Electrical Issue on Avionics Bus
1975 172N. I have two different voltages on the system bus and the avionics bus. When I turn on the master switch, the system bus reads 12.8v. The power side of the contactor (located inside the plane on the left side by your knee) also reads 12.8v. When I switch on the avionics master, the avionics master bus reads 10.6v and both sides of the contactor read 10.6v. However, the system bus still reads 12.8v.
Thinking it was a current draw issue, I turned off all radios/avionics but no difference. I also replaced the contactor (thinking it was corroded inside and causing too much of a draw), but same effect. It's causing my EDM-700 which is powered with the avionics master, to read a low voltage which is what set off this investigation. I've tested with a Fluke multimeter and the EDM is accurate.
I've also put a jumper wire connecting the two busses (bypassing the avionics master switch and contactor) and it will then read 12.8v on both buses. If something was pulling so much current on the avionics bus, I would think when I jumped them, it would cause the system bus to drop also, but it doesn't.
I don't understand why there would be such a big voltage drop on the avionics bus when I turn on the avionics master switch. I'm concerned that I'm not supplying enough voltage to my avionics equipment but when I turn them all on, the voltage stays at 10.6 or so.
Any ideas or troubleshooting tips?