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Home >> Chrysler >> 2009 >> 300 Signature Series >> Repair and Diagnosis >> Heating, Ventilation & A/C (HVAC) >> HVAC Control Systems >> (HVAC Systems (MTC) - Electrical Diagnostics) >> DTC Troubleshooting >> DTC 74: Ac Request Output Signal Too Low >> Diagnostic Test

Diagnostic Test

  1. CHECK (C105) HVAC MUX CONTROL CIRCUIT VOLTAGE AT A/C HEATER CONTROL C1 HARNESS CONNECTOR 
    Fig 1: Checking HVAC MUX Control Circuit Voltage At A/C Heater Control C1 Harness Connector
    GC0096413Courtesy of CHRYSLER LLC
    1. Turn the ignition off.
    2. Disconnect the A/C Heater Control C1 harness connector.
    3. Turn the ignition on.
    4. Measure the voltage of the (C105) HVAC MUX Control circuit.

      Is the voltage below 4.8 volts?

      Yes 

      • Go to step  2

      No 

  2. CHECK (C105) HVAC MUX CONTROL CIRCUIT FOR A SHORT TO GROUND 
    Fig 2: Checking HVAC MUX Control Circuit For A Short To Ground
    GC0096414Courtesy of CHRYSLER LLC
    1. Turn the ignition off.
    2. Disconnect the Instrument Cluster C2 harness connector.
    3. Measure the resistance of the (C105) HVAC MUX Control circuit between ground and the A/C Heater Control C1 harness connector.

      Is the resistance below 10K ohms?

      Yes 

      No 

      • Go to step  3
  3. CHECK (C105) HVAC MUX CONTROL CIRCUIT FOR AN OPEN 
    Fig 3: Checking HVAC MUX Control Circuit For An Open
    GC0096347Courtesy of CHRYSLER LLC
    1. Measure the resistance of the (C105) HVAC MUX Control circuit between the A/C Heater Control C1 harness connector and the Instrument Cluster C2 harness connector.

      Is the resistance below 5.0 ohms?

      Yes 

      No