fog light headscratcher

[Paul_Hedley]

I've known for a while what my rear fog was not working, but hadn't got round to investigating properly until this afternoon (MOT due!). I made some progress, but have hit something that I really don't understand.

 

With the light not working, it seemed sensible to put the multimeter across the connections on the light unit as a starting point, and then trace wires back if necessary looking for any short/damage along the main wiring run. However, I get a (slightly fluctuating - to be expected I guess) 10.5v-11v at the terminals going into the light unit. Voltage is slightly higher when the engine is running, unsurprisingly. I assumed therefore that all was well in the wiring run. However, with what visually is a working bulb (I've tried several, including the working one from the reversing light on the other side) I never get a lit bulb. Even measuring the voltage across the plates on the light unit (i.e. where the bulb connections would be) I still get that 10.5v but no light. As soon as there is a bulb in the circuit, no go. It doesn't seem to be the connections either (though they did need a tweak to make proper contacts with the bulb).

 

I think I'm right in saying that the earth is shared with the rear lights, so I've cleaned that up, and the rear number plate light seems to be fine too (other posts indicate that the two often fail together).

 

As a non electrickery person, I'm stumped as to what the problem might be, or what to try next?

 

Any pointers would be gratefully received.

 

Cheers

 

Paul

[Jonathan Kay]

That is strange and you've done the right tests. I'd play with the connection of a known to be working bulb in the bulbholder yet again.

 

Has it ever blown fuses?

 

Jonathan

 

Edited by - Jonathan Kay on 27 Apr 2013 17:15:37

[7 wonders of the world]

-bulb,

-All circuit connections

-make sure headlamps are in dip position NOT main beam

 

you'll find it

 

 

[Jonathan Kay]

I've wasted a lot of time in an MoT inspection and in my garage trying to make the foglight work when the headlamps weren't on and dipped. But I thought that seeing >10V across the unit excluded that... ?

 

Advice please: In general do you ever get the potential difference but can't get enough current because of poor connections on low-current circuits the way you often do on the high-current starter circuit?

 

Jonathan

 

Jonathan

 

Edited by - Jonathan Kay on 27 Apr 2013 18:11:52

[sparky]

the best thing you can do is replace your multi meter for a test light.

Multi meters do not require large current to operate and give a reading.

Your process is correct but a bad choice of tools, ( my opinion of course)

continue on your train of thought and work back from the light i suspect you will find either a corroded connection or poor connection within switch.

 

[7TOM]

Hi a multimeter is a great tool but you need to load the circuit to test (bulb in switched on) check with one side of the meter on body earth. Check voltage live side of bulb do you get bat voltage if not look into why and trace back to switch if yes chances are you have a bad earth return this can be confirmed if you have bat voltage on the neg side of the bulb .

[The Pikey]

It's a voltage drop due to current draw.

 

With the fog light in the on position (and headlamps if you need them on to activate the fog??) run a wire from the chassis somewhere to the negative side of the lamp, if it lights up you know it's a bad earth. If it don't then you know it's the poss side at fault. If the lamp didn't light up then I'd cut an inch off the red/blue? wire at the lamp end,reconnect and try again. If that didn't work then see if the switch is at fault by pulling the plug off the bridging the two contacts, if that lights the lamp the switch is at fault. If that fails to sort it, run a long wire from the other wire on switch plug red/orange? directly to the red/blue terminal on the fog lamp. If that works then the wire if broke between the switch and lamp.

 

Jason

[CharlesElliott]

I'd agree with bad earth. Meter shows voltage but as current draw is required through bulb the poor earthing cannot flow what is needed.

[Jonathan Kay]

Quoting Jonathan Kay: 

Advice please: In general do you ever get the potential difference but can't get enough current because of poor connections on low-current circuits the way you often do on the high-current starter circuit?

Thanks: people clearly think this can happen.

 

Jason's "bypass with a good connection" method should identify this. I'd use that on both the earth and feed (both upstream and downstream of the switch) sides before changing anything else.

 

Jonathan

[elie boone]

It can be that u need to clean and bend the + contact inside the fog light to make sure it makes good contact, also clean the inside of the ring witch provide the negative to the bulb.

[Jonathan Kay]

Any progress?

 

Jonathan

[simon metcalfe]

Hi, I've had similar problems with my fog light in the past - in my experience it was one of two things (or possibly both!)

