Join Date: Mar 2002
Location: Austin, TX, USA
There are two types. The light version is a switch. 12Volts is supplied to the dash warning light. The other side of the light is connected to the switch. When the engine gets hot, the switch closes and illuminates the light.
Guages are wired similarly in place of the lamp, but the sender is a thermistor (resistor that changes value with heat). The thermistor is a negative temperature coeficient type meaning it's resistance decreases with increasing heat. A cold sender will measure around 1K ohm depending on the season. When hot, the resistance drops to a few hundered ohms (~300ohms). At near 300 degrees, the resitance is almost zero. Guages and sending units are matched. Often, an aftermarket guage's / sender will not work with a factory sender / guage.
In both types, ground returns through the sensor body via the engine block and a bond strap to the firewall. Good grounds are a must to acurrate guage readings.
I just finished helping a friend on his 82 camaro do a TPI swap. Seems like the wire was black-tan for the guage? Beware that there is a temp switch also that is wired into to the fuel pump relay circuit. It allows the fuel pump to come on when oil pressure builds. I'd recommend getting a schematic to be sure which sensor you're dealing with.
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI