Page 322 - Understanding Automotive Electronics

2735 | CH 9 Page 309 Tuesday, March 10, 1998 1:24 PM

AUTOMOTIVE INSTRUMENTATION 9

Figure 9.12
Coolant
Temperature
Sensor

FPO

To measure coolant tem- The sensor output voltage is sampled during the appropriate time slot and
perature, the sensor out- is converted to a binary number equivalent by the A/D converter. The
put from a thermistor is computer compares this binary number to the one stored in memory that
converted into a digital corresponds to the high temperature limit. If the coolant temperature exceeds
signal and compared to a the limit, an output signal is generated that activates the warning indicator. If
maximum safe value the limit is not exceeded, the output signal is not generated and the warning
stored in memory. message is not activated. A proportional display of actual temperature can be
used if the memory contains a cross-reference table between sensor output
voltage and the corresponding temperature, similar to that described for the
fuel quantity table.
OIL PRESSURE MEASUREMENT
Oil pressure warning Engine oil pressure measurement is similar to coolant temperature
systems use a variable- measurement in that it uses a warning message display rather than an indicated
resistance sensor as part numerical value. Whenever the oil pressure is outside allowable limits, a
of a voltage divider. This warning message is displayed to the driver. In the case of oil pressure, it is
arrangement provides a important for the driver to know whenever the oil pressure falls below a lower
varying voltage that cor- limit. It is also possible for the oil pressure to go above an allowable upper limit;
responds to changes in however, many manufacturers do not include a high oil pressure warning in the
oil pressure. instrumentation.
The simplest oil pressure warning system involves a spring-loaded switch
connected to a diaphragm. The switch assembly is mounted in one of the oil
passageways such that the diaphragm is exposed directly to the oil pressure. The
force developed on the diaphragm by the oil pressure is sufficient to overcome

UNDERSTANDING AUTOMOTIVE ELECTRONICS 309