I think some of the confusion on this part is due, in part, to the fact that GM changed the name of the part, as well as the part number down through the years. In the 1970 PDM, sheet 480841 #2, the part is called a Transducer Assembly, #1513321, with option U30.
The 9/1/70 Pontiac MPC has it as #1513321 also, but calls it a thermostatic gauge switch (round single conn.) (w/rally gauges or aux. gauges). But the part does not function as a switch. A switch is all on or off, make or break. A gauge sending unit is a thermistor, giving a varying reading. Thermistor or transducer is more accurate.
But switch would be an accurate description of a sender for an idiot light, since they simply trip to close the circuit & turn on a light.
Sooooooooo,...all this is to say that when you use an idiot light sender in a car equipped with a gauge, the gauge will read no temp, until the sender (switch) reaches its' pre-set temp at which it will trip & make an internal connection. (Often around 248 degrees) Then the gauge will quickly go up to near max reading.
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