GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 1997 🢒 Catera 🢒 DTC 🢒 DTC 27 Blower Motor Voltage High

Object Number: 242520 Size: LF

Circuit Description

Operation of the blower motor is determined by the mode selection of the following components:

    • The heater and A/C control
    • The blower controller
        The blower controler can select any of 5 speeds.

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

The blower controller provides battery voltage from terminal 6 on a continuous basis to the blower motor.

The actual fan speed is achieved by controlling the ground side of the blower motor at terminal 7. Voltage on the ground side varies according to the following criteria:

    • 10 V--When a low blower speed is requested.
    • 0.5 V--When a high blower speed is requested.

The blower controller provides a feedback CKT XM112 in order to ensure the following conditions:

    • The blower motor speed stability.
    • The indication of any failure in the input circuitry to the blower controller.
    • The indication of any failure in the output circuitry to the blower controller.

Conditions for Setting the DTC
    • The heater and A/C controller detects an open or short to ground feedback (voltage greater than 3 V) in CKT XM112.
    • All of the above conditions must be met for less than 1 second.

Action Taken When the DTC Sets
    • The controller internally disables the voltage output at terminal 11, 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 after 20 consecutive ignition cycles if the conditions for the fault are no longer present.

Diagnostic Aids
    • If the DTC is a history or an intermittent, perform the tests shown while moving the wiring harnesses and the connectors. This may cause the malfunction to appear.
    • When moving related harnesses and harness connectors, inspect the wiring and the connectors for the following conditions:
       - Dirt or corrosion on the terminals. The terminals must be clean and free of any foreign material which could impede proper terminal contact.
       - A damaged connector body resulting from the following conditions:
   • Exposure of the terminals to moisture and dirt.
   • Not properly maintaining the terminal orientation with the component or mating connector.
   • Rubbed through wire insulation. This could cause an intermittent short when the bare area contacts other wiring or components.
    • The following is a list of typical voltage readings taken at terminal 2 of the blower controller 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)
    • 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 inspects for a short to voltage in CKT  XM112. Ensure that the measurements are made with the ignition switch in the OFF position and the key in the ON position.

    2.

  2. This step tests for a possible intermittent short to voltage.

    3.

  3. If DTC 27 continues to reset after a circuit repair or no circuit condition is found, the most likely cause is a failure internal to the heater and A/C controller.

Step

Action

Value(s)

Yes

No

1

Did you perform the Blower Controls System Check?

--

Go to Step 2

Go to HVAC Blower Controls System Check

2

  1. Press the OFF button on the heater and A/C control unit.
  2. Place the ignition switch in the OFF position.
  3. Locate the blower controller and remove the large connector.
  4. Use a digital multimeter (DMM) in order to measure and record the voltage at terminal 4.
  5. Turn the ignition switch to the ON position.
  6. Measure and record the voltage at terminal 4.

Are both of the voltage readings less than the specified value?

0.1 V

Go to Step 3

Go to Step 4

3

The condition may be intermittent at this time. Perform Step 2 while moving the related harnesses and the harness connectors.

Is there voltage present?

--

Go to Step 4

Go to Step 5

4

Locate and repair the short to voltage in CKT XM112 (BRN/WHT).

Is the circuit repair complete?

--

Go to Step 5

--

5
  1. Reconnect the connectors that were previously disconnected, as needed.
  2. Install the scan tool in order to clear any HVAC DTCs.
  3. Turn the ignition switch to the ON position.
  4. Cycle through all of the blower speeds.
  5. Inspect for DTC 27.

Does DTC 27 reset as current?

--

Go to Step 6

System OK

6

Replace and reprogram the heater and A/C control. Refer to Control Assembly Replacement .

Is the repair complete?

--

Go to Step 7

--

7
  1. Install the scan tool in order to clear any HVAC DTCs.
  2. Inspect for DTC 27.

Does DTC 27 reset as current?

--

--

System OK