1) Bad earth - the earth is likely to be corroded from all the debris inside the wheel arch - disconnect and either clean or if necessary cut wire and attach a new connection.

2) Faulty switch - because it is rarely used the connections within the switch can become "dusty" - it can help to just switch on and off several times.

 

Hope this helps.

[Paul_Hedley]

Some progress.

 

After much fiddling about it seems that the problem was at the switch end. CC sent me a replacement switch which means I have working fog but it doesn't have a tell-tale which I think is still an mot fail.

 

I feel an wiring mod coming on A la Jonty Lyons to take the fog switch off the normal dash illumination and to only light when the fog is on.

 

Thanks for all of your help - I certainly understand the wiring (and some more general electrickery) better than I did.

 

Paul

[Jonathan Kay]

Well done.

 

MOT Inspection: Manual here. Section 1.3 says a coloured tag on the switch is acceptable!

 

Jonathan

[Paul_Hedley]

Indeed but I can't work out what they mean by that.

 

Maybe a coloured sticker on the top edge of the switch would suffice.

 

Thanks

 

Paul

[Jonathan Kay]

That's how I read it: surprising.

 

Jonathan

[Ivaan]

Does the new switch have three terminals on the back? Are you only using two?

 

I've just refitted my fog for the MOT, and whilst it was working fine the tell tail light in the switch didn't work. I had to run a earth wire from the third terminal to the chassis. Then a bit of swapping terminal to get the right sequence of lights on/off.

[Paul_Hedley]

Thanks for this comment - I seem to recall when I swapped the switch that there are only two terminals being used in the block with 4 (I think) coming out of the switch.

 

I'll have a play with an earth wire, as that would be a better solution than keying into existing wiring, and less 'heath robinson' than a sticker!

 

Paul

 

 

[The Pikey]

From memory there are only two pin connections on the rear of the switch but there are also two spades on the sides for the illumination.

 

 

Edited to add link to switch pic

 

 

Edited by - Jason Fletcher on 1 May 2013 22:55:48

[Paul_Hedley]

That does indeed look like the switch in question, though my switch (not the loom connections) has 4 terminals on the back. I can't find anywhere what those terminals might do, and if they might be useful in this situation.

 

The side spades are indeed illumination, hence me looking at Jonty's mod so that the illumination only works when the switch is in the 'on' position rather than being illuminated all the time.

 

On a slightly different subject, there seems to be quite a bit of hostility towards scotchlocks. Has anyone used Posi-taps for this sort of thing (or indeed for the famous k-series relay mod)?

 

http://uk.rs-online.com/web/p/crimp-butt-splice-terminals/6666022/

 

They look like a nice product for tapping into an existing wire in this sort of fashion. I've used their Posi-twist connectors before as an alternative to crimp ones (they are removable and claim to be waterproof, if you buy the sealed ones). Any other blatchat experience here?

 

Thanks

 

Paul

[John Vine]

Quoting 7 wonders of the world: 

make sure headlamps are in dip position NOT main beam

I've just checked my rear fog lamp(s). On the 7 (2008), it comes on with headlamps in dip or main position (and with ignition on or off). On the Mondeo, they come on with dipped or main headlamps, but only when the ignition is on. The 7 has one fog (on the offside), but the Mondeo has one each side. Why the differences, I wonder?

 

JV

[7 wonders of the world]

Mine was a 2006 MY and the fog light only worked on dip

 

 

[The Pikey]

The fog should be on with both the dipped and main beam lights but not the sides. If it don't then you will fail IVA so I guess if the MOT man noticed it you would fail that too.

 

Jason

 

 

 

Edited by - Jason Fletcher on 4 May 2013 20:19:20

[Paul_Hedley]

Just so everyone is clear, it's not the light operation which is now my problem (the fog is working both on dip and main beam). It's the tell-tale on the switch that is now the problem from the point of view of the MOT.

 

Paul

 

 

[John Vine]

Quoting Paul_Hedley: 

Just so everyone is clear, it's not the light operation which is now my problem (the fog is working both on dip and main beam). It's the tell-tale on the switch that is now the problem from the point of view of the MOT.

 

Paul

 

As a stop-gap to get through the MOT, perhaps you could stick a piece of dayglo tape on top of the rocker so that it's visible when the switch is on? Then you could sort out the wiring/switch at your leisure.

 

JV