For 1990-2009 cars

### Circuit Description

The solar/twilight sensor is a photo-electric cell that provides two inputs to the heater and A/C control system. The input for the sunload is at terminal 3 of the heater and A/C control. This is used by the controller for temperature control compensation based on additional heat from the sun (sunload.) The input for the twilight sentinel is at terminal 2. This input is processed by the controller and sent to the Body Control Module (BCM).

### Conditions for Setting the DTC

The heater and A/C control continuously monitors CKT X812. The circuit needs to stay at or near zero volts for 200 ms.

### Action Taken When the DTC Sets

• | Stores a DTC 38 in the heater and A/C control memory.

• | The system will default to a assumed value of no solar heat (darkness).

### Conditions for Clearing the DTC

• | The conditions for the fault are no longer present.

• | Using a scan tool.

• | A history DTC will clear after 20 consecutive ignition cycles if the condition for the fault is no longer present.

### Diagnostic Aids

• | If the DTC is a history or intermittent. Try performing the tests shown while moving the wiring harnesses and connectors, this can often cause the malfunction to appear.

• | Visually inspect sensor connector and harness for damage, corrosion or water intrusion.

• | Check for adequate terminal tension, mis-routed harness, rubbed through wire insulation, and broken wire(s) inside the insulation.

### Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

1. This step checks for available reference voltage from the heater and A/C control. A normal reading should be approximately 5.0 - 5.5 V. A significantly lower voltage reading indicates a circuit problem.

2. This step isolates the circuit and a check is made for a short to ground.

3. If no apparent circuit problem was found in the previous steps and the DTC resets, the most likely cause is a heater and A/C control malfunction.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Was the Air Delivery System Check performed? | -- | Go to Step 2 | Go to HVAC Air Delivery System Check
2 |

1. Remove the solar/twilight sensor and disconnect the connector.
2. Turn ON the ignition switch.
3. Using a DMM, measure the voltage at harness terminal 1.

Is the voltage reading within the specified value?

5.0-5.5 V

Go to Step 8

Go to Step 4

3 | Check the solar/twilight sensor connector for damaged terminals and proper terminal tension. Repair as needed. Was a problem found with the connector and a repair made? | -- | Go to Step 5 | Go to HVAC Air Delivery System Check
4 |

1. Remove the heater and A/C control. Refer to Control Assembly Replacement .
2. Remove connector C1 from the controller.
3. Connect a DMM from terminal 7 and a good ground.
4. Measure the resistance.

Is the resistance reading within the specified value?

OL

Go to Step 5

Go to Step 6

5 |

1. Reassemble any connectors/components removed.
2. Install the scan tool and clear codes.
3. Recheck for a current DTC 38.

Is a current DTC 38 present?

--

Go to Step 7

Go to HVAC Air Delivery System Check

6 | Locate and repair the short to ground in CKT X812. Is the repair complete? | -- | Go to HVAC Air Delivery System Check | --
7 | Replace and program the heater and A/C control. Refer to Control Assembly Replacement . Is the replacement and programming complete? | -- | Go to HVAC Air Delivery System Check | --
8 | Replace the solar/twilight sensor. Is the replacement complete? | -- | Go to Step 5 | --