The Cessna 120-140 Association Forum

Hi all,

I have an issue and thought one of you electrical savvy folks might be able to assist. I have a 20 amp Delco generator and Zeftronics G1200N voltage regulator (just under 2-years old) installed in my airplane.

A few weeks ago, I was getting night current and after a while of flying around with everything on (radio, transponder, panel lights, beacon, landing light) I noticed that my bus voltage had dropped to battery voltage (around 12.3V, normally it runs about 14.2V). I cycled things off and saw no improvement in the bus voltage. The same thing happened to me when I was flying back from OSH on Friday. Again, I had everything noted on. I then turned everything off except the master and noted no change in the voltage. I varied the rpm and noticed no change in bus voltage and the ammeter showed a slight negative charge no matter what the engine rpm.

Normally, if I fly around with just the radios on (no lights) this doesn't happen. It only seems to happen when I load the system up. I have LED landing and nav lights, incandescent panel lights, and dual Whelen 70509 beacons. My total amp draw with everything on is 12.4 amps...up to 13.9 amps when the radio is transmitting.

Would needing to polarize the generator by flashing the field cause this kind of problem? I was thinking of trying that, but had some questions about how to do it. Any other thoughts on what might be wrong?

I thought I would try the forums first before taking my plane to a mechanic. Hoping for some simple and obvious solution (flashing the field?) that can be used to sort it out.

Thanks in advance for any advice.

You should never need to flash the field of a generator that was installed and working. Flashing the field restores the residual magnetism to the field poles, and the only time they would loose that is if the generator was dropped on the floor or overhauled.

Make sure the connections to the generator are tight and clean. Make sure the generator fuse or breaker is good. Make sure the wiring between the generator and the regulator is in good condition. Your generator field wire should pass through the master switch. Make sure the master switch is completing the field connection from the regulator to the field terminal on the generator.

If you're still stuck, you can test the generator. You will need a volt meter (not the one in the plane) and a secure way to connect it to the generator.

Remove the field and B+ (ARM) wires from the generator. Insulate both from the airframe and each other. Connect the voltmeter plus (red) lead to the generator B+ terminal and the negative (black) lead to the engine case. Set the meter to DC volts and a range that accommodates at least 5 volts. Run the engine and note the reading. It should be 1-2 volts. Expect at least 1.6 volts at 1300 RPM. If you get a negative voltage the polarity of the generator is reversed. This can be corrected by flashing the field, but you have to wonder how it could have gotten that way...

Next, install a secure jumper from the field terminal to ground. Leave the volt meter connected and select a range that allows at least 20 volts. Now, run the engine and note the voltage. It should follow engine RPM and exceed 13 volts by about 1500 RPM.

If you pass these two tests the problem lies in the wiring or regulator. (Note: this is the party line. I think bad brushes may be able to pass the second test but not carry current to run the accessories and charge the battery. If all else fails, check the brushes.)

If you fail the first test, you can try flashing the field, but, as previously stated, operating generators don't typically loose their residual magnetism.

Well, I think I found what might be the problem. I went flying and turned on the electrical systems in sequence. Everything was stable until I got to the Whelen flashers when the voltage started to drop off and the ammeter started jumping around from - 10 to + 10 amps. Turned the flashers off and the voltage and amps stabilized. I landed, pulled the lamps, and found this on the belly light.

6930 wrote: ↑Sun Jul 30, 2023 2:11 pmthe ammeter started jumping around from - 10 to + 10 amps.

Yup! That's a sure sign of an overload somewhere.

Thanks for the outstanding info John (as always)! I think, as you saw, I might have uncovered the issue ie corrosion in the lower flasher. I cleaned the pin connectors on the base with DeoxIT and a tiny brush and will get a new bulb installed. I think that must be it because everything else seems to be operating properly. However, if that doesn’t solve the problem I’ll dig in as you advised.

Again, thank you so much! Great info!

By the way, the top flasher is fine. I think the oil along the belly of the airplane gets into to light housing. Maybe some water? Looks like whatever it was (oil, water or both) caused massive corrosion and the load issues.

Most likely water. Oil doesn't promote corrosion.