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23 Oct 2021, 14:41 [ UTC - 5; DST ]


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 Post subject: Cessna - Electrical fails when engine is running
PostPosted: 17 Feb 2020, 09:57 
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Joined: 09/17/12
Posts: 249
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Location: Addison, TX (ADS)
Aircraft: 1977 Bonanza V35B
My buddy had a mechanical after he flew me back to KADS yesterday evening that's leaving our heads scratching.

Short story - Electrical works fine with the engine off, the moment you turn it on it *all* fails except the courtesy light under the wing.

He had a pretty substantial amount of panel work completed recently, so there's a lot of "new" in this plane that could be at play.

    Battery
    Alternator
    Voltage regulator
    Dual G5
    JPI EMD 930
    GMX200
    420 and 530
    Garmin AP

The only anomaly that we noticed in our flights this past weekend is that volts go up and down approximately +/- 0.25V when the beacon is on.

As he went to depart yesterday, just before takeoff, every screen went out. That includes everything on the avionics bus and on the master switch (EDM930). He taxied it back and we checked that there wasn't corrosion on the battery terminals. We did find a broken plastic housing on the avionics switch so we had that replaced last night. It was a let down after five or ten minutes of working fine with the engine off, to only have it fail immediately once the starter was disengaged.

- Electrical fails w/o the alternator on
- It's becoming more frequent, it was failing after a few minutes, to now immediately.
- We tried to isolate by turning everything off on the avionics bus, turning them on one by one and sometimes that works and sometimes it doesn't.
- Connections on the master switch behind the panel look firm.
- There is no electrical burning smell

I thought the JPI wasn't recording, but, then realized just now that it was but Savvy by default doesn't show you short flights. I did notice this oddity:

Yesterday's long flight (showing the +/- 0.25V):
Attachment:
Screen Shot 2020-02-17 at 7.55.06 AM.png


Electrical failure just before T/O... high amps after starting then falling towards normal. Could be nothing but everything is suspect now.
Attachment:
Screen Shot 2020-02-17 at 7.56.38 AM.png


Any ideas on where we should look next?


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 Post subject: Re: Cessna - Electrical fails when engine is running
PostPosted: 17 Feb 2020, 10:19 
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Posts: 1448
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Location: Columbus, Ohio
Aircraft: Baron 58, Lear 35
Check where the negative battery cable attaches to the airframe. Also check where the positive cable attaches to the battery relay and the other side of that relay which supplies power to the bus and starter relay.


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 Post subject: Re: Cessna - Electrical fails when engine is running
PostPosted: 17 Feb 2020, 10:27 
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Joined: 12/12/07
Posts: 7178
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Location: Dallas, TX (KADS)
Aircraft: 1969 Bonanza V35A
I believe that the courtesy light bypasses the Master Relay.

Since you have an "everything" failure, I look hard at the "everything" components - teh Master Relay is top of my list. They do fail, you know. And, of course, careful examination of all the cables associated with the Master Relay - battery to relay, relay to Main Bus.

A bad battery ground would take out the courtesy light, as well.

My money's on the relay.

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PP, ASEL, Instrument Airplane, A&P
Texas Construction Law: http://www.TexasConstructionLaw.com


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 Post subject: Re: Cessna - Electrical fails when engine is running
PostPosted: 17 Feb 2020, 10:45 
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Company: Midwest Chemtrails LLC
Location: KPTK (SE Michigan)
Aircraft: C205
Start with the cheap tasks first.

With that much work, I am guessing that something was not tightened, or waaay over tightened.

Also; a cross-threaded fastener can easily be confused with tight.

_________________
Want to go far off road? Land Rover. Want to go far off road & return? Land Cruiser.


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 Post subject: Re: Cessna - Electrical fails when engine is running
PostPosted: 17 Feb 2020, 14:11 
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Joined: 12/12/07
Posts: 7178
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Location: Dallas, TX (KADS)
Aircraft: 1969 Bonanza V35A
Username Protected wrote:
Start with the cheap tasks first.

With that much work, I am guessing that something was not tightened, or waaay over tightened.

Also; a cross-threaded fastener can easily be confused with tight.


All this, of course! :thumbup:

_________________
PP, ASEL, Instrument Airplane, A&P
Texas Construction Law: http://www.TexasConstructionLaw.com


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 Post subject: Re: Cessna - Electrical fails when engine is running
PostPosted: 17 Feb 2020, 21:45 
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Joined: 01/29/09
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Company: retired corporate mostly
Location: Chico,California KCIC/CL56
Aircraft: 1956 Champion 7EC
Did I miss what kind of Cessna this is?

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Jeff

soloed in a land of Superhomers/ Cessna 150, retired with Proline 21/ CJ4.


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 Post subject: Re: Cessna - Electrical fails when engine is running
PostPosted: 18 Feb 2020, 00:59 
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Joined: 09/17/12
Posts: 249
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Location: Addison, TX (ADS)
Aircraft: 1977 Bonanza V35B
No, it’s a R172K.

We installed a new master relay, master switch, avionics switch and ground cable on the battery.

Voltage is normal on the line out of the master relay. At the bus bar, it reads as low as 3V and then other times at 13V.

We’re stumped. We’ve replaced everything in line from the battery to the bus bar, with the exception of the long wire itself. Any ideas on what would cause the bus bar to become energized but have such low voltage, intermittently?


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 Post subject: Re: Cessna - Electrical fails when engine is running
PostPosted: 18 Feb 2020, 07:13 
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Joined: 10/07/18
Posts: 1448
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Location: Columbus, Ohio
Aircraft: Baron 58, Lear 35
Loose terminal at the bus bar or corrosion between the terminal end and the cable. Or a bad cable. Not too many possibilities.


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