Circuit Description
Operation of the blower motor is determined by the blower selection made at the heater and A/C control. Up to five blower speeds can be achieved.
When you select a blower speed, the heater and A/C control 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 for a low blower speed request to 0.5 V for a high blower speed request.
The blower control module provides a feedback circuit, on circuit XM112, to the heater and A/C control, 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 control. |
Conditions for Setting the DTC
| • | The heater and A/C control 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, the blower control module disables the blower supply voltage ending all blower motor operation. |
| • | When CKT A465 is open or shorted to ground, the blower control module disables the blower supply voltage ending all blower motor operation. |
| • | The heater and A/C control 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. |
| • | The DTC can be cleared with 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. |
| - | 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 is low speed and position 5 is high speed: |
| - | Position 1--1.5 V |
| - | Position 2--2.1 V |
| - | Position 3--2.8 V |
| - | Position 4--3.4 V |
| - | Position 5--5.0 V |
| • | You can monitor the voltage at the blower control module with a scan tool. Select blower controller 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.
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This step tests for an open in CKT XM112.
-
This step tests for a short to ground in CKT XM112.
-
This step tests for an open in CKT FP812.
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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.
-
This step tests for a short to ground in CKT FP812.
-
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.
-
When you are inspecting for a possible intermittent condition, moving the harness may cause the condition to reappear.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
Schematic Reference: HVAC Schematics | ||||
1 | Did you perform the HVAC Systems Diagnostic System Check? | -- | Go to Step 2 | |
2 | Does the blower motor operate at any speed? | -- | Go to Step 3 | Go to Blower Motor Inoperative |
3 | Test circuit A75/A465 at the blower control module for a short to ground, a high resistance, or an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 19 | Go to Step 4 |
Does the resistance measure less than the specified value? | 0.5 ohms | Go to Step 5 | Go to Step 6 | |
Measure the resistance from CKT XM112 to a good ground. Refer to Circuit Testing in Wiring Systems. Does the resistance measure at the specified value? | OL | Go to Step 7 | Go to Step 8 | |
6 | Locate and repair the open in CKT XM112. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 19 | Go to Step 9 |
Measure the resistance of circuit FP812. Refer to Circuit Testing in Wiring Systems. Does the resistance measure less than the specified value? | 0.5 ohms | Go to Step 10 | ||
8 | Locate and repair the short to ground in CKT XM112. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 19 | Go to Step 9 |
Does DTC 27 reset as current? | -- | Go to Step 12 | Go to Step 19 | |
Measure the resistance from the blower speed output circuit to a good ground. Refer to Circuit Testing in Wiring Systems. Does the resistance measure at the specified value? | OL | Go to Step 14 | Go to Step 15 | |
11 | Locate and repair the open in the blower speed output circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 19 | Go to Step 9 |
12 | Inspect for poor connections at the harness connector of the blower control module. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 19 | Go to Step 9 |
Replace the blower control module. Refer to Blower Motor Control Module Replacement . Did you complete the replacement? | -- | Go to Step 16 | -- | |
Did you find and correct the condition? | -- | Go to Step 9 | Go to Diagnostic Aids | |
15 | Locate and repair the short to ground in the blower speed output circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 19 | Go to Step 9 |
16 |
Does DTC 27 reset as current? | -- | Go to Step 17 | Go to Step 19 |
17 | Inspect for poor connections at the harness connector of the heater and A/C control. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 19 | Go to Step 18 |
18 |
Important: Perform the reprogramming procedure for the heater and A/C control. Replace the heater and A/C control. Refer to HVAC Control Module Replacement . Did you complete the replacement? | -- | Go to Step 19 | -- |
19 |
Does the DTC reset? | -- | Go to Step 2 | System OK |
