12v Rooflight won't switch off

Discussion in 'Tech/Mech General' started by Goose, Sep 1, 2011.

  1. Goose

    Goose Read Only Funster

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    I'm completely stumped with this one. Logically, I just cannot figure it out.

    Yesterday, we noticed that the rear 12v rooflight over the bed was on. No matter how many times I flicked the switch, it stayed on. If it was a faulty switch I could understand the light not come on...but staying on permanently?

    I dismantled the switch and discovered (I think) that no electrical current is coming to the switch. I assume I should still be getting a small 12V jolt in touching the wires :Eeek: ?? But there appears to be nothing. So the switch is no longer acting as an interrupter.

    No electrical work has been done to the motorhome, so what would cause the circuit to bypass the switch?
     
  2. Swift

    Swift

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  3. Goose

    Goose Read Only Funster

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    :Rofl1::Rofl1::Rofl1:

    Enjoyed that...made me chuckle!
     
  4. Wildman

    Wildman Read Only Funster

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    the chances of you feeling a 12V jolt is extreemly rare, measure it with a meter, if there really is no current and the switch is disconnected to prove it then power is coming from elesewhere, like a leak connecting the lamp to some other live feed. I suspect a broken switch end of story.
    disconnect the wire from one or even both sides of the switch does the light go out, yes then switch is stuck short circuit. No then are you sure it is the correct light switch
     
  5. Dodgey

    Dodgey Funster

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    Short circuit in the fitting?

    Carefully remove the fitting and check wiring and bulb holder.
     
  6. Goose

    Goose Read Only Funster

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    Thanks Wildman.

    I did measure it with my trusty volt-meter and could get no current to register. But, you've made me think with your comment about not feeling a jolt from a 12V supply. (as that was my backup test).

    Normally, one would (as I did) insert the red tip of the voltmeter onto the live current and the black tip onto the return/neutral. But, of course, the two wires to the switch are all the same wire :Doh:. So, naturally, the voltmeter won't register anything. I guess I need to find another neutral source for the black tip to get a proper reading. Thanks for that. Hope is it, as you suspect, simply a faulty switch.
     
  7. Johns_Cross_Motorhomes

    Johns_Cross_Motorhomes Trader - Motorhome & Accessory Sales

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    If you disconnect it and the light goes out its the swiitch!

    Peter
     
  8. pappajohn

    pappajohn Funster Life Member

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    as others have said, simply disconnect one wire at the switch.

    if the switch has 'welded' closed, this will have the same effect as turning it off under normal circumstances...IE: breaking the circuit.

    if the light remains lit then it has to be getting power from another, unrelated, circuit between the light and switch.
     
  9. Goose

    Goose Read Only Funster

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    Thanks chaps.

    I have just finished completely dismantling the switch again. I've even opened the switch up to see its innards and get a handle on how the thing works (thinking it might operate completely differently to any switch I've ever known). But no, it has a toggle which either connects metal with metal, so completing the circuit or interrupting it when toggled the other way.

    But here's the thing...with the two switch wires free of the switch and completely separated from one another...their is still permanent current to the light ie the bulb stays on.

    If I hold the two wires together (thinking it may be some sort of reverse interrupter) the same holds...light is still getting current.

    So the switch is serving zero purpose. Wires connected = current to light. Switch wires disconnected = current to light.

    And, yes, this time I did insert the neutral black tip of the voltmeter in a neutral source and the red tip onto the red switch wire and the voltmeter registered the incoming 12V.

    Eeesh! What magic is this? I'm beginning to think Swift's ghosts may indeed be at play here!:Eek!:
     
  10. pappajohn

    pappajohn Funster Life Member

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    not knowing where the switch is in the bedroom area, is it possible a second switch has been fitted near to the bed for convenience.

    you may not be aware of the switch and accidently turned it on.
     
  11. Swift

    Swift

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    A voltmeter at the bulb end should solve it, also the number of wires going to it re second switch suggestion :Wink:
     
  12. pappajohn

    pappajohn Funster Life Member

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    Am i missing something Swifty mate.....the light is on so the bulb holder is permenantly live, a volt meter at the bulb holder will simply confirm the obvious....the bulb holder is live :Doh::Wink:
     
  13. Goose

    Goose Read Only Funster

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    Thanks Pappajohn and Swift.

    Our motorhome is the rear French corner-bed configuration with the bathroom next to it. The light switch for the 12V light over the bed is on the thin wall that divides the bedroom from the bathroom. It is a simple 2-way switch...not a three-way operated from multiple locations.

    I did insert the voltmeter into the two wires leading into the light receptacle (which is, by the way, a simple 12V halogen receptacle for a 10W two-pin push-in halogen bulb). The current correctly measures 12V coming in...

    ...but from where? That's the question.

    The switch interrupter is no longer acting as a switch. (Just in case you're wondering...we've been in the motorhome pretty much permanently for the past 15 months and it's the same switch we use every day...until now, because it's no longer interrupting power supply to the light).

    ???????

    I haven't been doing any handyman stuff, knocking nails or screws into any walls so the wiring has been safe from the introduction of any new foreign objects at my hands. And we've had precious little rain recently so I can't see how it could be water ingress causing a short.

    Any other ideas?
     
  14. Swift

    Swift

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    I was following your advice re a possible 'other source' wire, that's all, I think some tests need done to ensure the wire at the switch is the live wire that appears at the bulb, turning off the power then checking that wire for continuity switch to bulb would help to ensure it is the same wire by just putting some 12v power through it :Wink:
     
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