MTC 74

74-AC REQUEST OUTPUT SIGNAL TOO LOW (MTC)

74 - AC Request Output Signal Too Low (MTC):

WHEN MONITORED
With the ignition on.

SET CONDITION
If the A/C Heater Control detects voltage below 1.51 volts on the (C105) HVAC MUX Control circuit.

POSSIBLE CAUSES
- (C105) HVAC mux control circuit shorted to ground
- (C105) HVAC mux control circuit open
- Instrument cluster (CCN)
- A/C heater control

NOTE: This DTC must be active for the results of this test to be valid. Do not perform this test if this DTC is stored. Refer to HVAC System Test (MTC) for stored DTC test procedures. HVAC System Test (MTC)

DIAGNOSTIC TEST

1. CHECK (C105) HVAC MUX CONTROL CIRCUIT VOLTAGE AT A/C HEATER CONTROL C1 HARNESS CONNECTOR




Turn the ignition off.
Disconnect the A/C Heater Control C1 harness connector.
Turn the ignition on.
Measure the voltage of the (C105) HVAC MUX Control circuit.

Q: Is the voltage below 4.8 volts?

YES: Go To 2
NO: Replace the A/C Heater Control.
Perform BODY VERIFICATION TEST - VER 1. Body Verification Test

2. CHECK (C105) HVAC MUX CONTROL CIRCUIT FOR A SHORT TO GROUND




Turn the ignition off.
Disconnect the Instrument Cluster C2 harness connector.
Measure the resistance of the (C105) HVAC MUX Control circuit between ground and the A/C Heater Control C1 harness connector.

Q: Is the resistance below 10K ohms?

YES: Repair the (C105) HVAC MUX Control circuit for a short to ground.
Perform BODY VERIFICATION TEST - VER 1. Body Verification Test

3. CHECK (C221) A/C SWITCH SENSE CIRCUIT FOR AN OPEN




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.

Q: Is the resistance below 5.0 ohms?

YES: Replace the Instrument Cluster (CCN).
Perform BODY VERIFICATION TEST - VER 1. Body Verification Test
NO: Repair the (C105) HVAC MUX Control circuit for an open.
Perform BODY VERIFICATION TEST - VER 1. Body Verification Test