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Avionics Bus Dropping Offline?

ssokol

Well Known Member
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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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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, I’ve seen poor connections do strange things. This will show up as a voltage drop or different voltage across a connection.
 
Sounds like your automotive avionics relay is the problem, I would start by replacing the relay.
^^^^^THIS^^^^^

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.

:cool:
 
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
 
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.

-S
 
alternator

...
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...
The only things that can make this happen are bad readings, or a bad alternator.
 
Without detailed circuit architecture it is impossible to offer real help. There are just too many variables.

I believe you have a high resistance connection somewhere. Likely where your busses, alternator B lead, master power, master contractor, or etc. are connected. The alternator seems to not be getting full battery voltage when the engine is running. This is why you have the high charging voltage.

The battery is not low. 12.9 volts when not running is OK. A fully charged automotive battery is 12.6 volts and an odyssey PC680 is 12.85 volts.

There is the remote possibility of a ground issue, but would look at other first.

You could power everything up, like the plane is running and hook a battery charger to the alternator B lead at the alternator to simulate the circuit under real conditions. If you can reproduce the problem then check voltage at each major junction. Everything 12 volt should be about the same voltage.

If the meter is connected to the battery negative everything that is ground should have very little to no voltage on it. If there is more than .5 volt (I know a general high number) at any grounds there is a ground problem.

You stated you had the engine running, with the master off, and the alternator was charging. This is not good. It is not a good idea to have the alternator running with the battery off line. I would recommend that the master switch be a dual pole switch that also shuts off the field or control lead to the alternator and to be sure that the alternator does shut down when the field or control circuit is disconnected. If not you may not be able to shut down the electrical system with the engine running.

Also, do you have any high voltage protection. My guess is no. If no, it would be a good idea to install something for high voltage protection.

GM
 
From the logs...

I pulled the EMS logs for yesterday's flight. It appears that about four minutes after startup (about a minute before takeoff) the voltage climbed above the 14.1 v set point on the alternator.

At the same time, the log shows the value from the amperage shunt - which measures amps into and out of the battery - went to zero.

From that point the voltage goes up to / bounces around at 14.6 - 14.8 for the next 16 minutes. Amperage reads zero all this time.

At 2:02:50 PM, about 20 minutes after startup, the voltage suddenly jumps to 15 and then bounces between 15.1v and 16.9v. At 2:03:28 I pulled the power to the avionics bus, ending the log.

If I recall correctly - and that's a bit of an if, as I was focused on safely operating the airplane - I popped the alt field breaker to try to switch to battery-only before I shut down the avionics. If so, I don't see any indication that doing so made any difference in the situation: voltage remained high, battery amperage remained zero.

If anyone cares to look at the log, I have it posted here:

https://falkenavionics.com/files/voltage_issue.log

First 13 characters are the timestamp. Voltage is "vbat". Amperage is "ishunt" (multiply by 1000 to get amps).
 
Still Grounded

Between work and the weather snap here in Texas I haven't had a chance to fight with this for a while. Here's where things stand:

I have replaced all of the potential bad actors in this farce: the automotive relay, the PC680 battery, and the Denso internally regulated alternator. The three replacements are in verified working condition. Replacing them did not solve the problem.

The system appears to work as expected when safely sitting on the ground powered by the battery. The avionics master switch grounds the automative relay. The automotive relay feeds power from the main bus to the avionics bus. All the avionics are happy.

Things do not work as expected when the engine and alternator are on. Snapping on the avionics master switch does nothing. The relay doesn't click. The bus does not power up. But other, odd stuff does happen...

I have a stand-alone volt meter (not the one in the EFIS) that I'm using to measure voltage at various places in the system. With the engine off and the master contactor on (so battery only) the master bus shows 12.7 volts between the master bus bar and the negative terminal on the battery. With the master off, it show 0.0v. So far, so good.

When I start the engine, I can see the voltage step up from battery voltage of 12.7 to alternator voltage of 14.2. The voltage then begins to climb rather quickly. When I ran it this afternoon, it climbed to 15.6 within one minute.

So I shut down and started measuring things with the engine off, and heres's where it gets really weird...

I disconnected the master bus (the bus bar on the panel) from the system - it's not connected to the battery or to the alternator.

  • With the master contactor open (off), I measure 0v between the master bus bar and the negative terminal on the battery.
  • When I turn the master switch on, I see about 0.7 volts between the master bus bar and the negative terminal of the battery.
  • When I unground the magneto, the voltage jumps to over 6v.
  • When I disengage the alternator field breaker it jumps to about 8v.

This is with the bus bar completely disconnected from the feed. There should be no path for power to get to the bus. But there's power there. So how?

The master contactor on this airplane has two outputs - the high amperage feed to the starter relay and the (now disconnected) to the bus. That's it.

Weird, eh?

One other oddity that may or may not matter here: for reasons I've never understood, the starter on this airplane won't budge if the alternator field breaker is open. That one is not new - it's been that way since I got it.

[EDIT: Update the above - found where the power to the starter switch comes from the alternator field breaker.]

