I managed to get my hands on a copy of the wiring diagram for the fog lights and as a bonus the headlight circuits.* I’ve had it for a while, but today I decided to attempt the mod again (my first ended in failure.)
With careful study of the circuit wiring diagrams it looked like for the fog light relay to trip, turning on the fogs, it needed +12v coming from the headlight relay and a ground available when the headlight switch is in low. My ideal situation would be to have an ignition on twelve volts applied to one end of the relay and a ground applied to the other whenever the lights are off. This would disable the fog lights as fog lights, but make them daytime running lights – always on when the car is running and off at night.
First thing I decided to do was cut the red/yellow wire. I really didn’t want to, but I could never get the relay loose from the connector, so snip I did. Hooked up the barrel splice to the +12v wire I found in my original attempt and the red/yellow wire running into the relay connector. Figuring that this would not change any operation as the ground from the headlight switch was still missing, I tested the setup. Much to my surprise when the key was turned to IGN the fogs came on. Not only were they on, but the light in the switch was on, even though the switch was not depressed. I tried turning the lights on, another surprise awaited, as when they came on, the fogs went off (along with the light in the switch.) So I pushed in the fog light switch and the fogs came on, they also went out when I turned on the high beams, just like they are supposed to.
Following the diagram out, I must be getting a ground from somewhere I’m not supposed to be, through either the left front low beam or somewhere in the instrument cluster. Seeing as my +12v is only applied when the ignition is on I won’t be running down the battery or anything. The current draw to trip that relay is probably small enough that I won’t do any damage to anything, but I would still like to know where it is coming from. When we go for a drive tonight I’ll see if I can figure out which one by looking at the headlight and the instrument cluster in the dark.
*Because I write in here each day what is happening in a small segment of the Miata world, I consider this blog news. Consequently, that makes me a reporter. Therefore don’t ask where I got the wiring diagrams from, I can not reveal my sources.