Wiring into rear light failure telltale for fog light switch

[mawby]

It's been a few years since I did anything with electronics so I want to double check something...

 

My J-Spec car currently has a fog light installed which is switched by a horrid rocker switch with an integrated telltale. I'm currently replacing the switch with a proper UK Spec fog light switch. However, UK Specs have a telltale on the dash which J-Specs don't have, so I want to use the rear light failure telltale as my fog light telltale so it'll pass an MOT.

 

Looking at the attached wiring diagram I can see I need to earth wire 18 when the fogs are on. I haven't checked yet but I'm guessing my fog light set up will be switching the live and not the earth. If I connect wire 18 to the input side of my switch I'm guessing it will short circuit up 18 and down 11 which is the MIL. So I think I need to put a diode in there so the current can't go that way. Then, when the switch is turned on, hopefully the rear light failure telltale will earth through the switch and illuminate.

 

Does that sound about right or am I talking bolloxs and, respectively, can anyone recommend a suitable diode to use or tell me what should I be doing.

[kjgreen3]

Why not wire the Fog light so you switch the earth instead of the live (positive). Assuming you have two wires going to the fog light that is. Use one as permanent live (fused accordingly say 10A off ignition or headlights feed so will only switch on when they're on).

 

The other will be a switched earth via the fog light switch, the output of the switch side paralleled to 18 on your schematic as I'm sure the warning lights are switched earth to light. But don't hold me to that.

[Andy Blyth]

You can do the conversion from switched positive to switched negative with a single relay. The type you buy at Halfords will be fine. What you are looking to do is connect the coil of the relay in parallel with the fog lights. The contacts are used as a switch to ground.

 

In text form: Connect terminal 85 to the wire going to your fog lights. Connect terminal 86 to a ground point. There's a good one in the drivers footwell if you remove the plastic trim next to the door. While you're there, connect terminal 87 to the ground also. All you need to do then is cut wire 18 / VIO and connect the display side to terminal 30 of the relay. There's a couple of diagrams below that may help, let me know if you want a quick diagram of the wiring needed.

 

 

 

Edit: There's a way of doing this and also keeping the original function of the bulb failure warning. If you get a 5 terminal relay that includes terminal 87a, connect that to the other end of the violet wire you cut. This way, the bulb failure warning will light up if either you have a failed rear bulb or if you have the fog lights on.

Edited July 20, 2010 by Andy Blyth (see edit history)

[mawby]

Thanks for the ideas.

 

You can do the conversion from switched positive to switched negative with a single relay.

I'll be giving that a try later on.

[mawby]

Just to say thanks Andy, I wired the switch in as per your diagram and it works perfectly.

 

Having 87a connected also means the light still illuminates during start-up to show it's working.