A friend and neighbor with an RV will be coming over to take a look this weekend. I would appreciate any suggestions as to what we should check.
 
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I think your starter / alt field breaker issue is probably just some laziness on the part of the original builder. You want the starter circuit to come from the main bus and be circuit protected. This circuit normally is on a separate fuse or breaker which is supplied from the switched main bus. It appears that the builder used the alternator breaker for protecting both circuits. Not a great idea. You should investigate adding a separate fuse or breaker for the starter and remove this dependency. It should be pretty simple to fix this problem.

Not sure how to comment on your primary problem. Too many unknowns as others have expressed.
 
Some installations using a relay to control the avionics bus are using the relay differently. Using is Normally closed with the switch activating the relay (and opening the contact) in the apparent «*off*» position would avoid losing avionics when there is an issue with the relay.
 
It seems that if the alternator test good then there is a problem in the path that senses the bus voltage. I'm just thinking out loud here, but if there is some voltage drop in that path then the alternator tries to put out more voltage because it isn't sensing correctly.
 
SNIP

When I start the engine, I can see the voltage step up from battery voltage of 12.7 to alternator voltage of 14.2. The voltage then begins to climb rather quickly. When I ran it this afternoon, it climbed to 15.6 within one minute.

This is a classic “failing ND internal regulator” indication. As you have muliple stuff going on, it could also be a weird “sense voltage” going to the alternator - weird as in a bad contact, bad breaker or such so the alterntor is seeing a sense voltage input lower than actual buss voltage. Note, for internal regulated alternators you provide a sense voltage, the alternator field voltage is provided by the internal regulator). The alternator, if working properly, will adjust voltage output to match the sense voltage, but within the bounds of the alternator capacity. This is why normal indication is for buss voltage to be low after engine start, then climb to 14.1vdc or so after the battery gets charged.

I would first verify the sense voltage is really buss voltage. Do some voltage measurements (sense voltage at the alternator is really buss voltage) and some resistance measurements between the buss and the alternator sense line input.

If all good, then here is a simple alternator test, after engine start and voltage starts to climbe above 14.2vdc, turn on all your heavy loads (landing lights, pitot heat). If voltage stops climbing this is a strong indication the alternator internal regulator is going south.

Carl
 
It seems that if the alternator test good then there is a problem in the path that senses the bus voltage. I'm just thinking out loud here, but if there is some voltage drop in that path then the alternator tries to put out more voltage because it isn't sensing correctly.

With everything working well, a rising alternator voltage output is often due to resistance in it's voltage sense circuit. I would speculate that you have a very loose connection somewhere on the main buss or battery that is causing increased resistance and this would also explain the brown outs happening on your avionics.

Larry
 
Or, your master contactor has high resistance under load.
Try bypassing the master relay with a heavy gauge jumper.
 
Found It

TL;DR - bad ground causes much weirdness

This past weekend I pulled the panel - a relatively painless process in this airplane, and also a good way to limit help isolate the potential causes of the issue. I was fairly sure that the issue was somewhere in the panel wiring, and that turned out to be a good guess. My friend Luis, an EE and RV-7A builder, came over and we started looking for a high resistance connection somewhere.

It only took a couple of minutes to track down the problem. Several ground links, including the switched ground that activates the avionics master relay, were connected to a bolt. The bolt was not making solid contact with the aluminum of the panel. A few seconds with some sandpaper and the resistance dropped to zero.

I reassembled the panel, fired up the engine, and switched on the avionics. Everything worked as expected. The voltage stepped up from 12.8 to 14.4 with the alternator field engaged. I let it run for 15 minutes and everything looked good.

I'm still not quite sure why the bad ground caused the alternator to ramp up the voltage. I'm going to guess that the bad ground somehow impacted the voltage its internal regulator was reading on the field input? (There is no separate "sense" input on this one.)

In any case, I _think_ I have it worked out. I have to fix one more issue (slight leak in the pitot line) and I'll be ready to flight test it.
 
TL;DR - bad ground causes much weirdness

...

I'm still not quite sure why the bad ground caused the alternator to ramp up the voltage. I'm going to guess that the bad ground somehow impacted the voltage its internal regulator was reading on the field input? (There is no separate "sense" input on this one.)
...
Glad you found it. I have heard/read probably 100 stories like this. Anytime you combine "weird behavior" and "electricity" there's a high likelihood that it's a bad ground. Looking back at my suggestion to you earlier in this thread shows that I missed the possibility of a bad ground. I guess we should treat checking for bad grounds like we do "reboot / update" in IT.
 
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...I'm still not quite sure why the bad ground caused the alternator to ramp up the voltage. I'm going to guess that the bad ground somehow impacted the voltage its internal regulator was reading on the field input? (There is no separate "sense" input on this one.)

In any case, I _think_ I have it worked out. I have to fix one more issue (slight leak in the pitot line) and I'll be ready to flight test it.

Your voltage regulator was trying to make up for the voltage drop across the bad ground.
 
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