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?

Comments

  • Scott ShererScott Sherer COO Forum Moderator

    12.8 is right on the money and with the engine running at reasonable RPM that's what you should see with all of the avionics powered up.
    Here's what you should do next. Pull all of the circuit breakers and push them in one at a time while reading the voltage. The two voltages should match with all of the breakers out. My guess is that there's something that's turned on when you turn on the avionics master. This should help.

    Scott Sherer
    Wright Brothers Master Pilot, FAA Commercial Pilot
    Aviation Director, Cessna Owners Organization Forum Moderator and Cessna Owners Author.

    Need help? Let me know!

  • Thanks Scott. Unfortunately, only two of the seven breakers on the avionics bus are "pullable" (XPDR and AutoPilot), the rest are reset only. I've tried those two and they aren't the culprits. Other than unscrewing the terminals from the breakers behind the panel, is there anything else I can try? I can pull the devices out of the trays which should have the same effect of removing them from any draw?

  • Scott ShererScott Sherer COO Forum Moderator

    My guess is that it's something that's not in a tray or very obvious, like an altitude encoder hidden behind a panel somewhere. Pull out your parts and service manuals and look at the electrical drawings and see if anything looks obvious to you.

    Scott Sherer
    Wright Brothers Master Pilot, FAA Commercial Pilot
    Aviation Director, Cessna Owners Organization Forum Moderator and Cessna Owners Author.

    Need help? Let me know!

  • Thanks for the input. The issue was a corroded/burned connection terminal on the relay attached to the top of glove box. It is the feed to the avionics switch contactor. Since the system bus never dropped voltage, even when the avionics bus did, it was clear the supply to the avionics bus contactor was the problem. We cut the old connector off, up to good wire, and crimped on a quality terminal connector. Both busses read the same 12.8v now.

    I expect the voltage to the avionics has been low for a long time, perhaps many years. Would low voltage cause any damage to my avionics stack? (2) KX-155, Garmin GPS500W, KN62 DME, KMA 20 Audio Panel, Transponder?

  • Scott ShererScott Sherer COO Forum Moderator

    Newer avionics have voltage sensing and will turns themselves off. Older devices might be damaged by low voltage. However, low voltage is alot better than too-high voltage. I'm really glad you found the problem.

    Scott Sherer
    Wright Brothers Master Pilot, FAA Commercial Pilot
    Aviation Director, Cessna Owners Organization Forum Moderator and Cessna Owners Author.

    Need help? Let me know!

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