Wiring info on aftermarket gauges

I'm adding electrical volt/temp/oil pressure gauges and needed a quick double check on my info before I start.

If I move the dummy light wire from the old oil switch to the new sending unit I can than move the dash wires from the dummy light to the gauge? Is the wiring basically going to be 2 wires on the light get moved to the 2 terminals on the gauge or do I need to pick up a hot source somewhere else?

Same thing on the temp gauge. I've heard that I should bypass the green ignition wire to prevent a full sweep test at startup, but I'm not sure why since every car I've had with a factory gauge had the test on start to make sure that the gauge actually functions. Also is the temp sensor on the 455 at the front passenger side port? I know there's a sensor there but I'm not sure if that's the actual temp sensor or for the blower disable feature or both.

I know I'll hear from people saying to leave the dummy lights functional, but that means I can't run the wiring through the stock connector unless I start removing other circuits to open ports in the bulkhead connector. Also my gauges are getting mounted on top of the dash right in my line of sight, so they'll be easier to see than the dummy lights anyway.

 

Your typical aftermarket pressure or temp gauge is a three wire affair.

1. 12V+ from a source that is switched with the ignition.
2. Permanent ground- wire that goes to a good ground on the dash or chassis.
3. Variable ground- this is the wire that runs to the sending unit, which is grounded to the block.

During "normal operation" there is no current flowing through the variable ground, because it has extremely high resistance. All the current flows through the permanent ground, and the gauge does not move. When the sending unit is receiving a signal, such as warmth from the coolant, or pressure from the oil, the sending unit becomes less resistant, and some of the current flowing into the gauge will flow out through the sending unit, which causes the gauge to move.

So... it would seem to me that if you hooked up the 12V+ lead from the idiot light to the + terminal on the gauge, and hooked up the sending unit lead to the sending unit terminal on the gauge, and then ran a line to ground from the ground terminal on the gauge, you would be in business. I don't think you'd have anything to lose by trying, anyway.

I will say, that I ran my wiring through the plastic grommet that is just outboard of the main fuse block connector. Drilled a 1/2" hole, put a rubber grommet on it, ran the wires through, sealed it with a little roofing tar... Looks like its supposed to be there, and it can run with the main harness all the way up until the last 2 inches of travel or so. Do what you feel though.

Main temp port on the 455 is the one on the passenger side of the front water crossover in the intake. The one on the driver's side is for the thermostatic vacuum advance junk. The blower-turner-offer goes in one of the head ports. I don't know which one, because I never noticed it until I had my heads off. I think it was only an option/feature on the big cars.