Theory Of Operation
The Electronic Voltage Regulation (EVR) system maintains the system voltage at a desired level by turning the Pulse Width Modulated (PWM) Alternator Field Control circuit (A) on and off. When the Alternator field is turned on, the system voltage increases. When the Alternator field is turned off, the system voltage slowly drops. The rate at which this happens is dependent upon the existing electrical loads, ambient under hood temperature, and the engine speed. A constant system voltage (B, C, D) can be maintained only when the Alternator field is switched on and off at a duty cycle that very accurately emulates the existing electrical loads given the existing ambient under hood temperature and engine speed.
During normal operation: The voltage reading at the Alternator output stud will be very close to the target charging voltage viewed on the scan tool. This is the system voltage and is sensed by the Powertrain Control Module (PCM) through the Fused B+ circuit (B). With the Alternator connector plugged in, the voltage reading on the Alternator Sense circuit will be approximately 3.5 volts less than the voltage at the Alternator output stud due the resistor inside the Alternator. This is the Alternator Sense (E) input to the PCM. These two voltage sense inputs are used and compared during the different diagnostics performed on the EVR System by the PCM.
Diagnostic Mode: When the PCM detects that the output voltage is too high or too low, the PCM runs a series of diagnostics to determine the cause. When the diagnostic is initiated, the PCM will change the field control circuit and look for a change in the output voltage. If no reaction is detected, the PCM rationalizes that it has lost control of the Alternator Field Control (A) due to an open or shorted condition. If the Alternator Field control is active, and the engine speed is above a calibrated threshold, the PCM will run a series of tests that turn the Alternator Field control off and on for a brief period of time and monitors the system voltage (B) for a calibrated amount of change in the voltage, to determine a failure of the Alternator. This diagnostic test requires repeated failures to insure that an erroneous fault is not set.