[JivenJim]

I don't believe that Vdo Gauge housing had a ground Tab on them? I'm confused. So if I understand, you soldered a lug and then connected that with a ground wire. Is this a new wire or did you use a floating ground wire? If new, where did you ground connect??

The Instrument lights are Daisy Chained (Wired in Parallel) They are thin two tone Gray and I think Blue strips. I believe that the Led Bulb holder with attached wires has a grounding contact that spring contacts the inner feed thru light hole/gauge housing) So the light bulb grounding/return would be through the long Daisy chain wires.

Does your Tach Light Bulbs have two wire or just one?

Also remove the light Bulbs from Tach and Test that both wires for Pin 2 (Ac Pulse wave of Ignition coil - green) and pin 4 (Blue/Yellow) go to grounds.

I think, that the Pin#4 on the Tach is Ground and Feeds the ground of the Temp Gauge, so by altering the Ground path you may have introduced some stray potential Voltage.. thus effecting your Temp reading.

So If the gauges both work with out the Light bulb attach attached. You could install a light bulb that does not make metal contact with gauge might patch this. (maybe put tape on ground case lightbulb contact)