I have this exact same unit on my '49. Works great through the cigarette lighter socket. But oddly it will not drive a 12v oil temp gauge. It will test the gauge when the sending wire is fully grounded - maximum sweep - but not display any values whatsoever through sender resistance. I guess a EE could tell us why. Never found any explanation on the web.
That's interesting. I can't imagine that an oil temp gauge would use very much power.
I'm not an EE, but it's 2 wires in (POS/GND) and 2 wires out (POS/GND), 6v in->12v out.
How many connectors on on the gauge?
