Thermistors are variable resistors whose resistance changes in relation to temperature. Thermistors can have either a negative temperature coefficient (NTC) or a positive temperature coefficient (PTC). A thermistor with a negative temperature coefficient will decrease in resistance as the temperature is increased. On the other hand, a thermistor with a positive temperature coefficient will increase in resistance as the temperature is increased. The thermistor has two terminals, one for power and one for ground. A reference voltage is supplied to one terminal through a fixed series resistor located inside the computer. The other terminal of the thermistor is connected to ground, usually back through the computer. The computer monitors the voltage after the internal fixed resistor and compares this voltage to the reference voltage to determine the temperature of the thermistor. The relationship between the two voltages changes as the temperature of the thermistor changes.
The coolant temperature sensor and the air temperature sensor in the air flow meter are both NTC thermistors. Thermistors are also used as sending units for temperature gauges such as the coolant temperature gauge. The TCCS ECU uses data from the coolant temperature sensor and air temperature sensor to help determine the proper amount of fuel and how long to open the fuel injectors. The ECU also uses this data to determine how much the ignition timing should be advanced as well as the proper setting for the ISC to maintain the proper idle speed. When either the air temperature or the coolant temperature is low, the respective thermistor's resistance increases and the computer receives a high voltage signal at the respective sensor wire. Conversely, a high temperature at either sensor results in a low voltage signal due to the lower resistance of the thermistor.
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