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Old 01-25-2021, 06:39 PM
ssokol ssokol is offline
Join Date: Jan 2016
Location: Cupertino, California
Posts: 118
Default Avionics Bus Dropping Offline?

I tried to go for a brief flight this evening and found that gremlins have apparently invaded my electrical system. I last flew about a week ago - a brief 30 minute flight around the area. Everything worked as expected.

When I fired up this evening, the battery was a bit weak - took a few more blades than usual. I had done a bit of avionics testing and chalked the weak battery up to having used a few more amps than I thought.

Once the engine was turning as normal I flipped on the avionics master and... crickets. The avionics master controls an automotive relay that feeds power from the main bus to the avionics bus. (See the attached diagram.) Apparently the relay didn't close, so no juice to the avionics bus.

I snapped the avionics master back off and then turned off the master switch. The master bus remained hot (the alternator was live, and the ALT Field is controlled by a separate breaker). I snapped the avionics master back on and all the avionics went live.

Snapped the master switch back on, re-connecting the battery. Then I tried to turn on the strobes (powered from the master bus) and the avionics all went off.

I repeated this several times, and it seems that with the batter and alternator both connected to the master bus, any significant change in the load causes the avionics master relay to open - and remain open until cycled.

I shut down, pushed it back into the hangar and charged the battery for an hour. Tested everything off of the battery (engine / alternator off) and no problems: I could add and remove load without having any impact on the avionics relay. Everything worked as expected.

I hauled it back out, fired it back up (started immediately) and... same thing. Avionics don't want to come on line or stay on line.

I tested the relay and it checks out on the bench - closes at about 8v. Draws about 0.1 amp at 12v. No resistance on the switched path.

I tested the avionics master switch - grounds the relay coil as expected.

This same configuration has been working for about two years now with no issues.

Anyone have a guess what's going on?
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Steve Sokol
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Old 01-25-2021, 06:50 PM
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Jpm757 Jpm757 is offline
Join Date: May 2013
Location: Sherman, CT
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Sounds like your automotive avionics relay is the problem, I would start by replacing the relay.
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Old 01-25-2021, 09:18 PM
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GalinHdz GalinHdz is offline
Join Date: Mar 2010
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Originally Posted by Jpm757 View Post
Sounds like your automotive avionics relay is the problem, I would start by replacing the relay.

I have a similar setup but with two automotive relays in parallel, each with it's respective switch. This provides redundancy in case one of the relays goes bad on me. FWIW, after 8yrs and almost 900hrs they are both still working fine.

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Old 01-25-2021, 10:09 PM
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PaulvS PaulvS is offline
Join Date: Aug 2019
Location: Western Australia
Posts: 559

Try charging the battery for longer, i.e. overnight. Perhaps an hour on the charger may not have been adequate? Easy to do!
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Old 01-25-2021, 10:25 PM
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MCA MCA is offline
Join Date: Jun 2005
Posts: 706

Intermittent problems are always challenging.

At first blush this looks like a bad connection somewhere that has vibrated loose over the years. I've also seen bad switches that click but don't close the connection.

So possible problem areas include:
- bad relay
- bad switch
- loose switch to relay wiring
- loose bus wiring
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Old 01-26-2021, 07:40 AM
Mich48041 Mich48041 is offline
Join Date: Apr 2008
Location: Riley TWP MI
Posts: 3,161

If there is no avionics bus, then it can not fail. Avionics in my plane are connected to the main bus.
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Old 01-25-2021, 06:55 PM
rapid_ascent rapid_ascent is offline
Join Date: Mar 2008
Location: Dublin, CA
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Steve, This sounds like a head scratcher from your description. I think it would help if you measure the voltage on these different busses as you go through your testing. That additional information should help.
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Old 01-25-2021, 07:11 PM
tthrew tthrew is offline
Join Date: Jan 2015
Location: Peoria, IL
Posts: 90
Default Voltage on main bus

Hi Steve, you need to know what the voltage is doing on the Main bus.
Also, Iíve seen poor connections do strange things. This will show up as a voltage drop or different voltage across a connection.
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