The Cessna 120-140 Association Forum

Hello All,

I have a 1948 Cessna 140 with an O-200 engine and the ER14-50 Hartzell Plane Power Alternator conversion STC. Today when I turned on the master switch, the "ALT INOP" light didn't come on like it is supposed to according to Hartzell. Have any of you troubleshot this issue? I was able to start the engine using starter button as per normal, and when I turned the engine off using the mixture control, the ALT INOP light briefly illuminated.

Upon reading the instructions for continuing maintenance from Hartzell, I found several references to a "field switch", or "alternator switch". I don't have this in my panel, just a master switch and ALT INOP light, and a breaker for the alternator.

Thanks, Steve

Alternators require a switch to take them off line should they run amok. Is the breaker low amperage, like 2A or 5? Can you pull it to open it, or switch it off?

It's common practice to turn the alternator off prior to shutting the engine down, and leaving it off until the engine is started and running smoothly.

look at the back of your master switch. it may be closing both the master solenoid circuit and the alternator field circuit. i.e. the master switch may very be the alternator field switch.

I did that install on my aircraft about 7 years ago. Your set up should have a separate alternator switch, that is specifically called out in the instructions.

My guess is they used the "old set up"(when the airplane was new and used a generator) whereby the generator field ran through the master switch which was a DPDT switch. In other words when you turned on the master the switch performed two functions, A. it engergized the main battery soleniod activating the electrical system to the whole aircraft and B. it turned on the field for the generator, making it come alive........once it got enough RPM's.

You're going to have to do some digging in your wiring.

I would do two things, find a point on the wiring behind your panel with the C/B's or your fuses to measure voltage and see if you measure about 14volts(with the airplane running at or above 1100RPM's.). That will tell you if your system is operating properly.

Then I would start with the simple stuff first, check all your wiring connections, especially the grounds and the wiring to the light.

If you come up empty, it's probably your regulator(that controls the light).

The "ALT INOP" light is now illuminating when it should, and not illuminating when it shouldn't. Ammeter is slightly positive in flight as per usual. I didn't do anything. The logbook entry and 337 for the alternator installation note that the STC was complied with. I'll trace the wiring back from the circuit breaker/ master switch and see if there is another switch under the panel that I am unaware of.

Thanks all, Steve