal of approximately 5 volts.
When the trailing edge of the pulse ring (shutter) leaves the sync signal generator, the following occurs: The change of the magnetic field causes the sync signal voltage to switch low to 0 volts.