I've seen crusty gauges that did work but not OK. Crusty build up ( carbon? tar and nicotine from the ash tray below? whatever it is) restricts movement much like a ace bandage on a limb. A bi metal beam should be arrow straight. If it has a bow at room temperature has been overheated. The needle position has already changed some, and that beam wont respond properly to temperature changes. So for these 50 yr old thermal gauges, passing ohms tests says no more than still electrically complete. What once was fairly accurate and fairly fluid like needle movement is wrong and/or sticking then jumping needle movement.
