DTC 027

Schematic:

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 pen 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

Steps 1-5:

Steps 6-16:

The numbers below refer to the step numbers on the diagnostic table.
3. This step tests for an open in CKT XM112.
4. This step tests for a short to ground in CKT XM112.
6. This step tests for an open in CKT FP812.
8. 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.
9. This step tests for a short to ground in CKT FP812.
11. 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.
12. When you are inspecting for a possible intermittent condition, moving the harness may cause the condition to reappear.