Odd lamp behavior in tachometer

[kwalters]

Greetings,
On my engine control panel I have a tachometer. The lamp in the gauge is always on when the ignition switch is on - that is until I flip the switch that turns on the lamps in the other gauges (Volts, Temp, Oil, etc) at which point the tach light goes out but the tach continues to work. As I recall from the last time I had the panel off the tachometer had a bundle of three wires in a gray plastic cover feeding it. I think the tach may be the ISSPRO available from MMI which would mean there was a white (coil), black (ground), and Red (Ign) wire in the gray cover.

I am curious if the red 12v wire to the ignition serves any purpose other than to light the lamp? Based on the fact that the tach appears to continue to work when the light goes out I am guessing this is the case. If so then I could simply move the red wire to the same source as the other gauge lamps to solve my problem of the light going off when I turn on the other lights, no? The reason it goes off currently remains a mystery to me...
Ken

[hanleyclifford]

Sounds like ground may be insufficient to handle all the bulbs. Remember the old "hit the brakes on the car and one tail light goes out issue". Same idea. Check ground on panel.