The water temperature gauge on my 69 charger is pegging to the maximum at warm idle. I just put in a new thermostat (180°F) and coolant and it is still reading high. I left the radiator cap off and watched the fluid in the tank from a cold start. The thermostat does not appear to open until the temperature gauge is pegged to the right. I checked the resistance from the temperature sending unit to the battery and the ground strap on the firewall at idle. I'm measuring ~23.5 ohms while the temperature gauge is pegged. The oil pressure gauge also appears to be pegged to the right, but the gas gauge is working ok. I checked the resistance from the battery to the engine, and from the battery to the firewall and got less than an ohm of resistance. I unplugged the bulkhead connector from the firewall and measured the resistance from the temperature sending unit plug to its pin on the firewall harness connector and got less than an ohm. The engine does not appear to be overheating. It does not leak out the radiator's overflow tube. What should I check next? Could the gas gauge be working if the dash's 5v voltage regulator was going out?

All 3 gauges are dependent on the 5 volt feed from the limiter. Two simple tests would to verify your oil pressure with a mechanical gauge and a pyrometer on the coolant or a mechanical gauge. I have the tester for the gauges. I can measure the readings and you could use resistors to duplicate my testers E-1/2-F values.

the gas gauge reading "correct' could just be low on gas.
I would run a test on the VR first. see if it is pulsing with a test light or reading 5 volts with the meter.

The ohms actually are about what I would expect for a pegged gauge, so it may just be the sensor bad. but I would still test the vr first since it is more than 1 gauge.

23.5 ohms at the sensor should be at or just left of center on the temperature gauge. For a pegged gauge I would expect at or near 10 ohms at the sensor.

Sounds like a pinched wire to me

Thanks guys. I just bought a solid state instrument voltage regulator from real time engineering. I'll give it a try.

If in fact the fuel gauge is reading correctly the replacing of the limiter will not help. As said a simple voltage reading at any gauge input would verify if you have the correct 5 volts.

My over the counter stat didn't open until 195...so upon a suggestion I bought a milodon high flow 180 stat and it opens right on the money. (they are pricey but I feel worth the money) I also drilled a 1/8 hole through it.

If you have a solid state replacement IVR of some sort you can read 5 volts (or whatever the exact voltage output seems to be). If you have the original electromechanical model, it gives a pulsing 12 volt output that averages about 5 volts over time. But you cannot read 5 volts.

I put in the new voltage regulator. Now I think my gauges are reading low. My temperature gauge reads ~170 degrees Fahrenheit when running. It does go up to 180 degrees when the thermostat opens up, but then holds 170 steady. Oil pressure looks a little low. Gas gauge is a little weird. I put gas in the tank until I saw fuel in the filler neck and it read about 3/4 to 7/8 full. It did go up to full when I parked downhill, so maybe the filler neck was still draining into the tank or the float needs adjustment.

Is it possible for the temperature gauge to read lower than the stat temperature? I just installed a 180 degree high flow Edelbrock thermostat and new coolant. The radiator is huge, it was the thickest Griffin 26" radiator they sold ten years ago. I didn't have much clearance with that radiator so I put a Flexalite fan in. I think I might be picking up extra resistance from the bulkhead connector, which is causing the temp gauge to read low. I don't remember if I installed pipe tape on the temperature sensor. That could be effecting the reading too.

The factory gauge isn't all that precise or accurate.

they can be if calibrated. A royal PITA BTDT

No indication the OP did or had that done though.

I didn't say he did or didn't
You made a statement that they are not that precise or accurate, I made a statement that said they can be if calibrated.

I've never calibrated the gauges. Thanks, I'll look into that.