Temp gauge not working at all

[BigBadJohn]

The temp gauge in my 75 doesnt move at all. even after idling to full temperature. Just stays on the stop peg
Wheres the sensor for this and what does it look like
Theres some sort of senser in the stock water neck but its not used anymore. Also one on the left side of the block thats hooked up to a black wire
How do these circuits work?
Does the gauge have a full + 12 volts and then the temp sensor has a certain resistance and that resistance changes with temp and thats the ground voltage and moves the needle or what?
Id rather not install an aftermarket if I can fix it easily

[VileZambonie]

It's a single wire sensor located in the drivers side head. Turn the key on (engine off) and pull the wire off. Observe the gauge. Then ground the wire and observe the gauge. Post your results.

[BigBadJohn]

Unplugged the needle didnt move.
Grounded w/ no resistance the needle was pegged
So I guess the switch decreases resistance when hot. Interesting
Oreillys is still open I think. Theyll probably have the switch. Either that or for some reason the switch doesnt have a good ground maybe

[BigBadJohn]

theres .3 ohms resistance from a grounding strap on the front clip section to the outside threaded area of the switch so it does have a good ground. Guess the switch is screwed up.
Ill see about replacing it
Thanks

[DnStClr]

I copied this system description from an on-line article -it's so much easier for them to explain it..
http://www.secondchancegarage.com/public/653.cfm
Basically, an electric temperature gauge is a voltmeter. The scale on the gauge face is reading temperature but the instrument itself is reading voltage. The gauge itself is comprised of a bimetallic (two different metals fastened together) "hairpin" assembly. This assembly is attached to the needle.

The gauge requires an electric circuit and a sending unit in order to read temperature. The sending unit is a temperature-sensitive material that is part of a variable resistance, water-sealed unit that sits in the coolant stream in the engine. As the engine warms up the resistance in the sending unit is lowered gradually until the system reaches maximum heat. The sending unit is the "ground" portion of the circuit.

In the completed circuit the battery voltage passes from one side of the gauge, through the bimetallic spring and onward to the sending unit, which is grounded to the engine. When the engine is cold the resistance is high, so little current passes through the gauge. This small current doesn't heat up the bimetallic spring, so the gauge reads a low temperature. As the engine warms and the sending unit's resistance lowers more current passes through the gauge and the needle reads higher and higher because the bimetallic spring expands further.

Electric gauges can fail to read accurately because the sending units fatigue or rust over, or simply lose their connection to ground. The bimetallic spring can also fatigue over time, rendering the gauge inaccurate or inoperable.