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Old 02-23-2019, 08:24 PM
Jonathan Alvord Jonathan Alvord is offline
Join Date: Feb 2018
Location: Prosser, WA
Posts: 123
Default Avionics failure during flight

Went for fuel filling trip today and lost all avionics including transponder, EIS, GRT displays. Thankfully had steam gauges for backup. That is the short and sweet. Here comes all the details leading up to it. The plane has roughly 460 hours on it and I am the second owner. Two months ago we replaced the alternator when the displays turn off when coming in to land. No real issues otherwise up until today. Run up normal, flew to Oregon for gas and avionics shut down while on final and rebooted. Filled up, run up good, Voltage 12.9 amps 10 I believe all in the green on the EIS and displays. Half way back in VFR all the avionics turned off, no contact with ATC and I was on flight following, called Flight services on cell to ask them to relay to ATC that I had lost comms and would continue VFR direct to my local airport.

Now why did it fail? It appears the PTT on joystick was pulling power from the avionics, that's my theory. I and two others have looked through the plane most of the day. All the solenoids in the engine compartment appear to working. WE have power at the master switch, we no longer have power to Avionics (GRT, Radio stack) and Fuel pump. I have checked all the fuses and can't find a single one that is bad. We were able to jump the power to the avionics and it still works , just not from the switch. When testing the panel for endurance fuses (fuel pump, manifold pressure, avionics, EIS, and inst lights) we could only get 2.7v, All of the Main fuses were working (nave lights, auto pilot, pitot, strobe, trim ignition, trim switch, landing, pwr source 1, interestingly Power source 2 had no fuse in it).

I am unable to find a wiring diagram, although I know it would help we drew out one and I have a picture but can't post here. I did not pull the floor panels off and have had no problems with the fuel pump in previous 250 hours.

Any ideas? There are no mech or avionic specialist at the airport, this is new territory for me as to how to get it repaired. Any Advice would be greatly appreciated. BTW I am near Yakima/Tri Cities Washington.
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Old 02-23-2019, 10:19 PM
rv7charlie rv7charlie is online now
Join Date: May 2006
Location: Pocahontas MS
Posts: 3,888

If volts were 12.9 in cruise flight, it's likely the alt was dead again, or the regulator was off line. It's unlikely that the PTT line could cause signifcant drain on the system, unless there's a catastrophic wiring fault somewhere. The PTT circuit is by nature very current-limited.

As you know, you really do need a wiring diagram. But to start, if it's wired conventionally, and the battery is fully charged, you should see ~12.5V at the 'top' of the master switch *when it's off*. When it's *on*, it completes the master contactor coil's circuit *to ground*, so you'll see zero volts on both switch terminals when it's *on*.

From that point, troubleshooting is checking for 12+V, starting at the load side of the master solenoid, then the various buses.

Hopefully, that will get you started.

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Old 02-23-2019, 11:29 PM
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N804RV N804RV is offline
Join Date: May 2013
Location: Mount Vernon, Wa
Posts: 828

If all of your avionics (i.e. your avionics buss) are drawing current through a single "avionics master" switch, I be curious about how its rated. Or, if its some kind of switch/relay set up, how that is wired.

If you were at Arlington, I'd offer to take a look next week. But, being in Yakima, I don't know when I could get over there.
Ken W.
Mount Vernon, WA
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Last edited by N804RV : 02-23-2019 at 11:32 PM.
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Old 02-24-2019, 06:46 AM
Red Mtn flyer Red Mtn flyer is offline
Join Date: Jul 2017
Location: Carson City, NV
Posts: 128
Default +1 Ken

Saw a repetitive failure like this some yrs back - owner kept replacing an avionics relay switch with incorrect unit. Would work long enough to get him off the ground....
With low voltage, check your avionics switching.
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Old 02-24-2019, 06:58 AM
sailvi767 sailvi767 is offline
Join Date: Sep 2010
Location: Charlotte NC
Posts: 1,388

12.9 volts on runup means your charging circuit was not working and you were operating on the battery. Sounds like as the battery voltage dropped you went below the minimums to operate the glass. What voltage does the battery show now. Voltage on runup should be 13.8 or better. Usually about 14.2.
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Old 02-24-2019, 07:34 AM
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Alan Carroll Alan Carroll is offline
Join Date: Jul 2007
Location: Madison, Wisconsin
Posts: 778

