Speedo / Tach / Gauges: How to test temperature gauge/sending unit

[mjeansonne]

My temperature gauge pegs all the way to hot, but the engine is not overheating. Installed a new sending unit a few months ago when I rebuilt the engine.
How can I test the gauge/sending unit to see if its working properly, without purchasing a new sending unit?

[Chucks Stude]

Purchase an infared thermometer, check what the temperature gauge says, and aim the thermometer at the thermostat housing, and you should get the same reading. If not, you may have some grounding problems. Make sure that you have good ground from the engine to the frame, and from the small gauges to the dash support. If the grounds are good, and the temp is still off, you might have a bad gauge. But it all starts with the infared thermometer. You can also use it too test the operation of the thermostat, and radiator.

[Jackson]

I had a temp gauge problem on both a 63 289 Lark and 64 259 Lark. In both cases I found that the internal temp gauge windings had shorted (grounded) to the case. I isolated the gauge windings from the case with a piece of cardboard and solved the probelm. The temp gauges in both Larrks now read correct as verified by an IR.

[doug]

Use a cooking thermometer in the radiator and check the water temp against the gauge reading. Start with the radiator cap off, don't remove it from a hot engine.

[greyben]

Sending units seldom fail. Dash units have frequently failed after fify years.
1. With the key off disconnect the wire at the sending unit and then measure the resistance to ground at the sending unit terminal. With the engine cold it should measure about 200 ohms if hot about 50. These values are very approximate. If it measures zero the unit is bad due to an internal ground.

2. With the wire still disconnected measure the resistance from the end of the wire to ground. It will probably measure zero indicating an internal short to ground in the dash unit.

3. Replace the dash unit with a good unit. This requires patience, a flexible back and prehensile arms. Also, be sure to disconnect the battery before poking around or disconnecting wires in the dash.

I recently went through this on my 55. The engine was out of the car and I measured the resistance with the sending unit removed at room temperature and at boiling. I forget the actual resistances I came up with. A 12 volt car would probably have higher resistanc values.