Power Surge

I am posting this problem to see if any of the VAF brain trust can help steer me to the answer. I was flying on a two hour XC yesterday in my -9A (steam gauges) when I heard static in my headset. The amp gauge was bouncing peg-peg (+/- 40 amps). Voltage was constant at 14.2. I shut off the strobes; no change. Then I shut off the XLS 250 NavCom; no difference. I then switched off the alternator field; again, no change. Next I shut off the avionics switch, which stopped the bouncing amp gauge. After waiting several minutes, I switched the avionics back on, intending to turn off the Garmin 327 transponder, as this and the intercom are the only other devices on the avionics switch. However, the system was quiet and stayed that way all the way home. No smell, no smoke.

I am concerned that I had an intermittent short somewhere, causing the violent amp gauge movements and noise in the intercom. However, no CB's were tripped. I am not sure it was the transponder, as I could not reproduce the problem.

Anybody have any advice what could have happened? Is there anything I can do other than wait for it to happen again and resume troubleshooting in the air?

Thanks.

That sounds like a main wire carrying current prior to any of the breakers - check your primary power from where it penetrates the firewall and comes aft into the avionics bay to the avionics master bus. That will be after the ammeter shunt (so you'll see the current surges) but before any of the breakers (so nothing trips). If the insulation rubs through on the sharp stainless steel firewall edge and starts arcing, or you have a loose wire arcing between there and the avionics master bus, it would cause what you saw.

Doug, my first suspect would be a failing avionics master switch. Easy to swap, might be a good first troubleshooting step. If this happens again, I would feel the avionics master switch and see if it's overheating.

I have a steam guage equiped RV-4, with the CAM 40 amp alt. A couple weeks ago I picked up static in the intercom, but no AMP guage fluctuation as you are experiencing..but my AMP guage always bounces when I transmit with PTT...Im just used to it. What I found (the smoking gun) was a broken spade plug(cracked in half inside the plastic plug) at the alternator for my "idiot light" which was also slightly flickering and only seen during a late evening flight. I never had any issues with my radio or any lighting..just static in headset that drove me nuts trying to isolate. Pat Hatch is who I would listen to, but Im letting you know what mine did..similar event.

Thanks to all who chimed in. I will check out the avionics switch, which is really a combo switch/breaker, the next time it happens. I will also carefully check the main power feed through the firewall to the main bus.