If there is an avionics master switch I would try bypassing it and see that gets power back to avionics. (could be the builder put the fuel pump on the same bus with the idea that it is an essential item).
Alan Carroll
RV-8 N12AC
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Old 02-24-2019, 07:44 AM
AaronG AaronG is offline
Join Date: Sep 2009
Location: Hartford, CT
Posts: 107

My first guess would be failed charging system, either alternator or something else. In flight, voltage should be around 14 volts to charge 12 volt battery. Anything less, and the battery will not take a charge. Start by putting a volt meter straight across your battery. If it?s less than 11 or 12 volts, it is likely the charging system. If low voltage, charge battery and recheck. Once at 12v, turn on system and verify radios are working. With recharged battery, fly to somewhere they can trouble shoot.

If the battery is reading the normal 12 to 12.5, I would focus on a loss of circuit continuity. With the system turned on, check the voltage from your essential bus or after the fuse going to your radio. On the volt meter put the positive on the fuse, and the negative on any exposed aircraft metal (no paint). If it reads 12 volts, then turn on the radio, and it should work. If no voltage, start working upstream with the volt meter until you find the issue. Check at any switches or solenoids and work all the way back to the battery. At some point, you should find either a loose connection, broken switch, or broken solenoid (or potentially a switch that is driving a solenoid).

I?d be very surprised if it was the PTT switch. The current generated by your system would likely overheat and start a fire if that wire was really drawing all that power.

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Old 02-24-2019, 08:04 AM
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maniago maniago is offline
Join Date: Aug 2014
Location: Bowie MD
Posts: 890

What everyone else said about your charging circuit.

14.3V is what your batt should be seeing. Below that your batt is providing all the power. I'd bet that wrt the PTT switch, since thats the transmit side of the radio, the radio pulls a lot of amps to do that, and when you transmit, if the batt voltage is already on the edge, it prob pulls down the batt voltage just enough to make your master solenoid field to collapse, and thus you lose all power.
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Last edited by maniago : 02-24-2019 at 08:18 AM.
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Old 02-24-2019, 08:11 AM
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Jpm757 Jpm757 is offline
Join Date: May 2013
Location: Sherman, CT
Posts: 991

The PTT switch is a simple radio transmit key that when pushed, grounds the circuit allowing comm radio transmission through Mic circuit. Normally very small gauge wire that would not support the amperage load of all the avionics.
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Old 02-24-2019, 08:21 AM
74-07 74-07 is offline
Join Date: Dec 2016
Location: Greenville, SC
Posts: 607
Default Charging

I went through a similiar issue. Tracked wires for days trying to first, determine what went where. Next, the problem was absolutely intermittent. Also had just installed a new alternator. Plane Power with me all the way and completely backed their product. Every time we thought we’d found the problem, (suspect crimps, etc.), we’d fly it and all would seem good until it failed again. We checked every single wire by checking continuity while tugging and pulling. We didn’t pull and tug on the alternator output wire as afterall, we had just reolaced the alternator. The wires were all really well supported by adel clamps and we didn’t take every clamp loose. Yes, we did replace the field/sense connector. I finally placed one voltmeter on the field wire (to see if I was losing field voltage) and another on the output side. This way, we could determine if we were losing field voltage or if the output voltage was failing (internal regulator). After two hours of ground runs with no failure, I was just about to give up when it failed! The field voltage was good at 12.4 volts but the output went to 15.1 volts! I quickly shut off the alternator. Now, we knew the alternator was working as was the voltage regulator. We were losing bus voltage somewhere downstream of the alternator. I decided to go back to the basics and start with the wiring at the output side. and check every connection to the bus. The first was the output connector which
Was well secred with an adel clamp. I took the adel clamp off and once disconnected, I tugged on it and the connector came off in my hand. Put a new connector on, recrimped it and 20 hours of flying later, everything is, once again, perfect. Moral of this story.........start with checking the very basics and don’t assume anything,

Last edited by 74-07 : 02-24-2019 at 08:23 AM.
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