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.
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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.
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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 |
|---|---|---|---|---|
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 | |
Does the resistance measure less than the specified value? | 0.5 ohms | Go to Step 4 | Go to Step 5 | |
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 | -- |
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 | ||
7 | Locate and repair the short to ground in CKT XM112. Did you complete the repair? | -- | Go to Step 8 | -- |
Does DTC 27 reset as current? | -- | Go to Step 11 | System OK | |
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 | -- |
Replace the blower control module. Did you complete the replacement? | -- | Go to Step 14 | -- | |
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 |
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 |
Does DTC 27 reset as current? | -- | -- | System OK |
