57 model 172 generator light

[Doug Reid]

Is the generator light on that model an over voltage indicator or a failure indicator ?

Received a phone call from a friend with this issue. I will of course check it out with my voltmeter, but I think the owner might be misinformed as to the function.

 

[Dan Thomas]

Is the generator light on that model an over voltage indicator or a failure indicator ?

Received a phone call from a friend with this issue. I will of course check it out with my voltmeter, but I think the owner might be misinformed as to the function.

It does the same thing the old idiot lights did in generator-equipped automobiles. It's not an overvolt indicator. It lights up when the generator's output voltage falls below the battery voltage.

Even in alternator-equipped vehicles, including airplanes, it doesn't indicate an overvolt condition. It indicates that the voltage regulator (and therefore the alternator) has been shut off for some reason. In the car, it's because the alternator stator voltage has fallen far enough that the regulator relay has opened. In the airplane, it's because the regulator has been shut off (with master on) either because the pilot did that, or the overvolt sensor shut the regulator off because it sensed an overvolt condition.

 

[Tom-D]

It does the same thing the old idiot lights did in generator-equipped automobiles. It's not an overvolt indicator. It lights up when the generator's output voltage falls below the battery voltage.

Dan got it right

 

[flyingbrit]

Normal operation:
Light is on if the master switch is on and the engine not running, or at a slow idle.

Abnormal operation:
Light is on with engine at normal operating speed. This can be caused by a generator failure such as worn brushes, or a field circuit failure in either the generator or regulator. In theory, the cut out relay (part of regulator) could fail but the generator operate normally. In this case the light would gradually increase in brightness as the battery discharged. I've never seen this condition so I expect it is quite uncommon.