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Reversed Polarity - Need Help!

Work2Fly

I'm New Here
Morning All,

Have an RV-6A with a slider canopy finished in 2002. Like an idiot, when I was replacing the battery (different model) I hooked the positive to the negative and the negative to the positive. When I flipped the Master Switch, wires immediately started to smoke.

I traced the fried wire (only one) and it appears to be the ground wire that runs from the bottom of the right side of the Master Switch to a ground at the top of my panel behind my GPS.

I replaced the wire and flipped the Master (this time with battery correctly installed). Wire still smokes. Not nearly as hot or as quickly as when polarity was reversed, but still no good. And no power to instruments. Figure I must have a ground fault still in place, but not sure where.

Looking for some educated help as to what I should be looking for ... seems like no power is making it past the Master Relay, so I am assuming maybe that is the problem, but I'm not certain...nor am I sure how to test that. Or perhaps the Master Switch is the problem.

Any ideas you might have are welcomed. Also, I am in the San Diego area and have tried to find a good mobile avionics service to assist, but so far no luck. If you happen to know anyone good and available in the area, that would also be great.

Thanks in advance!
 
Reverse polarity.

What you did has the potential to do a lot of damage. Check with local EAA Chapters for recommendations for expert help.
 
For starters, do you have a ‘back emf suppressing’ diode across your master contactor? If so it is surely toast. Any wires that ‘smoked’ should be replaced. Hopefully no avionics were energized?
 
Hey Bob,

Not sure, I will check, and I will snap some pics of what I am working with as soon as I can get back out to the plane this week.

No, no avionics were energized.

Thanks...John
 
Do you have fuses, circuit breakers, or some magic box like the VPower?
I'm assuming some instruments come on with the master... like engine instruments?

I'd definitely pull all the circuit breakers (assuming you HAVE pullable breakers) before any more power-up tests.
 
Morning All,

I replaced the wire and flipped the Master (this time with battery correctly installed). Wire still smokes. Not nearly as hot or as quickly as when polarity was reversed, but still no good. And no power to instruments. Figure I must have a ground fault still in place, but not sure where.

Thanks in advance!

Disconnect the diode and try again.
 
Ditto!

I did the same stupid thing in the middle of the night….I caused damage to my SL40, and VP Unit. Had a local shop fix my VP unit as they specialize in fixing circuit boards and electronics.

The one thing you want to check is your ANL fuse….mine blew and had to be replaced. I got lucky…somewhat! Had to run my flaps outside the VP system as it couldn’t be fixed….all good now!

Don
 
I think that the master contactor have been damaged and needs to be replaced. Also the diode connected to the coil of the contactor needs to be replaced.

Good luck.
 
I’ve often thought about preventing this by having a diode in the circuit of the master contactor actuation circuit but worried about having the diode fail and the contactor open unintentionally.

The excellent thread recently in ground power circuits shows a diode protected ground power contactor but of course that wouldn’t matter in flight.

A bolt-on diode might work ok? But I’ll just be super careful making the battery connections and not change anything.
 
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I would be concerned that one of the wires that smoked also damaged a wire next to it, leaving an existing short that you may not see. Check the other wires that were next to the one(s) that smoked.

-Marc
 
Do you have an electrical power schematic for the AC?

What's the PN of the master contactor? What I want to know is whether its flyback suppression diode is internal or external. Commonest master contactor is PN 24115 with diode external. (Curious Vans OP-10 schematic shows contactor 24115 with no diode.)
I would have thought the diode would open with the excessive current it saw but what you describe is consistent with the diode:
  • Forward conducting with the battery backwards and fusing that ground wire open.
  • Becoming shorted during the excessive current event.
  • Fusing the new wire open when the battery was connected with the correct polarity.
  • In this scenario the master contactor would not have closed in either case so only the diode and the wire would have been damaged, unless there is an always-hot battery bus.
  • I've read that diodes generally fail shorted due to excessive reverse voltage and open due to excessive forward current but maybe you got lucky and it shorted in your excessive forward current case.

The master contactor would not be damaged unless it's coil flyback diode is internal, its coil doesn’t care about polarity.
 
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johnbright has offered very good advice. If it is desired to protect against
future reverse battery polarity, an inline fuse (3 Amp ?) could be inserted in the
wire that goes to the master switch, (the same wire that burned open). A diode
MUST be installed across the contactor coil in order for the fuse to offer
protection. Connect the banded end of the diode to positive.
 
Help reverse polarity

I think if the earth wire was hot the diode being small would have exploded. I’d be looking at the alternator as the hot wire to the battery may be un protected and perhaps blew the alternator diodes. Just try it with the alternator disconnected. Just a possible thought👍
 
Reversed Polarity - The Logic

Been there and done that. The attached info from Vans shows the diodes that are fixed across the master (and starter) relays. This note refers to the master diode.

Without them, when the master switch is turned off, stopping the flow of electricity through the relay coil, the relay moves back to its open position and the movement acts like a generator, causing a big spike of voltage in the reverse direction. That flows up the wire to the master switch where it arcs across the still opening contacts damaging them. Repetition causes the master switch to fail.

To prevent that, a diode (Vans ES Diode Master Yellow) is fixed from the relay terminal to ground as per diagram. With normal positive operating voltage applied, no current can flow through the diode but when the master switch is turned off, the reverse voltage creates a short spike of current that passes harmlessly through the diode to ground so protecting your switch.

If you connect the battery the wrong way, when you switch on, 12v is applied across the diode in the forward direction and it melts, shorting power to ground until the wire melts too.

You can buy the diodes with the correct fittings from Vans. The master and starter diodes need different sized fittings. You can make up your own using regular diodes and fittings. Then replace any damaged wires. The circuit involved goes from the battery through your distribution panel to the battery contactor (thick wire so probably ok) and then through thin wire to the master switch and then to ground (that will certainly have failed). Worth checking the master switch contacts for evidence of damage.

John RV-8 was RV-6A
 

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The diode protecting any solenoid switch would be toasted and perhaps the solenoid itself. As others have said, look very carefully for any wires that have heated up and damaged insulation on adjacent wires. Look for wires that have laid across something metal that might have melted thru and shorted. If you only turned on the Master, perhaps the avionics buss has it's own switch and was never energized. Fingers crossed.
 
All fixed, thanks for the help!

Thanks for all the help. Turned out to be the relay diode that was the problem. Replaced the Master Switch, the Master Relay, and a few nearby wires just to be on the safe side, and wired in a new diode on the relay and all is well.

Really appreciate all the input and quick responses!
 
Mach2 & Dad's RV-10, the Vans page show the diode is only for the RV-12/12is, but wouldn't it be applicable to any airplane?

TIA


-correction: the page for the Master Diode shows it is only for the 912/912is, but the starter has no exception, website typo? (Since I'm building a SeaRey with a Rotax 914 my starter relay is different, so I don't think I'd be using one there.)
 
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he Vans page show the diode is only for the RV-12/12is, but wouldn't it be applicable to any airplane?
Yes, any relay or contactor should have a diode connected across the coil, banded end to positive.
 
Thanks you, sir. This was one of those "I'm really jetlagged and should not work on my airplane but really want to get something done. " days. Simple project, done.
 

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thanks to all posters above!

I experienced a negative polarity event (...) lately, and luckily only the diode was fried... and that was a cheap and easy to replace :)
 
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