Interesting Electrical Failure (and repair tip)

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Steve
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Interesting Electrical Failure (and repair tip)

Post by Steve »

So, went out early a couple of days ago to get a flight in before it got too hot here in S. TX (no, not before sunrise). Pre-start checklist normal. When I engaged the starter, the prop actually rotated backwards about 20 degrees, all of the electrics (annunciator, artificial horizon, and engine monitor) died, and there was a sizzling sound coming from the firewall on the copilot side. :shock:

Master switch OFF, and the sizzling sound ceased (thank goodness). So, instead of flying, I pulled it back into the hangar for some electrical diagnosis. My first thought was a bad Master relay. As some of you already have found out the hard way, all of the power relays in the DA40 are on the firewall, behind a fire shield, which is extremely difficult to remove. First one has to dismount the oil pressure sensor from the motor mount, and disconnect the oil temperature and RPM sensors. Removing the fire shield is extremely difficult, thanks to the small amount of space between the firewall and motor mount in that area. There are two 10mm bolts, one of which, the inboard (easy), has to be removed and the outboard (almost impossible) which has to be loosened a turn or two to free up a slot in the cover.

The space is so restricted that I never have found a good wrench or pliers that can both reach in there and actually turn the bolt - until today (desperation is the mother of invention).
Double-jointed bent needle nosed pliers (H/F)
Double-jointed bent needle nosed pliers (H/F)
Pliers approach angle to reach outboard bolt
Pliers approach angle to reach outboard bolt
Pliers on bolt head
Pliers on bolt head

With this setup, you can actually turn the outboard bolt enough to loosen it, and slide the cover off. Of course, getting it out of the way to allow access to the panel requires more contortions...
Relay panel with cover off and Master relay removed
Relay panel with cover off and Master relay removed
Once off the airplane, I tested the relay just to be sure it was bad before shelling out $150 for a new one. The relay would indeed pull in, but the contactor remained open. When I brought it home, I decided to cut it open:
Master relay (open)
Master relay (open)
Contactor
Contactor

The contacts don't really look that bad (for 21 years of use). I guess I just hit a bad spot (unlucky), but at least I was at my home field (lucky). So, now waiting on a part (my mechanic didn't have a 24 volt one, only 12 volts).
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Rich
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Re: Interesting Electrical Failure (and repair tip)

Post by Rich »

I'd also be inclined to check into the starter and its circuit, looking for a short. The reason being it seems that things were OK until you hit the starter.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Steve
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Re: Interesting Electrical Failure (and repair tip)

Post by Steve »

Yes, I had that thought as well. Starter and associated wiring ohms out OK.
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