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By mr spog
We are having problems with the landing light on our Fox. It is an MR16 domestic lamp, and we are using LED.
At first I assumed it was a wiring problem, then after testing and getting 12 through the system, I assumed both lamps had failed. (Unlikely for LED), but they are fine.
After much testing and frustration it appears that firewall forward I am getting 12v with a meter (Open circuit voltage) but when a load such as the LED light or even a basic 12 torch bulb is put across the connections the voltage drops to near zero.
I am wondering if the circuit breaker could be causing this problem as everything else seems to be working fine.
Any ideas from those of you in know?
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By Flyin'Dutch'
Dodgy mass connection?
By ChrisRowland
Something in the circuit is high resistance. High being something like 100 ohms. The voltmeter takes practically nothing but even a small power drain is enough to cause the voltage to drop.

Start at the battery terminals, both ones, and try the voltage drop under load. It should be fine but if it isn't the problem is the battery. Work away from the battery, adding one thing or wire at a time, and checking the voltage until you see the problem. The last thing you added will be the problem, or one of the problems.

Or you could guess, changing things and trying it. In that case start with the most expensive.
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By PaulSS
Yeah, that's my initial hunch. To have 12V suddenly drop to 0 smacks of a dodgy earth to me.

Are you measuring 12V across the landing light terminals (at the bulb) or are you getting 12V with the negative of the multi-meter earthed to something other than the bulb's -ve terminal?

Since you know you are getting 12V through the CB then, in my very amateur opinion, the problem is likely to be in the vicinity of the bulb housing/terminals. I would be checking in particular the earth wire from the bulb terminals and that the +ve is not being grounded to the airframe in the vicinity of the bulb.
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By mr spog
Thank you for your suggestions.
Yes I did get the 12v across both of the bulb terminals, and I have done a basic test of the Earth lead, but maybe to check the resistance of this properly.
Paul and Chris your suggestions make sense and give me something to work on. I was just wondering if this is a recognised problem with a faulty CB. I work through the system with a small bulb as a load and use the voltmeter to see where the drop arises from.
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By PaulSS
I warn you in advance of my electrical numptiness but the fact that you get a drop of voltage to zero when the circuit is under load indicates to this numpty that you have a short after the CB i.e. that current has found a far nicer place to go than through your bulbs. I would be upside down under the panel and checking for continuity between the 'out' of the CB to the switch and then from the switch to the light itself. I would then be tempted to bypass the switch to prove the short is not in the switch itself, or you could just test the continuity across the switch I suppose but that's not as much fun :D

It is my incredibly amateur opinion the CB is not to blame but I was wrong once before :oops:

PS: A Eurofox or a Kitfox?
By ChrisRowland
A short would cause things to be wrong all the time, not depending on load. A resistance will cause exactly what Paul describes because of the relationship between volts, current and resistance V = IR.
Rearranging this gives R = V/I. If a load of a 5W side light bulb - 0.5A - gives a voltage drop from 12 to 2V that's a drop of 10V across the resistance. 10/0.5 is 20 ohms.

I'm a big fan of working through this methodically rather than guessing about what is wrong, it may be the CB but it could equally well be a bad join in the wiring or the battery. A battery that's on it's last legs will do this, that is why I suggested starting there.
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By PaulSS
@ChrisRowland For my education, if it was the battery wouldn't every load have the same result e.g. if Paul (Mr Spog) turned on his strobe lights then wouldn't they also drop the voltage to 0 ?

PS: I'm guessing the switch :D
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By Sooty25
@mr spog as mentioned previously, this will be caused by a high resistance most likely at a joint. Fit the lamp and work your way from the circuit breaker towards the load.

Assume nothing is right though, I've just cleared build faults in wiring in a 25 year old aeroplane, so it could be something that looks okay, but isn't

what part of the country are you in?
By johnm
Remake every connection in the circuit whether it needs it or not, starting at the bulb end and working your way back. If you have it switch cleaner application may help. but just pulling and pushing a spade connector can make big a difference. My experience is based on the similarity between aircraft wiring and the Morris Minor MrsJohnm used to drive :-)
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By VRB_20kt
The methodical approach is undoubtedly best. Clamp one lead of your volt meter securely to earth, switch on the load and use the other voltmeter lead as a probe. Everything north of the lamp should be 12V and everything after it 0V. If you find more than 0V on the earth side then you have an earth fault; less than 12V on the plus side and there is a supply fault.

If the CB is at fault, you'll see a different voltage on each of its terminals

Sent from my K6000 Pro using Tapatalk
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By mr spog
It is a Eurofox and we are near to Brighton. I will go back over later in the week and work through methodically with the voltmeter (and Ohmmeter) to find where the voltage drop is occurring.
I'll feedback when I have had another go.
Many thanks
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By cockney steve
Do not use the ohmmeter whilst the battery is switched in circuit...use the master switch/isolator. Trying to measure the resistance of a battery lets all the magic smoke escape from the test-instrument and it then ceases to work. :oops: I know about these things.
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