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?
Sounds like your automotive avionics relay is the problem, I would start by replacing the relay.
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.
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.
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.
Try charging the battery for longer, i.e. overnight. Perhaps an hour on the charger may not have been adequate? Easy to do!
Intermittent problems are always challenging. :mad:
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
If there is no avionics bus, then it can not fail. Avionics in my plane are connected to the main bus.
The plot thickens...
So I took the relay out of the path and routed the connection from the main bus to the avionics bus through the rocker switch - rated for 20 amps. Powered on the master bus, then the avionics bus and all was well: the avionics powered up.
I tested out different load scenarios and everything seemed relatively happy.
Shut down the avionics bus and lit the engine - started on two blades. Brought the avionics online and everything still looked good. Voltage read nominal (14.1) and after a minute or so the amperage dropped to nominal (1) indicating that the battery was happy.
Took off and flew for about 10 minutes. Voltage apparently started to climb. I noticed it when it was at 14.9 and immediately turned back to home. A couple of minutes later the voltage was reading 15.6 - well into the red zone, but well within the tolerance of all my avionics (everything I have is built for 12v - 28v). But...
I keyed the mic to announce my entry into the patter (fortunately empty) and the radio and audio panel glitched - a kind of "stuttering" pattern that happens when they're not getting enough power. Voltage was up to 15.9.
I cut the alternator circuit, thinking that the alternator was the issue, but the stuttering / brown-out condition continue which would seem to indicate the battery had been draining throughout the flight (?).
Turned off all the switches. Shut down the avionics bus. Put in some flaps, shut down the main bus and landed uneventfully.
Measuring the battery voltage just now and it shows 12.9v.
Bad alternator? (It's a Denso clone with internal regulator - about 19 months in service, probably somewhere south of 200 hours.)
Bad battery? (It's an Odyssey PC680, about 3 years old. Was run down once by leaving the master switch on for 24 hours, but recovered.)
Short somewhere? (No breakers are popping. No wires appear to damaged. No smell of overheated Tefzel.)
Any thoughts / suggestions would be much appreciated.
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