[kevin]

Hi folks,

I am having an interesting experience with my electrical system. You know, like the chinese curse: "May you live an interesting life."

Aircraft is a '73 P337. Electrical system is all stock, plus an EI volt/amp meter wired so that current can be measured for each alternator individually, or for the system. Voltage cannot be measured individually, except by turning off one alternator or the other. Oh, and I have an EI Superclock with voice annunciator that monitors bus voltage all the time.

Last week, I started to see the rear alternator drop off line after landing. This is visible ONLY on the EI gauge, the stock Cessna stuff was happy, but the EI gauge would show only .1 or .2 amps and after a bit the whole system would go into discharge (again, only on EI gauge (yellow discharge light), the Cessna idiot lights were all out. I checked the idiot lights by turning off one, then the other, then both alternators, and all three lights (left, right and discharge) work as expected. Anyway, I can't remember all the symptoms, but everything just seemed sick and wrong.

So my mechanic and I attempted to rebalance the regulators, which is the first thing I look for when the system is sick.

BTW, I have one really old regulator (three adjusting screws) and one relatively (1 year) new one (one screw).

Anyway, in the course of troubleshooting, we discovered that the rear alternator was in fact going off line, as evidenced by an external voltmeter. After it went off line, whacking the regulator (the new one) would get it to start working again. We decided to replace the regulator (and balance the system). After doing that, all seemed well for two or three flight hours.

On my flight to Canada and back today, I noticed the rear alternator was only putting out 0.1 amps. I did not screw with it, as I was IFR, and needed my electrical system. But I did determine that it seemed unaffected by load. The front alternator was taking up the slack just fine, putting out about 14 amps.

I shut down when I arrived, to drop some pax at the terminal. When I started the airplane again, everything was suddenly working correctly. The rear was not putting out a lot of current, maybe 2 amps, but it was contributing. Mumbling under my breath (because I had already called my mechanic and he was on his way to the airport to work with me on it), I shut off the avionics master in preparation for shutting down. The FRONT alternator went completely offline, zero amps, and a front alternator idiot light. Turn the avoinics master back on, the idiot light goes out.

My first theory was that the regulators were unbalanced again. The voltage on the front alternator was 27.4, the rear 28.2. So we started to balance them again, by changing the adjustment on the new regulator. (We have been trying to avoid touching the old one, as they are harder to adjust.) We reduced the voltage on the new regulator to 27.4 or so.

Starting both engines, now on the ground the airplane behaves exactly as it did in the air. Here is the sequence:

1.) Both engines running 1500 RPM. Front alternator putting out 6 amps or so, rear 0.1 amps. Turn off front alternator. Rear alternator does NOT pick up the load, rather, it rises to 0.9 amps for a few seconds, then drops to a 1 amp discharge. Turn on the front alternator, the system returns to original condition, 6 amps on front, 0.1 on rear.
2.) Whack the brand new regulator. Rear comes up, starts putting out expected amperage (4 amps or so), front is putting out 2 amps.

We tried some more things which I don't remember, but basically it always came back to 1.) and 2.) above.

I am dubious, but we decide that we suppose we could have gotten two bad regulators in a row. We decide to swap the front and rear regulators. We do that.

Again, both engines running 1500 RPM, at first, the regulators just look way out of balance. The rear (now the old regulator) is putting out about 27.8 volts, the front (new) 28.9. The front alternator is completely offline (no amps). So we adjust the front (new) regulator down to 28 volts.

Now, at least on the ground at 1500 RPM, everything works perfectly. (No whacks on either regulator required, nearly even current draw, correct system voltage.)

Why? I don't get it. I have not really looked at the circuit, and I don't know more than the basics about electricity and electronics. I don't understand why it works now, and I don't understand what whacking the regulator was about. Neither does my A&P. I pulled the regulator we replaced out of the garbase, as it may be good after all.

I will fly it tomorrow (oh good, an excuse to go flying!), and we'll see how it does.

Any thoughts anyone?

Kevin