For 1990-2009 cars
HVAC Components
HVAC Module Power, Ground, Serial Data, and Blower Controls

### Circuit Description

Operation of the blower motor is determined by the mode selection made at the heater and A/C control. Up to five blower speeds can be achieved.

When you select a blower speed or mode; the heater and A/C controller provides a variable output voltage in the range of 0 to 5 V from terminal 11 to terminal 2 of the blower control module.

Terminal 6 of the blower control module provides the battery voltage for the blower motor.

The actual fan speed is achieved by controlling the ground side of the blower motor at terminal 7 of the blower control module. Voltage on the ground side varies from 10 V (low blower speed request) to .5 V (high blower speed request).

The blower control module provides a feedback circuit (XM112), to the heater and A/C controller, in order to ensure the following.

• | Blower motor speed stability.

• | Indication of any failure in the input circuitry to the blower control module.

• | Indication of any failure in the output circuitry to the heater and A/C controller.

### Conditions for Setting the DTC

• | The heater and A/C controller detects an open or short to ground (voltage less than 0.5 V) in CKT XM112 , CKT FP812, CKT A75, or CKT A465.

• | All of the above conditions must be met for less than 1 second.

### Action Taken When the DTC Sets

• | When CKT FP812 is open or shorted to ground, the controller internally disables the blower speed voltage output at terminal 11, ending all blower motor operation.

• | When CKT XM112 is open or shorted to ground, blower motor operation is still functional.

• | When CKT A75 is open or shorted to ground, disables the blower supply voltage ending all blower motor operation.

• | When CKT A465 is open or shorted to ground, disables the blower supply voltage ending all blower motor operation.

• | The heater and A/C controller stores DTC 27 in memory.

### Conditions for Clearing the DTC

The following conditions must be met in order to clear the DTC:

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

• | Using a scan tool.

• | A history DTC will clear immediately if the conditions for the fault are no longer present.

### Diagnostic Aids

• | If the DTC is an intermittent, perform the tests shown while moving the wiring harnesses and connectors. This may cause the malfunction to appear.

• | When moving the related harnesses and harness connectors, inspect the wiring and connectors for the following conditions:

- | Broken wiring inside the insulation.

- | Poor mating of the connector halves or a terminal not fully seated in the connector body (backed-out).

- | Improperly formed or damaged terminals.

- | All connector terminals in the related circuits should be carefully reformed or replaced in order to ensure the proper contact tension.

- | Poor terminal to wire connection. Inspect for the following conditions:

• | Poor crimps

• | Crimping over wire insulation rather then the wire

• | The following is a list of typical voltage readings taken at terminal 2 of the blower control module with the key ON and engine OFF and no fault present:

- | Position 1--1.5 V (low speed)

- | Position 2--2.1 V

- | Position 3--2.8 V

- | Position 4--3.4 V

- | Position 5--5.0 V (high speed)

• | You can monitor the voltage at the blower control module with a scan tool. Select blower control module VOLTAGE from the Heating and Air Conditioning data list in order to monitor the voltage.

• | Always inspect the power and ground circuits before you replace the heater and A/C controller.

### Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. This step tests for an open in CKT XM112.

2. This step tests for a short to ground in CKT XM112.

3. This step tests for an open in CKT FP812.

4. Always clear the DTCs after a repair is performed. Cycling through all of the blower speeds exercises all of the components and related circuitry in this system. Inspect for a current DTC 27 in order to verify the repair.

5. This step tests for a short to ground in CKT FP812.

6. If the DTC reappears after a circuit or no trouble is found in the circuits, the blower controller or heater and A/C controller may be at fault. Both components cannot be bench tested in the field. For test purposes, replace the blower control module first.

7. When you are inspecting for a possible intermittent condition, moving the harness may cause the condition to reappear.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the HVAC Systems Diagnostic System Check? | -- | Go to Step 2 | Go to Diagnostic System Check
2 | Does the blower motor operate at any speed? | -- | Go to Step 3 | Go to Blower Motor Inoperative at Any Speed
3 |

1. Remove the heater and A/C control.
2. Disconnect the heater and A/C control connector C1.
3. Disconnect the blower control module 5 pin black connector.
4. Measure the resistance of CKT XM112.

Does the resistance measure less than the specified value?

0.5 ohms

Go to Step 4

Go to Step 5

4 | Measure the resistance from CKT XM112 to a good ground. Does the resistance measure at the specified value? | OL (∞) | Go to Step 6 | Go to Step 7
5 | Locate and repair the open in CKT XM112. Did you complete the repair? | -- | Go to Step 8 | --
6 | Measure the resistance of the blower speed output circuit (FP812). Does the resistance measure less than the specified value? | 0.5 ohms | Go to Step 9 | Go to Step 10
7 | Locate and repair the short to ground in CKT XM112. Did you complete the repair? | -- | Go to Step 8 | --
8 |

1. Reconnect the connectors that were previously removed, as needed.
2. Install a scan tool in order to clear any HVAC DTCs.
3. Cycle through all of the blower speeds.
4. Inspect for DTC 27.

Does DTC 27 reset as current?

--

Go to Step 11

System OK

9 | Measure the resistance from the blower speed output circuit to a good ground. Does the resistance measure at the specified value? | OL (∞) | Go to Step 12 | Go to Step 13
10 | Locate and repair the open in the blower speed output circuit. Did you complete the repair? | -- | Go to Step 8 | --
11 | Replace the blower control module. Did you complete the replacement? | -- | Go to Step 14 | --
12 |

1. The condition may be intermittent at this time in either circuit.
2. Perform the above electrical steps again while moving related harnesses and connectors.

Did you find and correct the condition?

--

Go to Step 8

Go to Diagnostic Aids

13 | Locate and repair the short to ground in the blower speed output circuit. Did you complete the repair? | -- | Go to Step 8 | --
14 |

1. Install a scan tool in order to clear any HVAC DTCs.
2. Cycle through all of the blower speeds.
3. Inspect for DTC 27.

Does DTC 27 reset as current?

--

Go to Step 15

System OK

15 | Replace and reprogram the heater and A/C controller. Did you complete the replacement? | -- | Go to Step 16 | --
16 |

1. Install a scan tool in order to clear any HVAC DTCs.
2. Cycle through all of the blower speeds.
3. Inspect for DTC 27.

Does DTC 27 reset as current?

--

--

System OK