emotepix

Hi All

Wiring up some additional gauges on my 86 turbo 4runner and have a question about the gauge illumination (night time gauge light) circuits:

I've found the wire that goes to a 12v +ve when the headlight stalk switch is turned on - so far so good, I'll call this wire point A.

However, when I check out the rest of circuit with my voltmeter to find the dimmer wire, what I get is that the instrument dimmer control output (wire point B) gives me a negative graduation instead of a positive one.

In other words, instead of going from 0 volts +ve to 12 volts, wire point B goes from 12v full dim to 0v full bright. All other wires have no voltage output at all. The dimmer bulbs on the existing gauges do work properly.

So, for this arguement's sake, since I know it doesn't make any difference which way you wire up a conventional bulb:

does this mean that it's the negative terminals on the gauge bulbs that's being fed by wire point B? (i.e. connect the +ve side of the bulb to wire point A and the -ve to point B?

This would float the bulbs at +12v but the potential difference would be the same as if you went from a 0v ground to a varying 12v +ve.

In theory.

In practice, shorting could be a major issue unless the instrument gauge illumination circuits were well insulated away from the rest of the vehicle ground system.

For the time being I have my illumination bulbs wired conventionally (i.e. -ve to ground, and +ve to wire point A. This means that they won't dim when the dimmer pot is turned, but also that they're a part of the rest of the system and won't short anything either.

Anybody here done this before?

Thanks!
Cheers
Chris

ps/ my add on gauges come from different maufacturers and have differing bulb illumnation strengths. Do I have to add resistors or VRs into their circuits to match brightnesses? If so, what kind of values should I start with? (I want to err on the side of safety). Or do just I buy matching illumination bulbs/LEDs?