Alternator Troubleshooting

This guide covers step-by-step procedures for diagnosing and isolating common issues with Plane-Power aircraft alternators and voltage regulators. An experienced mechanic can perform most of these steps with a hacksaw blade and a Volt/Ohm Meter (VOM), also known as a Digital Voltmeter or Multimeter. For a complete visual reference, download the troubleshooting flowcharts at the bottom of this page.

Quick Alternator Field Check

Energizing the aircraft alternator circuit while the unit is not running creates an electromagnet. Holding a hacksaw blade against the alternator housing will produce a magnetic pull if the alternator is creating a field. If there is a pull when the alternator rotates, electrical output should occur.
*** If there is a pull, proceed to the output load test below. If there is no pull or it is very weak, proceed to the field verification steps.

Alternator Output and Load Test

Even with a confirmed magnetic field, the alternator must be verified to carry a load under operating conditions.

1. Run the engine with a typical electrical load applied.
2. Connect a VOM to the B+ terminal and measure voltage against ground.
3. For 12-volt systems, voltage should be 13.5 volts or above 1,500 RPM. For 24-volt systems, voltage should be 25.5 volts or above. If voltage is below these thresholds, alternator output is low.
4. If voltage checks out, switch the VOM to AC volts and verify a maximum of 1 volt AC into the bus. More than 1 volt AC indicates a rectifier issue.
5. Failure of either test is cause to remove the alternator for inspection or overhaul.

Field Verification (No Pull or Weak Pull)

If the hacksaw blade test produces no pull or a weak pull, follow these steps to isolate the cause:

Step 1: Verify Field Input Voltage

Check that the voltage at the field terminal (F1) is approximately equal to bus voltage. If not, inspect the voltage regulator and associated connections, wiring, circuit breakers, and switches for faults.

Step 2: Check Field Resistance

With the F1 and F2 (if provided) field terminals isolated from the circuit, measure the resistance across them. Expected resistance is approximately 4-8 ohms, depending on whether it is a 12-volt or 24-volt system.
If resistance is not within range, remove the belt or alternator (if gear-driven) and slowly rotate the output shaft by hand while measuring. Using an analog VOM, watch for:

• High resistance, unsteady resistance, or an open circuit (infinite): possible bad brushes, slip ring contamination, or an open rotor.
• Zero resistance: the field circuit is shorted. The rotor should be considered suspect.

Voltage Regulator Troubleshooting

Most aircraft voltage regulators in use today are solid-state and do not use the contact points and coils found in older designs. As a result, regulator problems other than basic adjustment are minimal. Solid-state regulators typically either work or they do not.
However, as regulators have become more sophisticated, they are also more prone to false indications. Combined with modern glass cockpit warning systems, voltage fluctuation warnings may be triggered by transient conditions that do not represent a true fault.

Regulator Input and Output Voltage Check

1. Verify input voltage to the regulator. It should be approximately equal to bus voltage. A drop of more than 0.5 volts between the bus and the regulator input indicates a fault upstream of the regulator, such as a wiring issue or a poor connection.
2. Verify output voltage from the regulator. If the drop between the regulator input and output exceeds 1.5 volts, the regulator likely has a ground issue or an internal fault.

*** Important: Solid-state regulators are sensitive to electrical transients. Even a brief, inadvertent short to ground can destroy the regulator. Always verify wiring integrity before replacing a regulator.

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