Page 323 - Understanding Automotive Electronics

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9 AUTOMOTIVE INSTRUMENTATION

the spring and to hold the switch open as long as the oil pressure exceeds the
lower limit. Whenever the oil pressure falls below this limit the spring force is
sufficient to close the switch. Switch closure is used to switch on the low oil
pressure warning message lamp.
One of the deficiences of this simple switch-based oil pressure warning
system is that it has a single fixed low oil pressure limit. In fact, the threshold oil
pressure for safe operation varies with engine load. Whereas a relatively low oil
pressure can protect bearing surfaces at low loads (e.g., at idle), a
proportionately higher oil pressure threshold is required with increasing load
(i.e., increasing horsepower and RPM).
An oil pressure instrument that operates with a load- or speed-dependent
threshold requires an oil pressure sensor rather than a switch. Such an oil
pressure warning system is illustrated in Figure 9.13. This system uses a
variable-resistance oil pressure sensor such as seen in Figure 9.14. A voltage is
developed across a series of fixed resistance that is proportional to oil pressure.
It should be noted that this assumed pressure sensor is hypothetical and used
only for illustrative purposes.

Figure 9.13
Oil Pressure
Measurement

FPO

310 UNDERSTANDING AUTOMOTIVE ELECTRONICS