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Fluctuating Ammeters

by Don Maxwell

MAPA Log, Volume 28,

Number 3 (March 2005)

Fluctuating ammeters are a common complaint we receive from many Mooney owners, normally on a weekly basis. Callers tend to be frustrated after spending hundreds of dollars replacing alternators, voltage regulators, ammeters and wiring and the problem continues to exist.  During daylight flights, a fluctuating ammeter is just an annoyance. However, a night flight is a whole different story.  I once had a caller describe his problem as, “. . . it’s like attending a séance.” As the ammeter needle dances back and forth, the panel and instrument lights pulse and other engine gauges try to keep time with the ammeter. Alone at night, IFR, this phenomenon can be very discomforting.


Fluctuating ammeter needles are very common on all Mooney’s starting with the first M20J’s. In nearly all cases, the problem is caused by corrosion on the terminals of the master switch. An easy diagnosis is, while in flight,  to apply pressure to the connectors on the back of the master switch with your finger.  You will not receive a shock. The fluctuation of the ammeter should stop.


The master switch is a double pole, single throw switch. This simply means it is two separate switches in one case, and that both switches are either on or off at the same time. One side of the master switch controls the master electrical relay at the battery, thus supplying power to the aircraft buss. The other side of the master switch completes, or opens, the circuit between the alternator field and the voltage regulator. This switch arrangement has been used throughout Mooney production with both alternators and generators. The difference between earlier pre “J” models and later Mooneys is the actual master switch itself. Earlier models, (Figure 1) used a toggle type switch with screw on connector, (Figure 2) to attach the wiring to the switch, while later models use a rocker type switch, (Figure 3) with slip-on type connector, (Figure 4) to attach the wiring.

The voltage regulator can be compared to a control tower at the airport. It directs the alternator on what to do and when. The communication between the voltage regulator and the alternator is directed thru the field wire. If the connection between the two devices is good and corrosion free, the two communicate freely.  There is no delay from the alternator in response to the voltage regulator request. The result is a smooth and steady ammeter movement. Corrosion on the slip-on terminals of later model Mooney’s can cause resistance, or delay, in the communication between the


It’s kind of like flying into Love Field with all the kids in the back seat talking at the same time.  voltage regulator and alternator. The delay between the request of the voltage regulator and the response of the alternator is what causes the ammeter fluctuation.

The tower says “expedite,” you miss the call due to the fuss in the back, the tower calls again, now you start to expedite, and then they say “hold your position.” Anyway, hold, stop, hold, stop that’s what the ammeter is doing.

The good news is the fix is simple and inexpensive. Simply clean the terminal on the back of the master switch.

The switch is easily accessible on most models. Newer models have the master switch located above the ignition switch and are more difficult to reach. Sliding the connector back and forth on the terminal is all that is required in most situations.  Extreme situations may require the master switch to be replaced due to corrosion inside the switch.

As usual, feel free to contact me with your technical questions at (903) 643-9902. or

Another helpful web site for diagnosing electrical charging problems is


Figure 1


Figure 2


Figure 3


Figure 4

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