Foglights to DRLs - Mk3

  1. My model of MX-5 is: 2006 NC 2.0L Sport

I’ve recently repurposed my existing foglights as DRLs by replacing the bulbs with LEDs and adding a switching relay activated by ignition on and inhibited by headlights on. I removed the existing front fog relay in the fusebox and connected to the lights with a spade terminal into the relay base (i.e. to the blk/red wires shown on P44 of the Mellens / Mitchell diagrams).

This all works fine, however . . .
When the lights come on, the front foglight indicator in the speedometer illuminates and also stays on when I switch the headlights on (turning the DRLs off) showing it is not indicating the state of the DRL feed after the relay.

It should be pretty clear what’s happening if I could find the configuration at the instrument cluster in the Mellens diagrams, but I cannot see any reference to the fog light indicator in the external lighting (P44) or the instrument cluster wiring P47), or anywhere else.
Is this me going daft in my old age or is it actually missing?

No, and yes.

Bear in mind the Mellens wiring info is an early draft and for US market and there are a lot of differences in small details, not least because of L<->R swaps here and there.

I had a look in the 2017 ed. Haynes NC book I have. Fog Light Indicator is missing there too, although Haynes does have some things not mentioned in Mellens, as well as quite a few mistakes and mix-ups.

And I am puzzled by how the Fog Light indicator stays on when headlights come on if you are switching the Blk/Red wire.
Have you drawn out a diagram of the whole of your new (ex-fog) DRL circuit with the new relay? What power source are you feeding through the new relay to the DRLs?
I assume presence of headlight powers the new relay to open a normally closed contact to the DRL.

I added these two wires to do the job, keeping Fogs working as normal and adding DRLs.
The upper one with piggyback fuse is the Headlight signal to disable the DRLs.
The lower one with the in-line fuse is DRL power coming from the switched side of the IGN relay, simply by extending the hot contact of the 15A fuse.


Thanks for checking this out, Richard.
Too late to take pics today but my wiring is basically the same as yours; I’m taking the DRL power from the wiper fuse via a fused doubler and using the headlight signal to power the relay coil with the low side to chassis (ring terminal on the fusebox mounting bolt). The NC of the relay is connected to the DRLs.

My current thinking is there was previously some sort of residual pullup effect on the lt grn/blk wire via the relay coil which kept the indicator led driver disabled until it was pulled low by the fog light switch. (Although this does sound a little tenuous.)
I’ll get in there with a meter tomorrow and try and pin it down.

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Yep, going daft.
Checked again today and the fog light indicator does actually reflect the DRLs; it goes off with the DRLs when I turn the headlights on. No idea why I thought the opposite yesterday.

Still don’t know where the feed for this is coming from as I’m pretty certain there was only the two blk/red wires from the base of the original relay, but it must T off one of these downstream.

Not a problem - I’m happy with a DRL ON indicator on the console.

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To stop the fog indicator illuminating you need to take your feed from the wiper fuse to the new headlight relay and then from the relay direct to the drl and not use the existing fog light feed from the fog light relay base.

You will obviously have get a new connector or cut the existing fog light connector from the wiring loom.

Thanks, yeah, I can see that now.
I fully expected the indicator to be local to the switch but it’s been wired the long way round.
No great problem - I can live with it.

If I don’t mind losing fog light function is there a simple way to wire the ex foglights, now DRLs to switch on with ignition and off with headlamps ?
Tbh though just having them switch on with sidelights and manually switching off when using headlights isn’t a big problem.

I’ll post details with pics in a few days.
I used a larger fuse tap-off adapter than ideal and it’s a bit of a squeeze but I’ve got the low-profile mini type on the way and I’ll do a writeup when re-do the wiring.

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You’ll forget! I know I would, and SWMBO would be complaining all the time.

After thinking about my time involved in the various possibilities, I chose the simplest option and forked out for the Vinstar kit from MX5parts so I could have both Fogs and DRLs, and while a bit of a fiddle feeding the extra wire through the top cross-member from the fuse box, it does allow fully automatic operation, and the light cast is the correct different pattern for each. The car sailed through the MOT a week later.

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Those look great Richard. I did consider them but in the end went for cheap and cheerful.
I’ve found very few times in over 40 years of driving when fog lights gave better visibility than dipped headlights, so no great sacrifice in my opinion.


I agree.

However it was the time and effort involved which decided me.
And it is simple to put back the old halogen Fogs.
Also I will not be driving the NC in a Fog!

But one never knows, and it gives options on choosing the best light. On my old Astra driving through blowing snow on the A1 I needed to experiment between raising and lowering headlight beams, main or dipped, or Fogs. On that scary occasion I ended up with mains fully lowered.

Modern auto levelling headlights are an unmitigated pain - as on my Mazda3. So these days I’ll stay at home when the weather is that bad.