Fog lamp on by itself ???!!!

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cappers

cappers

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Just popped out for a moment and noticed the rear fog lamp on the 90 was on; it was hot as if it had been on for some time. I used it yesterday but didn't have the lights or fog lamp on. The fog lamp switch did not do anything i.e lamp stayed on. The side/headlight switch was off yet the fog lamp was on irrespective of the main or fog switch?!

As it was dark and late I decided to remove the fog lamp bulb and investigate tomorrow. Opened it up, all seemed dry, clean and uncorroded (new 3 years ago) and tried to get the very hot bulb out. Twisting the bulb caused it to go out so I went to the cab and tried the switch - nothing. Put on main beam and the fog lamp switch turned the lamp on and off, and off when hi-beam was on. Had another look at the fog lamp wiring and all looked good. Checked and fiddled with the bulb and holder and the fault didn't recur and all is working as it should.

Any ideas? I thought wires in the rear loom, a live must have been reaching the switched fog live and overriding the main light switch? It's strange it all behaved properly after twisting the bulb.

Don't want an exposed live causing issues...the lamp was so hot I wonder if it could have melted and caught fire? :eek:
 
cappers said:
Don't think an '86 Ninety rear fog lamp has a relay?
Something to do with the rear loom perhaps, tow socket has a perm live....raining today so I'll look tomorrow.
Click to expand...
Thought it had a relay…
As there is no other way of turning the fog light off when you use high beam.
Check fuses as well,pop out the fog light fuse to see if it still works. This will help fault find
 
Fog lamp came on again by itself!

1. Took bulb out but dash warning light still on even though main light switch and fog light switch were off.
2. Switching on the fog lamp switch caused dipped head lights to come on irrespective of main light switch. Side & tail lamps not affected.

What you reckon? Wiring behind dash - the only place where dipped headlight and fog light wires might meet.
Thought it the rear loom to tow bar but would that affect headlights?

Disconnected battery just in case.
 
Don't forget the warning light wires in the wiring loom inside the dashboard.

I had a similar problem a while ago which involved the heated screen warning light coming on when I switched the sidelights or headlights on. Similarly, if I switched the heated screen on, even with the ignition switch in the off position, the ignition came on. Now when thIs sort of thing happens everyone says 'earth', but it wasn't. On isolating the heated screen warning light wire (disconnecting it from the back of the switch and the warning light cluster) the problem went away. So I made sure the ends were insulated and ran another warning light wire, and it's been fine since. I suppose I should unwrap the wiring loom and see whether some insulation has chafed through somewhere, but that's a long and fiddly job and I haven't quite got round to it yet.
 
Pulled the fuse and the fog lamp and fog lamp indicator went out (as did the horn, interior light and clock as on same fuse). Headlights working normally and not affected by the fog lamp switch.

The wiring diagram for '86 4cyl petrol (now V8 but didn't do anything to the lighting) shows that the fog lamp switch (25) fuse (58) shares a feed with the headlights (10, 11) via a common UR wire to the steering column switches (28).
The light switch for side and headlights (56) has a feed to the fuse adj to the fog lamp fuse - note when fuse in fog lamp switch activates the dipped beam and fog lamp permanently on.

rear fog lamp wiring.png

Fog lamp or main light or headlamp switch fault? Has been suggested a live in the rear eg tow bar might be powering the fog lamp but would that light up the dash lamp and make the fog lamp switch affect the headlights?

Any help appreciated, cheers.
 
With fuse in taking out the bulb doesn't cure the problem i.e. dash warning light stays on and headlights come on with fog lamp switch, and with lights and fog switched off there is still power to the rear fog lamp.

With fuse out, no dash light, headlights not affected, no power to rear fog lamp

The fog lamp switch is a simple on/off, but since the headlights are linked so the fog light on works on dip I wonder if the issue is the main light switch and the wiring to it...???.........Or fuse box wiring?
 
The only live at the rear with just ignition switch on is for the wiper park. Had some funny goings on with mine last year .Ended up one day with the back filling with smoke as I was driving. Battery cut off came into use. Chafing had taken place in the flexi ducting for the cables to the rear door.
 
tottot said:
The only live at the rear with just ignition switch on is for the wiper park. Had some funny goings on with mine last year .Ended up one day with the back filling with smoke as I was driving. Battery cut off came into use. Chafing had taken place in the flexi ducting for the cables to the rear door.
Click to expand...
What about the purple ? Is this a switched live or permanent

Just had a look for a relay…. I don’t have 1 mine has been removed when I done a rewire so can’t help.
My fog lights can be switched on any time & stop on with high beam.
 
tottot said:
My 1990 truck does not have relay for fog lamp, on with main beam or dip.
Click to expand...
Mine's an 1985 so don't think it has a relay, never seen it?

Waiting for a light switch I noticed that the fog lamp, warning light and dip beam are behaving correctly - fog lamp only on when switched and the main light switch on dip; goes off with sides only or full beam and when switched to off.

Wonderful BUT that is without the 10A fuse in for the fog lamp/horn/int light/clock??!!

Fuse in - fog lamp always on, dip beam on when fog switched on
Fuse out - fog lamp & dip beam working as they should

Not sure now if this indicates problem with the switch or a wiring issue. All has worked ok since I've had it until now.
 
I recon it is picking up power from another line, only way it can light with fog switch off or faulty. Pull its connection at dash to check and if light still on has to be power from another source. Pull fuse for rear wiper, it is the only thing at the back that has live when just ignition on. If still on then has to be problem at dash.
 
tottot said:
I recon it is picking up power from another line, only way it can light with fog switch off or faulty. Pull its connection at dash to check and if light still on has to be power from another source. Pull fuse for rear wiper, it is the only thing at the back that has live when just ignition on. If still on then has to be problem at dash.
Click to expand...

That's the point, it lights up with the ignition off. The only other perm live at the back is to the tow bar socket.
It's getting power before the fuse because pulling the fuse stops it, and all works fine then. Fuse in, lamp on permanently not controlled by any of the switches.

I know before I bought it there had been wiring issues which were 'fixed' prior to purchase. The fact that the fog light system works without the fuse suggests it has alternative wiring? Just noted the clock works without the fuse so that must be wired differently too! No horn or interior light without the fuse.
 
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