Cruise Control Systems: Operation: Multiplexing

The PCM sends out 5 volts through a fixed resistor and monitors the voltage change between the fixed resistor and the switches. If none of the switches are depressed, the PCM will measure 5 volts at the sensor point (open circuit). If a switch with no resistor is closed, the PCM will measure zero volts (grounded circuit). If a resistor is added to a switch, the PCM will measure some voltage proportional to the size of the resistor. By adding a different resistor to each switch, the PCM will see a different voltage depending on which switch is pushed. Another resistor has been added to the at rest circuit, causing the PCM to never see 5 volts. This was done for diagnostic purposes. If the switch circuit should open (bad connection), the PCM will see the 5 volts and know the circuit is faulty. The PCM will then set an open circuit fault.