Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsCharging System Diagnosis and RepairMedium
A technician is performing an alternator output test. With the engine at 2000 RPM and all accessories on, the multimeter reads 13.8V at the battery terminals and 75 amps. The vehicle manufacturer specifies a minimum alternator output of 90 amps. What is the MOST likely cause of this discrepancy?
- AOvercharged battery
- BWorn alternator brushes
- COpen circuit in the field winding
- DFaulty charging circuit fuse
Show answer & explanationAnswer & explanation
Correct answer: B. Worn alternator brushes
The voltage (13.8V) is within a normal charging range, but the current output (75 amps) is below the specified minimum (90 amps). This indicates the alternator is producing some voltage but cannot deliver sufficient current under load. Worn alternator brushes are a common cause of reduced current output as they provide an intermittent or poor connection to the rotor's slip rings, limiting the field current and thus the alternator's magnetic field strength and output.
Why the other options are wrong
- A. An overcharged battery would typically show a higher voltage (e.g., 15V+) and is not directly related to low current output from the alternator.
- C. An open circuit in the field winding would completely prevent the alternator from generating any output voltage or current, leading to a battery light and no charge.
- D. A faulty charging circuit fuse would likely result in no voltage or current output at all, or a complete open circuit.
Alternator Brush Wear
Worn carbon brushes in an alternator can lead to poor contact with the slip rings, reducing the field current and consequently the alternator's current output capacity.
- Brushes supply current to rotor.
- Wear reduces contact area.
- Results in low or intermittent current output.
Memory trick: Low Amps, High Suspects: Check Brushes and Wires.