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1981 alternator 200

posted by  someone claiming to be John Frasaer on  Thu Mar 21 09:13 CST 2002 [RELATED]

The ALT light glows whenever the alternator output falls below the battery voltage. This is accomplished by having a diode in the instrument panel wiring that only allows current through the light in one direction. That is, the light does not glow when the alternator output exceeds the battery, which is the norm. The Alternator output should measure 13.75 to 14.25 volts depending on the ambient temperature. Lower temp., higher voltage. If this is ok then the alternator output has pulses in it which fall below the battery voltage for very short instances. Since the bulb is an incandescent, it responds to average current. Therefore, a dim glow represents intermittent current flow such as pulses. Such pulses occur in the alternator output when one of the internal diodes is shorted. Either the diode assembly in the alternator must be replaced or a rebuilt alternator installed. An oscilloscope is needed to see these pulses, a voltmeter will not respond fast enough to detect them.






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