Rear Fog Light Voltage Mystery

[PaulG]

Hi all,

1989 RHD genuine UK van

My rear fog light looked dim, so I started investigating.  I have discovered that the voltage between the feed wire to my rear fog light and the earth wire in the white plug on the back of the cluster is barely 4 volts.  All the other wires/lamps in the white plug are approx 12 Volts (all this with engine off and ignition on by the way).  Same tests straight to a spot of bare metal bodywork gives the same result.  Earth connection for rear lights is the one underneath the ignition coil, this was remade not long ago and looks fine but not taken it off (yet) to clean it - but the issue seems to be confined to the fog light feed only.  Wiring diagram (Haynes) shows a relay on the relay plate, but there isn't one.  Fuse is good.  Fusebox in good nick and dry etc.  Fitted a new bulb for completeness, 12V 21W as per the one which was in it, no difference.

Where should I look next guys please?

Thank you all, always appreciate the help, never take it for granted,
PaulG

[PaulG]

Hi again,

I need to get back to investigating this, but found a post by Kev the Rev who mentioned that his switch had an issue - which I think Kev was able to resolve.  However, new genuine/pattern replacement switches is not available, nor likely to be so I have started to think about a contingency plan.

First idea.  Does anyone have a known good one to sell please?

Second idea.  Has anyone used a generic switch for this replacement please?  I'm thinking along the lines of a red light-up rocker switch, but wouldn't know how to wire that?  Some I have seen have 4 tabs for instatnce.

Any ideas please, or help you can give always appreciated, never taken for granted.  Many thanks.
PaulG

[chewbacca]

I'm all for modifying, but in this case don't you just need ebay?

Linky to eBay

There may be more on there, but Google took me straight to that one.


Incidentally, there's a couple of other quick tests I'd do first...

Disconnect the multiplug on the back of the lights & measure the voltage for the fog light there. If still 4v, repeat your test to another earth point or 2 to double check this.
Next, rule out earth completely by running a wire from the battery negative terminal & measure to that as well. If still 4 volts, it's the feed, not the earth.

Next, pop the switch out and measure the voltage of the output feed wire to the lights, to your piece of wire still attached the negative battery terminal. This rules out the wire starting to fail as it runs down the bus. If that wire is nearly broken through it will introduce resistance and drop the voltage

Next, measure the input voltage against the battery negative terminal. If that's 12v, it's your switch. If that's 4v, it's time to investigate the feed from the fuse box.

[PaulG]

Hi again folks,

Many thanks to Chewbacca for the suggestions on some tests to carry out.  I have in the meantime studied and figured out the Current Flow Diagram in the Haynes Manual - and I understand these pretty well having had expert tuition in the past.  So I set out to try the tests, and decided to test the light again before dissecting the van so I could note exactly what it does, or doesn't do.  I found some "interesting weirdness", so thought I would post this in case others have the same effects, if anyone can confirm this is normal, or if it triggers any more ideas from folk before I set about investigating.

This time with the engine running for fear of harming the ignition by leaving it on too long - just in case.  The rear fog light behaves like this:-
1) Engine running, side lights off and headlights off = fog light/switch tell-tale do not illuminate on either of the two switch positions.  I would expect this.
2)Engine running, side lights on, headlights off = fog light/switch tell-tale light up dim on second switch position only.
3) Engine running, side lights on + headlights on (dip or full beam) = fog light/switch tell-tale light up dim on first position, fog light/switch tell-tale light up full brightness on second position.

Or, to summarise another way, fog light works full brightness as long as you have headlights on and switch in position 2.  I find this baffling with what I know so far, including what the diagrams seem to show, plus why do I have a two position switch in any case since this van does not have front fog lights (unless it once did have and they have now been removed - unlikely I think since it was born as a basic panel van and then converted into a camper by Holdsworths).  Or maybe someone has fitted a 'new' switch which is not the correct one?  Mullered the switch wiring?  Mullered the feed from the fusebox somehow?  I still need to do the tests though of course, but I suspect it is something to do with the switch because once the light gets what it needs (12 volts of juice) it, and the tell-tale, light up brightly.

I think my next step will be to remove and inspect the switch, compare it to the one Chewbacca found on Ebay, and do Chewbacca's tests, but any more info, or a comparison of how your rear fog light behaves compared to mine would be a great help to me please.

Diagrams clearly show a relay in Position 7 in the relay plate but I don't have this.  Could this be because it is a 'rear fog light only model'?

Always grateful for the help,
PaulG

[Robsey]

The first position is front fogs.
Little light should not illuminate.

Second position is for both front and rear fogs.
The little light should illuminate.

[PaulG]

Thanks Robsey, now I realise what the switch positions are supposed to do.

What my issue seems to boil down to is that with the switch set to what would be Front Fogs (if I had any, and I don't) the Rear Fog Light lights up dim (I have measured 4 volts at the bulb holder), and the tell-tale in the switch also lights up dim.  So, somehow 4 volts of power is getting into that circuit.

With the switch set to Front and Rear Fogs (and I only have the factory fitted Rear Fog remember) the Rear Fog lights up fine, as does the tell-tale in the switch.

I keep coming back to the thoughts that with no Front Fogs there should not be any power connection to the circuit for them?  I still need to start pulling things apart, not had chance yet.  Still need to try some or all of Chewbacca's suggestions.

Continued thanks for the help so far,
Paul G.

[chewbacca]

I think if the lights are coming on as they should be on switch position 2, it's unlikely there's a wiring issue between the rear lights & the switch.
Sounds more to me like the switch is starting to fail & is making a partial contact when in position 1.
I'll try to see what mine does tonight if I remember, I'm like you, 2 position switch, no front fogs. Be funny if it's the same & I've never noticed.

[syncroandy]

If both telltale lamp and foglamp are dim, then the source of the fault must be on the positive input to the switch, as the lamps have different current paths from the switch to earth. On a van without front foglamps (& the relay) the foglamp switch takes its power from one of the 'M' terminals on the back of the relay plate. Its the grey/yellow wire. Possibly the problem lies there. To quote Glen Sankey on this topic:

" UK spec vans are different though. Their foglamp on models without front fogs is fed from one of the M
terminals. Its the one thats wired into the headlamp circuit. This is where the fog switch takes its power from.
The other M terminal goes into dipped beam and is where the dimdip system joins the main lighting (this system
starts at terminal 56D on a british headlamp switch, goes round the back of the fusebox in a single conector and
into the front loom where it goes through the balast resistor behind the RH headlamp and then back into the CE
bord at M whatever. You should find somewhere behind the fusebox is an unused single blade conector with a
wiring colour that matches the fog feed on the M terminal. This unused wire is the output from the fog relay in
position 7. Plug the wire from the M terminal in, add a relay and walla, your foglamps should now work off the
relay with the position lamps only. My van already had the wireing in the front loom so after doing that mod I
could plug the foglamps into the extra conectors hanging loose tucked behind the main headlamps somewhere -
they are the funny plugs that have a male and female pin, thus the same molding is used for both halfs of the
conector. Before I changed this I noticed my van had the odity that when dimdip was on the rear foglamp, being
fed from the headlamps, would also glow on dimdip if the switch was switched on and the dipswitch was in
dipped position (its a bit like how you can get the heater fan sto start up without the ignition on if you put the
headlamps on and hold the flash position - I suspect this would alow you to shut the electric windows in such an
equipped van though its not designed to happen, basicly the flasher circuit will power up the X relay) Now of
course the fogs are powered from the relay so they come on full brightness when dimdip is on. "