A technician is performing an alternator output test. With the engine at 2000 RPM and all accessories on (high beams, blower motor on high, rear defroster), the alternator is specified to produce 120 Amps. The technician measures 95 Amps. The charging voltage at the battery is 13.8V. What is the MOST likely cause of the lower-than-specified output?
- AWorn or sticking alternator brushes.
- BA shorted diode in the rectifier bridge.
- CA faulty voltage regulator.
- DAn internal open in the stator windings.
Show answer & explanationAnswer & explanation
Correct answer: A. Worn or sticking alternator brushes.
Worn or sticking brushes would intermittently or consistently reduce the current flow to the rotor's field coil. This weakens the magnetic field, resulting in lower-than-specified current output, even if the voltage regulator is attempting to maintain the correct voltage. A voltage of 13.8V indicates the regulator is still trying to control, but the current capability is limited.
Why the other options are wrong
- B. A shorted diode in the rectifier bridge would cause excessive AC ripple, overheating, and often reduced output, but the primary symptom related to current output limitation at a stable voltage points more directly to the field current supply, which the brushes control.
- C. A faulty voltage regulator would typically cause either overcharging (high voltage/current) or undercharging (low voltage/current, often with a battery light), but 13.8V is a reasonable regulated voltage, implying the regulator itself isn't the primary fault for *low current* at that voltage.
- D. An internal open in the stator windings would significantly reduce or eliminate alternator output, likely causing the voltage to drop much lower than 13.8V and illuminating the battery light constantly.
Alternator Brush Issues
Worn or sticking carbon brushes in an alternator can lead to poor contact with the slip rings, resulting in insufficient current flow to the rotor's field coil. This weakens the magnetic field and limits the alternator's maximum current output, even if voltage regulation appears normal.
- Brushes supply current to rotor field coil.
- Worn brushes cause intermittent or low field current.
- Leads to reduced maximum current output capacity.
Memory trick: Output's low, voltage's fine, brushes wore out, that's the sign.