Tech Tip Corner
Erratic Warm Idle
Erratic Warm Idle Late Model General Motors
An erratic, high, or searching idle condition on late model General Motors fuel
injected vehicles can sometimes be caused by the presence of AC voltage from a leaking
alternator diode on the ECM's DC power lead.
The momentary voltage drop that occurs as the AC voltage crosses the positive/negative
voltage threshold appears to fool the computer into thinking that a drop in system voltage
has occurred. The ECM reacts by increasing engine RPM and boosting the fuel mixture to
compensate for the incorrectly perceived low voltage condition. Technicians can test for
this condition by attaching a voltmeter , set to display AC voltage, to the main alternator
output terminal. The presence of AC voltage greater than 2 volts would indicate that alternator
service may be required.
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