This is for an R56, but I thought you guys could help because it isn't really car specific.
I have installed water temp and boost gauges in my R56. The gauges work fine but I am having a problem with the lighting. One bulb in one gauge works fine, but the bulb in the other gauge is burning too hot and has killed two bulbs already. The good unit allows you to touch the bulb. The bad unit is so hot you can't even touch it. I'll describe the current wiring and maybe someone can help me troubleshoot the cause:

I have wired the illumination to the DSC illumination wire in the center console to allow dimming. this is the same wiring as suggested by ALTA for their gauges. I ran that up to the gauge illumination units with one wire. At the gauges I split the single power wire into two wires, with one running to each illumination unit/12v bulb power wire. I run the ground wire from each unit to one single wire attached to ground.

So, I have one power wire that splits between the two bulbs, and two ground wires that combine to a single ground wire.

Do I need separate grounds? Separate power lines? Why is one bulb running hot and the other is fine if they use the same power and same ground?

Thanks!

 

Can you test the voltage at each gauge? I'm betting that there's a problem with your harness. (that you made, not the factory)