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Making Sense of Sensors—Part 1 (cont.)
Thermistors Thermistors are variable resistors whose resistance changes in relation to temperature. Thermistors may have either a positive or a negative temperature coefficient (PTC or NTC). If the resistance increases as the temperature increases, it has a PTC. Coolant Temperature Sensor Vigors use a thermistor for the Coolant Temperature Sensor (called the TW Sensor). The Vig's TW Sensor has an NTC—the resistance decreases as the temperature increases. See the resistance curve at right. The thermistor has two terminals, one for power and one for ground. See the illustration below. The ECU provides a reference voltage through an internal, fixed value resistor in series with the thermistor. The thermistor also gets its ground connection through the ECU. The ECU monitors the TW Signal voltage and compares that with the internal reference voltage to determine the temperature of the thermistor. The relationship between the two voltages varies as the temperature of the thermistor changes. There's an important distinction here between a potentiometer and a thermistor. They both have a reference voltage from the ECU, but a potentiometer returns a separate signal voltage. A thermistor, on the other hand, pulls down the reference voltage to "create" its signal voltage.
At very low temperatures (below 0°F), the thermistor has a high resistance (somewhere around 20KW for this very cold engine), so the TW Sensor voltage will be relatively low—somewhere around 0.5V. As the coolant (and the thermistor) heats up, the thermistor resistance will decrease (to 20-400W) and the voltage will increase to somewhere around 5V. By monitoring the voltage, the ECU can determine the coolant temperature.
How to test the circuit? Same as with a potentiometer, except there's one less terminal to contend with. Check Vref and resistance. First, disconnect the connector and measure the resistance of the sensor. If the engine is cold, the resistance should be high. If warm, the resistance should be much lower, as described in the preceding paragraph. Use the resistance curve above to get in the ballpark of resistance values. A noted problem with Vigors is a no-start only when engine the engine is hot. That's because these sensors tend to "die cold." Here's what happens: when cold, the thermistor has a high resistance. As the engine warms up, the resistance decreases until the engine is fully warmed up. Then, the sensor "opens" and dies. You won't notice this while you're driving around, but your engine will be running rich because of this. If you turn off the engine and then try to restart it, the "cold" signal to ECU will cause it to flood the combustion chambers with gas, thinking it's trying to start a cold engine. It won't start until the TW sensor cools down and starts over. If the resistance is good, check the Vref on the harness side of the connector. With KOEO, there should be approximately 5V between the YEL/GRN (+) wire and the GRN/WHT ( - ) wire. If there is, you can assume the circuit is good. Again, a more "direct" way is to backprobe the connector and measure the Vref with the engine running. This way, you can see exactly what is being reported to the ECU. Slide the needle tipped probes in carefully and start the engine. With KOER, the sensor voltage on the Vref wire should rise from around 0.5V to somewhere around 4.5V for a fully warmed up engine. And it should stay there! If you measure 0.5V on a fully warmed up engine, you've got a TW sensor that "dies cold." Air Temperature Sensor The Vigor's Air Temperature Sensor (called the TA Sensor) works identically to the TW Sensor. The essential differences are...
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