Overview of the Alternator Charging System

The bus provides the power for the aircraft charging system through the alternator switch. The field and alternator circuit breakers protect against a short circuit in the charging system.

In this “type B” system the over voltage relay (OVR) provides power to the voltage regulator. If the system voltage exceeds the over voltage trip point the relay will open, removing power from the regulator.

The voltage regulator (VR) controls the excitation of the alternator field to keep the aircraft electrical system voltage at a specific level. An alternator control unit (ACU) combines the voltage regulator with other functions, such as over voltage protection. A “Type B” regulator excites the field of the alternator by providing controlled power to one side of the alternator field. The other side of the alternator field must be grounded.

The aircraft charging system uses the alternator to convert the mechanical energy produced by the engine to electricity used in charging the battery and running other electrical systems in the aircraft. As long as the field circuit of the alternator is fully excited the alternator’s output will continue to increase until it reaches its maximum.


Whenever there is an electrical charging system problem, it is tempting to begin replacing components until the problem goes away. While this method usually works, it can overlook the cause of the component failure. It is important that a systematic approach be used in locating and solving the problem. Charging system problems associated with the voltage regulator and alternator should be analyzed in this manner:


This is the concept behind the unique design of ZEFTRONICS' self-testing, self-protected, field fault/function indicating voltage regulators (Alternator Controllers) like the R1510L, R15V0L, R25400 and others. These units can alert the mechanic to problems before, at, and after the voltage regulator.

Refer to the labeled test points on the system diagram below.


Check the alternator switch (A-C), field circuit breaker (C-D), wires and terminals, and the OV relay (D-E) for contact resistance build-up: resistance should be less than 0.1 ohms.


Check that there is power coming into and going out of the voltage regulator. In this type B system: Without the engine running, with Master switch ON, measure battery voltage (Vbatt) at (E) and approximately 1.0V less than Vbatt at (H). If the two voltages are the same the VR is shorted or alternator field/wire is open. If the VR output is 0 volt, the VR is defective (unless it is self-protected and has turned itself off due to field over-current or grounded field).


  • Check your alternator by measuring the resistance of the field. Resistance from Fld to Gnd on the alternator should be from 3.5 to 5.0 ohms. Check the resistance of the meter leads before measuring field.
  • Check for a “flying” short and other intermittent problems by slowly rotating the alternator while measuring the field resistance. A drop below 3.0 ohms could indicate a bad alternator that might damage the voltage regulator.
  • Check the condition of the alternator's field, pulley, gear/belt, terminals, and wires connecting it to the VR and the aircraft charging system.
  • Check the battery relay for proper operation and connections.
  • Check the fluid levels and charging capacity of the battery.
  • Pay attention to the DIFFERENCES between measured voltages. For example, if 12.7 volts if measured on the input of a switch and 12.0 volts is measured on the output, the difference of 0.7 volts is excessive and the switch is bad.
  • Make sure that terminals, connections, and contacts are not corroded or oxidized.
  • After a problem is found, continue checking the rest of the system. A problem with one section of the charging system can be caused by a subtler malfunction of another section. Example: A shorted voltage regulator can be caused by an intermittent short or arcing in the alternator. Or, a bad ground connection can cause nuisance tripping of the OV relay.
After troubleshooting the system, finding and resolving any faults, run the engine at 1500 RPM. The bus voltage should be 13.8 – 14.3 volts. If a fault occurs, shut down the engine but do not turn off the master switch. Measure the system voltages at the points shown on the system diagram.

Zeftronics is available to help in troubleshooting any aircraft charging system problem. You can contact us at 903-758-6661 or ECSS@zeftronics.com. (Free of charge) Or meet us at Gregg County Airport (GGG), Longview, Texas. (By appointment only, hourly charges will apply)
Alternator Charging System
12.0 12.0 12.0 0.0 - - Not Charging Open OVR
12.0 11.9 11.8 11.7 0.0 0.0 Not Charging Open VR or ACU offline
12.0 11.9 11.8 11.7 ~10.7 ~10.7 Not Charging OK or open field circuit
12.0 11.9 11.8 11.7 0.0 0.0 Not Charging Open field wire
12.0 11.7 - - - - Ammeter/Panel light flicker Bad switch/connector
- 12.0 11.7 - - - Ammeter/Panel light flicker Bad switch/connector
- - 12.0 11.7 - - Ammeter/Panel light flicker Bad OVR/connector
12.0 11.9 11.8 11.7 ~11.5 ~11.5 Overcharging - Pegged Ammeter Shorted VR & bad OVR
12.0 11.9 11.8 11.7 ~11.5 ~11.5 Overcharging - Pegged Ammeter Shorted VR & OVR OK