Diagnostic Overview - MUX Switches

The fact that the Electronic Control Module (ECU) uses a common signal and sensor ground to detect several switch positions helps in determining if a fault is most likely due to a switch or wiring issue. This can be determined by monitoring the voltage on the circuit or resistance through the switch while cycling through the different MUX Switch positions (depending on what info is available). If available, monitor the ECU voltage input using the scan tool (preferred). Use the information below to determine the most likely failure possibility:

1. Monitor for the voltage to read 0 volts or 5.0 volts when cycling through the switch positions. Since most MUX switches typically use a resistor to detect the at rest position the MUX signal will not be open (typically 5.0 volts) or shorted low (0 volts) at any time. This switch has two separate MUX signals that should be checked individually. The On/Off function uses a typical MUX signal. 
   • An S/C On/Off MUX Signal that reads 5.0 volts continuously could be an open switch, but is more likely an open MUX signal or sensor ground circuit.
   • An S/C On/Off MUX Signal that reads 0 volts continuously would most likely indicate a short to ground in the MUX signal circuit.
   • An S/C On/Off MUX Signal that reads 5.0 volts or 0 volts for only some switch positions would most likely indicate a faulty MUX Switch.
   • The resistance can be measured between the signal and sensor ground circuit at the component. Read and record the resistance through the switch with the switch not pressed, and then while pressing each switch position. Since most MUX switches typically use a resistor to detect the at rest position, the MUX Switch should typically never measure open when checking the switch positions.
2. The other speed control functions are combined in the S/C MUX Signal 1. The S/C MUX Signal 1 is open when no functions are being pressed.  Therefore, an open circuit will be interpreted as the S/C Switch not pressed. This can cause the S/C system to be inoperative with no fault indication.
   • An S/C MUX Signal 1 that reads 5.0 volts continuously when any of the speed control functions is being selected could be an open switch, an open MUX signal or sensor ground circuit.
   • An S/C MUX Signal 1 that reads 0 volts continuously would most likely indicate a short to ground in the MUX signal circuit.
   • An S/C MUX Signal 1 that reads 5.0 volts or 0 volts for only some switch positions would most likely indicate a faulty MUX Switch.
   • The resistance can be measured between the signal and sensor ground circuit at the switch. Read and record the resistance through the switch with the switch not pressed, and then while pressing each switch position.

The failures described above are to help shortcut and eliminate causes that are unlikely based on the voltage reads. The circuits and MUX Switch should still be checked and confirmed faulty before any repairs are made.