Page 321 - Understanding Automotive Electronics

2735 | CH 9 Page 308 Tuesday, March 10, 1998 1:24 PM

9 AUTOMOTIVE INSTRUMENTATION

becomes the signal that drives the display. It should be noted that this is actually
a form of digital filtering.
COOLANT TEMPERATURE MEASUREMENT
Another important automotive parameter that is measured by the
instrumentation is the coolant temperature. The measurement of this quantity
is different from that of fuel quantity because usually it is not important for the
driver to know the actual temperature at all times. For safe operation of the
engine, the driver only needs to know that the coolant temperature is less than
a critical value. A block diagram of the measuring system is shown in Figure
9.11.
The coolant temperature sensor used in most cars is a solid-state sensor
called a thermistor. Recall that this type of sensor was discussed in Chapter 6,
where it was shown that the resistance of this sensor decreases with increasing
temperature. Figure 9.12 shows the circuit connection and a sketch of a typical
sensor output voltage versus temperature curve.

Figure 9.11
Coolant
Temperature
Measurement

FPO

308 UNDERSTANDING AUTOMOTIVE ELECTRONICS