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I have a Ford Mondeo MK4 Saloon from around '08, Ireland, Right hand drive. Last year I noticed that only one of the reversing lights was working, but the car test passed it anyway. However when the car went for test this year the rear fog light on the other side is not working, so I assumed it was just the bulb and I went to replace it, but the bulb is fine. I stripped the inside of the boot/trunk lining and took out the bulb, noticed the filament was good so I switched the fog light with the other side, and the bulbs are fine. The fog light is a dual filament (I think the other is tail light) but the fault stayed on the right (Driver's) side even when the bulbs were switched over, so it's not the bulb.

I put a multimeter on the bulb socket terminals, and I discovered that the fog light filament is getting 8.8 volts when the fog light is on and 2 volts when it's off. (The tail light is normal, getting 11.8 volts.)

8.8 volts is not enough to light the filament (Or it disappears when the filament loads it) so the fog light doesn't work.

When the fog lights are activated inside the car the dashboard reports a rear fog light bulb fault.

Last year I noticed a similar problem with the reversing light on the OTHER side, it tests as 4.3 volts irrespective of whether the car is in reverse gear or not, so the problem seems to have spread from the left (Passenger) reverse light to the right (driver's) rear fog light.

So, am I looking at replacing wiring looms, or is it the central junction box, would disconnecting the battery for an hour reset it?

Some updated readings

I was playing with this again today just to jury-rig it for the car test, and I took some other observations.

The grounds seem to be good, and there is 0 ohms between the ground wire one the working vs non-working side. Also since the faults are specific to fog light on the right and reverse light on the left, ground isn't a strong candidate.

On the right this fault exists:

Reverse light on = 1.8 v to ground, off = 1.73 volts to ground

On the left/passenger:

Fog light off = 1.9 v to ground Fog light on = 8 v to ground open circuit, falling to 0.047 volts when loaded across the bulb.

I know there is no fuse here, so I suspect it is a piece of control circuitry in the passenger footwell fuse/control box, but that's just a guess.

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    With the light turned on, try measuring the voltage supply to the bulb, but using a different earth point. Does that read any different?
    – HandyHowie
    Dec 2, 2023 at 15:39
  • Hi @HandyHowie, sorry took a while for me to get back to this. There is no difference. In addition I checked the impedance on both sides and they are identical (about 300 ohm because of the paint/laquer on the part of the car I chose to earth) Also the impedance between the earth on the left and write is 0 ohms, so they are both earthered perfectly.
    – spl
    Dec 19, 2023 at 18:32

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