DESCRIPTION SERVICE MANUAL NUMBER 22
Page 5A-14 90-860074--1 FEBRUARY 2002
SWITCH TYPES
Switched inputs (also known as discretes) to the computer can cause one bit to change,
resulting in information being communicated to the computer. Switched inputs can come in
two types: they are pull-up and pull-down types. Both types will be discussed.
With a pull-up type switch, the ECM will sense a voltage when the switch is CLOSED. With
the pull-down switch, the ECM recognizes the voltage when the switch is OPEN.
Discretes can also be used to inform a computer of FREQUENCY information.
PULSE COUNTERS
For the computer to determine frequency information from a switched input, the computer
must measure the time between voltage pulses. As a number of pulses are recorded in a
set amount of time, the computer can calculate the frequency. The meaning of the frequency
number can have any number of meanings to the computer.
An example of a pulse counter type of input is the Engine rpm Speed sensor reference pulse
input. The computer can count a train of pulses, a given number of pulses per engine
revolution, and determine the rpm of the engine.
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