Strange problem

[dekhelia]

Fitting a rear fog lamp to a LHD camper. Long story short, I appear to have lost the dipped headlamps. The strange part is that if I remove the headlamp earth from its connector behind the fusebox (not the earth crowns, it has its own earthing point), I get current back to the feeds at the back of the headlamp switch. If I affix the earth point to the chassis, no more 12V to the headlamps.

Needless to say, with the earth removed and 12V to the headlamps, I still don't get working lights as there is no earth to complete the circuit.

Anyone know what this might be?

[dave friday]

So you have full beam?

[marlinowner]

What happens if you return the wiring to how it was before fitting the fog light?

[dekhelia]

Not sure on both counts. Will try full beam and de-wiring the fog lamp and report back.

[marlinowner]

My best guess at the moment is that you have wired the fog light in using a relay on the lighting circuit that is earthed through the lighting earth. You have accidentally wired it so that when the relay is energised it interrupts the feed to the lighting switch, when you disconnect the lighting earth you disable the relay.

[dekhelia]

Update: unplugging the fog wiring makes no difference.

Full beam works fine, as do the sidelights.

Can't be the switch, as I have swapped out with a known good switch to make sure.

Really got me stumped, this one.

Why would earthing the lamps (as they have to be, obviously) mean that I lose the 12V feed at the switch? Weird.

[dekhelia]

My best guess at the moment is that you have wired the fog light in using a relay on the lighting circuit that is earthed through the lighting earth. You have accidentally wired it so that when the relay is energised it interrupts the feed to the lighting switch, when you disconnect the lighting earth you disable the relay.

This makes sense, in a way: but I don't think the fog light (single, rear) is wired through a relay, think it's just a fused 12V feed.

[marlinowner]

It sounds as though there is a high resistance in the circuit somewhere. When you disconnect the earth (I'm guessing earth on both lights has be disconnected) the voltage floats high right through the circuit. When the earth is connected, all the voltage is dropped at the high resistance. The problem must be before the switch otherwise it would stay at 12v there, maybe at multiway connector at relays? Looks like 12v feed to headlights is connector A, pin 9.

[marlinowner]

Hmm maybe forget what I just said as that would stop main beam as well?

[marlinowner]

Unless when you say main beam works, that's when using the flasher, that has a different 12v supply.

[dekhelia]

It sounds as though there is a high resistance in the circuit somewhere. When you disconnect the earth (I'm guessing earth on both lights has be disconnected) the voltage floats high right through the circuit. When the earth is connected, all the voltage is dropped at the high resistance. The problem must be before the switch otherwise it would stay at 12v there, maybe at multiway connector at relays? Looks like 12v feed to headlights is connector A, pin 9.

This sounds about right. What I've done as a workaround is connect from the headlight 12V feed to the 'output' to the headlights on the switch, thus energising the circuit whether it likes it or not.

I have a sneaking feeling that this could present a fire hazard ... but it'll do to get it through the MOT and then I'll have to work out what the problem really is.

[dekhelia]

Hm. The 'workaround' only solves part of the problem. The only 12V feed I've been able to use that doesn't have the original problem (no power when earthed) has been from the sidelight circuit. This means that when the lights are on, they're on. First increment on the headlight switch now illuminates both the sidelights and the headlights. Fog lamp and fog lamp warning light are both fine.

Is this an MOT fail?

[marlinowner]

You could fit another switch in the feed to the headlights.

[Mocki]

What age is the van?

[dekhelia]

That's a good question. To which I don't know the answer. According to the VIN plate in the cab and the one on the chassis, it's a 1982. According to the other sticker in the cab (which has a different set of numbers - don't ask), it's a 1981.

But it has a plastic fuel filler pipe, the later model ARB fitting, later calipers .. etc etc. I have a hunch it's a 1986, but I've got no way to verify this.

Why d'you ask?