Solved: Rear fog for JDM - help

MrT · July 7, 2023, 13:50

Hello all
My MR-S had a crappy switch and wiring for a rear fog and I've tried to swap to a LED and new switch but I cannot get it to work.

The switch is taking power from the left stalk lighting wiring (where it did before) and running a wire back to the rear light bulb which is then grounded to the rear lights ground.

It is not the bulb or LEDs polarity, isn't working with a filament bulb. The old switch isn't even working. I also tried wiring a relay from the switch to power the bulb directly from the battery. No dice. The power when measured fro the switch live to ground is about 12v but seems to have no current. Is it a weird circuit like the dip/main beam?

Searching the forum I couldn't find anything useful. Can someone advise my how to wire this please? MOT booked and I just realised this isn't working (I don't use the fog)

Thank you

MrT · July 15, 2023, 22:10

Can anybody help me please? I tried again today to wire it with a negative switch still no power to the bulb. For a moment the switch had power then I heard some relays trip and nothing, it makes no sense to me.

Carolyn · July 16, 2023, 09:54

As I recall, I cheated. I spliced off the main power going into the ignition switch, put a fuse in the line then to an illuminating switch and straight to the rear fog (which is led). So it isn't linked to the lights being on, but (as it's an import) the MOT man lets it go.

MrT · July 16, 2023, 10:07

Quote from: Carolyn on July 16, 2023, 09:54As I recall, I cheated. I spliced off the main power going into the ignition switch, put a fuse in the line then to an illuminating switch and straight to the rear fog (which is led). So it isn't linked to the lights being on, but (as it's an import) the MOT man lets it go.

Thanks Carolyn
I've tried to wire it like that, just directly off the battery even. For some reason the light won't illuminate in the car. Tested on the bench it works, as soon as I do it in the car nothing. Wires all have continuity, only difference is grounding to the chassis Vs direct to the battery on the bench. Ground has continuity also...

MrT · July 16, 2023, 10:09

Quote from: Carolyn on July 16, 2023, 09:54As I recall, I cheated. I spliced off the main power going into the ignition switch, put a fuse in the line then to an illuminating switch and straight to the rear fog (which is led). So it isn't linked to the lights being on, but (as it's an import) the MOT man lets it go.

PS the rear fog doesn't have to be connected to the light circuit:
https://www.gov.uk/guidance/mot-inspection-manual-for-private-passenger-and-light-commercial-vehicles/4-lamps-reflectors-and-electrical-equipment

But it does have to be there and working 😭

Carolyn · July 16, 2023, 10:18

Try a new independent ground.

MrT · July 16, 2023, 10:28

Quote from: Carolyn on July 16, 2023, 10:18Try a new independent ground.

I just did that and got the bulb to work, thanks Carolyn!

In spite of the wiring loom ground having almost identical continuity it wasn't working... Weird.

I just need to tap a power source now, ignition sounds nice but I have a factory option front fog loom in the car and thought I could tap that for light switched power. Seems it reads voltage on the meter but doesn't put any out when I connect a load again. I'll try again but if no luck I'll just steel ignition switched power.

This wiring is weird. I've had the same problem trying to modify my headlights to have dipped and main beam on when main beam is switched but only the main stays on despite both being connected. I thought I had a decent basic electrical understanding but I'm missing something clearly

Carolyn · July 16, 2023, 11:41

I used the power into the ignition switch so, not switched.

LEDs are very voltage sensitive, so a slightly dodgy ground can cause a fault that wouldn't show up on an incandescent tail-light.

I like to make sure the voltage is spot-on for LEDs.

MrT · July 16, 2023, 12:15

Quote from: Carolyn on July 16, 2023, 11:41I used the power into the ignition switch so, not switched.

LEDs are very voltage sensitive, so a slightly dodgy ground can cause a fault that wouldn't show up on an incandescent tail-light.

I like to make sure the voltage is spot-on for LEDs.

Thanks Carolyn. I was testing with an incandescent bulb. That wasn't working until the ground to chassis. Now all set up, switched from the ignition. I need a brighter led for normal use though, the one I have is a but naf!