4 wires, yet 3 lights up the mast!

[chrisbitz]

I have often puzzled about my mast lights setup, as there are 4 wires supplying 3 lights, the decklight, steaming, and tricolour.

It's probably obvious to everyone else, but I only just realised that it'll be a common earth and 3 positives!

So bearing that in mind, is there an easy way with a multimeter how I can identify which is the common earth, since everything seems to earth either through the wire or through a bulb! (one of which I suspect is blown and it's a big fuss to get up there!!) so all wires appear to have some continuity, except the blown bulb one, of course!

colours are no use, as there's two blue and two brown!

 

[Cardo]

I have often puzzled about my mast lights setup, as there are 4 wires supplying 3 lights, the decklight, steaming, and tricolour.

It's probably obvious to everyone else, but I only just realised that it'll be a common earth and 3 positives!

So bearing that in mind, is there an easy way with a multimeter how I can identify which is the common earth, since everything seems to earth either through the wire or through a bulb! (one of which I suspect is blown and it's a big fuss to get up there!!) so all wires appear to have some continuity, except the blown bulb one, of course!

colours are no use, as there's two blue and two brown!

Check each wire (with the lights turned on) against a known negative. Three will give a voltage, one should be 0.

 

[chrisbitz]

Thanks Cardo, I knew there must be a simple answer

Umm, I feel silly now, but I forgot to mention, that I think they're wrongly wired at the moment... so with two switches on, the tricolour lights, and when I turn one off, the steaming and the tricolour comes on, or vice versa... I'm guessing that one's earthing through another bulb or something?...

but I guess your test still works when they're both on?

I knew it wasn't going to be simple!

 

[nigelmercier]

Could also be a faulty negative, so when you have identified which one it is, check it against one of the positives.

 

[CreakyDecks]

They could also have been wired up with a common positive wire!

 

[nimbusgb]

Turn everything on.

Disconnect one wire at a time.

The one that extinguishes all the lights ( or the two with working bulbs ) is the common one. Use that as the reference and test to a known positive. +12v and the common is negative. 0v and you have common positive ( unlikely )

Won't work if there's a wiring fault though.