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On 03-04 trucks there is a fused and switched wire coming out of the wiring bundle just before it gets to the OBD-II connection. It is white with a light blue stripe and is powered by 10A fuse #29. I assume it exists on later model trucks, but is probably a different fuse #.
I doubt that thing draws enough current to need a relay.
It's just a voltmeter. Tie the power to a 12V wire that is switched off with the ignition. No need for relays or upfitter switches. Use whatever 12V source you are going to use for your other gauges. Your ScanGauge II also reads volts, in case you hadn't seen that.
I am looking for the most direct 12v source under the dash. The more wires you have between battery and gauge, the less accurate this gauge will be. I was hoping to pretty much read a direct battery voltage...but was trying to avoid running a wire directly from the battery pole.
oh well, easy enough to run a 4' wire to the drivers side battery.
I've been watching the voltage on my Scangauge II, but want to free up that slot for another parameter. Currently watching ECT, EOT, FICM, Volts. Also have a 4 pod on the pillar for LPOP, Fuel, Boost, EGT.
My OCD nature would drive me crazy if I use an upfitter to power this gauge. I would want it on all the time...and having 1 of the 4 up all the time...not good for my peace of mind