How to test if gauges are bad

[jollyjerry]

I want to figure out whether my gauges are reading wrong or whether something is actually faulty. The ones I want to know about are:

• Temperature
• Voltmeter
• Oil pressure

My temperature gauge never reads above 170, even after long trips. I'm getting pretty decent mileage (15ish on a 5.7), so I don't think it's running cold the whole time. Can I just stick a thermometer into the coolant and compare that temperature? What sensor controls the dash gauge?

The voltmeter I can test with a voltmeter against the battery and the alternator. When I'm driving with accessories on, it looks like it's barely reading above 12v. I'm going to check the alternator tomorrow while it's running to make sure it's charging fine. If the gauge isn't accurate, where's it reading from?

My oil pressure starts just above 40, then drops down after it's warmed up a bit. I've seen it go as high as 50. What's the operating range for you?

 

[Chevy-Art]

The sensor for the gauge is in the left cylinder head.
For the volts it is just reading from the truck, it doesn't specifically hook up anywhere. My volts are screwed up too, it always shows really low voltage. Buy it isn't. I was thinking about removing the cluster and cleaning all the contacts, but first disconnect the battery.