A/C Heater Control Will Not Enter Diagnostic Mode (MTC)

A/C HEATER CONTROL WILL NOT ENTER DIAGNOSTIC MODE (MTC)

A/C Heater Control Will Not Enter Diagnosic Mode (MTC):

Theory of Operation
An active DTC 33 will prevent the control from entering diagnostic mode and performing certain diagnostic functions, it will also prevent proper A/C mode switch and status indicator function. An active DTC 34 will prevent the control from performing certain diagnostic functions and it will prevent proper EBL mode switch and status indicator function. In addition, a short or an open (C850) Blower Switch Position Signal circuit will also prevent the control from entering diagnostic mode.

POSSIBLE CAUSES
- Object holding A/C mode switch/EBL mode switch in a pushed position
- Substance causing A/C mode switch/EBL mode switch to get stuck in a pushed position
- (C850) blower switch position signal circuit shorted to voltage
- (C850) blower switch position signal circuit open
- (C850) blower switch position signal circuit shorted to ground
- A/C status indicator/EBL status indicator inoperative
- A/C heater control

DIAGNOSTIC TEST

1. INSPECT THE A/C HEATER CONTROL FOR DAMAGE
Turn the ignition off.
Inspect the A/C Heater Control for damage.

Q: Is the A/C Heater Control damaged?

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

2. INSPECT FOR OBJECT OR SUBSTANCE CAUSING THE A/C MODE SWITCH OR EBL MODE SWITCH TO STAY OR STICK IN A PUSHED POSITION
Inspect the A/C Heater Control for anything that would cause the A/C mode switch or the EBL mode switch to stay or stick in a pushed position.

Q: Is anything present that would cause the A/C mode switch or EBL mode switch to stay or stick in a pushed position?

YES: Repair as necessary. Perform BODY VERIFICATION TEST - VER 1. Body Verification Test
NO: Go To 3

3. CHECK THE A/C MODE SWITCH & STATUS INDICATOR FUNCTION WITH THE BLOWER CONTROL IN THE OFF POSITION
Turn the ignition on.
Turn the blower control off.
Press the A/C mode switch on and off several times while observing the A/C status indicator.

Q: Does the A/C status indicator turn on and off?

YES: Go To 4
NO: Go To 7

4. CHECK THE (C850) BLOWER SWITCH POSITION SIGNAL CIRCUIT VOLTAGE




Turn the ignition off.
Disconnect the A/C Heater Control C3 harness connector.
Turn the ignition on.
Measure the voltage of the (C850) Blower Switch Position Signal circuit.

Q: What is the voltage?

Below 4.8 volts
Go To 5
Above 5.2 volts
Go To 6
4.8 volts To 5.2 volts
Replace the A/C Heater Control. Perform BODY VERIFICATION TEST - VER 1. Body Verification Test

5. CHECK THE (C850) BLOWER SWITCH POSITION SIGNAL CIRCUIT FOR AN OPEN




Turn the ignition off.
Disconnect the A/C Heater Control C1 harness connector.
Measure the resistance of the (C850) Blower Switch Position Signal circuit between the A/C Heater Control C1 harness connector and the A/C Heater Control C3 harness connector.

Q: Is the resistance below 5.0 ohms?

YES: Replace the A/C Heater Control. Perform BODY VERIFICATION TEST - VER 1. Body Verification Test
NO: Repair the (C850) Blower Switch Position Signal circuit for an open. Perform BODY VERIFICATION TEST - VER 1. Body Verification Test

6. CHECK THE (C850) BLOWER SWITCH POSITION SIGNAL CIRCUIT FOR A SHORT TO VOLTAGE




Turn the ignition off.
Disconnect the A/C Heater Control C1 harness connector.
Turn the ignition on.
Measure the voltage of the (C850) Blower Switch Position Signal circuit.

Q: Is the voltage above 0.2 volts?

YES: Repair the (C850) Blower Switch Position Signal circuit for a short to voltage. Perform BODY VERIFICATION TEST - VER 1. Body Verification Test
NO: Replace the A/C Heater Control. Perform BODY VERIFICATION TEST - VER 1. Body Verification Test

7. CHECK THE A/C MODE SWITCH & STATUS INDICATOR FUNCTION WITH THE BLOWER CONTROL IN THE ON POSITION
Turn the blower control on.
Press the A/C mode switch on and off several times while observing the A/C status indicator.

Q: Does the A/C status indicator turn on and off?

YES: Go To 8
NO: Go To 11

8. CHECK FOR NORMAL EBL MODE SWITCH & STATUS INDICATOR FUNCTION
Turn the blower control off.
Press the EBL mode switch on and off several times while observing the EBL status indicator.

Q: Does the EBL status indicator turn on and off?

YES: Go To 9
NO: Replace the A/C Heater Control. Perform BODY VERIFICATION TEST - VER 1. Body Verification Test

9. CHECK A/C HEATER CONTROL OPERATION BY ATTEMPTING TO ENTER DIAGNOSTIC MODE
If illuminated, turn off the A/C status indicator and the EBL status indicator.
Actuate the Display DTC function by performing the following:
- Turn the blower control on.
- Press the A/C mode switch down, turn the blower control to off, wait 3 seconds and then release the A/C mode switch.

When entry mode is recognized, the A/C status indicator and the EBL status indicator will illuminate for 5 seconds and then display DTCs, if present. The A/C status indicator displays active DTCs when the EBL status indictor is not illuminated and stored DTCs when the EBL status indicator is illuminated. However, if no DTCs are present, both status indicators will turn off and the system will exit diagnostic mode.

Q: Did the A/C Heater Control enter diagnostic mode?

YES: Go To 10
NO: Replace the A/C Heater Control. Perform BODY VERIFICATION TEST - VER 1. Body Verification Test

10. CHECK THE A/C HEATER CONTROL BY ATTEMPTING TO RUN THE ACTUATOR CALIBRATION FUNCTION
If illuminated, turn off the A/C status indicator and the EBL status indicator.
Actuate the Actuator Calibration function by performing the following:
- Gain viewing access to the linkage of at least one door actuator to verify that the calibration function runs when actuated.
- Turn the ignition on.
- Turn the blower control to off.
- Press the EBL mode switch down for 5 seconds and then release it.
- View the actuator linkage to verify that the calibration function is running. The process takes approximately 90 seconds to complete.

Q: Did the A/C Heater Control run the function?

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

11. CHECK THE (C850) BLOWER SWITCH POSITION SIGNAL CIRCUIT FOR A SHORT TO GROUND




Turn the ignition off.
Disconnect the A/C Heater Control C1 harness connector.
Disconnect the A/C Heater Control C3 harness connector.
Measure the resistance of the (C850) Blower Switch Position Signal circuit between ground and the A/C Heater Control C1 harness connector.

Q: Is the resistance below 10K ohms?

YES: Repair the (C850) Blower Switch Position Signal circuit for a short to ground. Perform BODY VERIFICATION TEST - VER 1. Body Verification Test
NO: Replace the A/C Heater Control. Perform BODY VERIFICATION TEST - VER 1. Body Verification Test