Operation of Alternator light

CapnDean · Nov 10, 2016

One bug to chase that would make me feel better.

When the engine is running, the alternator light is illuminated. A quick test with the voltmeter .... Battery is 12.3 volts with the engine off, and 13.6 volts with the engine running. This indicates to me that the alternator is working. Any idea why the light is on? What tells the light to light in other words?

I did replace the key switch....and the replacement was not exactly the same but I managed to figure that out with the help of Ron at Sundowner. Had to install a jumper wire to keep fire to the pole that opens the run solenoid on the injector pump when starting.

Any ideas appreciated.

dickfoster · Nov 10, 2016

CapnDean said:
One bug to chase that would make me feel better.

When the engine is running, the alternator light is illuminated. A quick test with the voltmeter .... Battery is 12.3 volts with the engine off, and 13.6 volts with the engine running. This indicates to me that the alternator is working. Any idea why the light is on? What tells the light to light in other words?

I did replace the key switch....and the replacement was not exactly the same but I managed to figure that out with the help of Ron at Sundowner. Had to install a jumper wire to keep fire to the pole that opens the run solenoid on the injector pump when starting.

Any ideas appreciated.

With the engine running it should be 14 something volts unless the battery is very low or there is some sort of heavy drain or you're at low idle speeds. What does it read if you rev the engine up? could be something out of whack in the alternator, bad diodes, worn brushes, bad voltage regulator etc. The most common problem if the alternator has a lot of time on it is worn brushes which can usually be replaced for just a few bucks. The brushes conduct current to the field winding in the rotor as demanded by the regulator so if the field isn't getting enough current, alternator output will be low.