It is not voltage, it is ohms that it measures. Everything on the car is 12 volts. The gauges measure resistance, the temp gauge will send 12 volts of current with a specific amount of resistance, the gauge then reads this resistance with the needle in the gauge. I hope this quick explanation makes sense. You can ground the wire and the gauge will peg one way or the other for example.
Here is my write up how to bench test the fuel gauge:
http://www.modsandrods.tv/2012/03/29/bench-testing-fuel-gas-gauge/ Oil pressure sweeps in a different direction than the temperature gauge, otherwise it's the same procedure to test the oil/temp gauges.
Oil Gauge1 Ohms = 1psi
45 Ohms = 40psi
88 Ohms = 80psi
If the wire is disconnected to the oil pressure sender, there will be nothing to read, oil pressure would measure excessive pressure (over 80). Ground the wire and the gauge will show no pressure.
Water Temp Gauge46 Ohms = 260 degrees
68 Ohms = 220 degrees
350 Ohms = 100 degrees
Note: 80/81 cars have a different ohm range
If the wire is disconnected to the water temp sender, there will be nothing to measure which would show as a very cold engine. Ground the wire and the gauge will show as overheated.
Fuel Level Gauge90 Ohms = Full tank
45 Ohms = 1/2 full
0 Ohms = empty tank
If the wire is disconnected to the fuel sender, there will be nothing to measure and the gauge will read beyond full. Ground the wire and the gauge will read empty.
You can think of a disconnection as providing infinity amount of ohm, and a wire that is grounded to have zero ohms.
Read the link to my site, you need at least a 500 ohm potentiometer to test the accuracy of the gauges. If you just want to test that the gauges work, you can use the ground/disconnected method which will test the extremes explained above.
I will warn you though that I suspect either a fuse or a bad power/ground to your gauge cluster. Chances of both not working are low. If someone swapped the motor, the gauges would at least read extremes because they are not connected, which is why I would eliminate that suspicion.
Factory gauges, specifically the temp gauge, are inaccurate from my testing. I would
not rely on the factory temp gauge. The fuel and oil pressure gauges are acceptable. Either way I added new oil pressure and water temp gauges (in addition to the factory ones) in my map pocket.