GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2004 🢒 Astro - 4WD 🢒 DTC 🢒 DTC C0305

Circuit Description

The front propshaft speed sensor is a permanent magnet (PM) generator. The PM generator produces a pulsing AC voltage. The AC voltage level and number of pulses increases as speed increases. The transfer case shift control module converts the pulsating AC voltage to a propshaft RPM which is used for calculations, and to monitor the difference between the front and rear propshaft speed. It is also used in the AUTO 4WD mode of operation to determine the amount of slip and the percent of torque to apply to the front axle. The propshaft RPM can be displayed with a scan tool.

This DTC detects an open, short to ground, short to voltage or faulty sensor.

Conditions for Running the DTC
    • The ignition is ON.
    • The system voltage is 9 - 16 volts.
    • The vehicle speed must be greater than 16 km/h (10 mph).
    • The transmission is NOT in Park/Neutral.
    • The engine is running.
    • The front axle is engaged.

Conditions for Setting the DTC

The system logs the DTC if the front propshaft input indicates a speed of less than 16 RPM for 30 seconds.

Action Taken When the DTC Sets
    • All Shifts to AUTO 4WD mode are disabled. If the current mode is AUTO 4WD, the system does not allow any further adaptive events, such as correcting wheel slip.
    • The SERVICE AWD indicator remains illuminated for the remainder of the current ignition cycle.

Conditions for Clearing the DTC
    • The transfer case shift control module clears the DTC when the condition for setting the DTC no longer exists.
    • A history DTC clears after 100 consecutive ignition cycles without a fault present.
    • History DTCs can be cleared using a scan tool.

Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

  1. This step tests to see if the malfunction is intermittent.

    2.

  2. This step tests the internal resistance of the propshaft speed sensor.

    3.

  3. This step tests to see if the propshaft speed sensor can generate a sufficient amount of AC voltage.

    4.

  4. This step tests the front speed sensor low circuit for a short to ground, a high resistance, or an open.

    5.

  5. This step tests the front speed sensor high circuit for a short to ground, a high resistance, or an open.

    6.

  6. This step tests the front speed sensor low circuit for a short to voltage.

    7.

  7. This step tests the front speed sensor high circuit for a short to voltage.

    8.

  8. This step tests the front speed sensor low circuit for a short to the front speed sensor high circuit.

Step

Action

Value(s)

Yes

No

Schematic Reference: Transfer Case Control Schematics

1

Did you perform the Diagnostic System Check - Transfer Case?

--

Go to Step 2

Go to Diagnostic System Check - Transfer Case

2
  1. Drive the vehicle for a short test drive.
  2. With the scan tool, observe the value of the Front Propshaft Speed data parameter in the Transfer Case data list.

Does the RPM value vary while driving?

--

Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems

Go to Step 3

3
  1. Turn the ignition OFF.
  2. Disconnect the transfer case front propshaft speed sensor.
  3. With a DMM measure the resistance of the transfer case front propshaft speed sensor.

Is the resistance reading within the specified values?

1300-2700 ohms

Go to Step 4

Go to Step 10

4
  1. Connect a DMM to the transfer case front propshaft speed sensor.
  2. Block the rear wheels.
  3. Place the transmission in NEUTRAL.
  4. Raise and support the front axle so that the wheels turn freely.
  5. Hold one front wheel while rotating the other front wheel.
  6. Observing the AC voltage reading on the DMM.

Is the voltage reading greater than the specified value?

0.5 V AC

Go to Step 5

Go to Step 10

5
  1. Disconnect the transfer case shift control module.
  2. Test the front speed sensor low circuit for a short to ground, high resistance, or an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find an correct the condition?

--

Go to Step 12

Go to Step 6

6

Test the front speed sensor high circuit for a short to ground, high resistance, or an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find an correct the condition?

--

Go to Step 12

Go to Step 7

7

Test the front speed sensor low circuit for a short to voltage. Refer to Testing for a Short to Voltage and Wiring Repairs in Wiring Systems.

Did you find an correct the condition?

--

Go to Step 12

Go to Step 8

8

Test the front speed sensor high circuit for a short to voltage. Refer to Testing for a Short to Voltage and Wiring Repairs in Wiring Systems.

Did you find an correct the condition?

--

Go to Step 12

Go to Step 9

9

Test the front speed sensor low circuit for a short to the front speed sensor high circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find an correct the condition?

--

Go to Step 12

Go to Step 11

10

Replace the transfer case front propshaft speed sensor. Refer to Transfer Case Speed Sensor Replacement .

Did you complete the repair?

--

Go to Step 12

--

11

Replace the transfer case shift control module. Refer to Transfer Case Shift Control Module Replacement .

Did you complete the repair?

--

Go to Step 12

--

12
  1. Use the scan tool in order to clear the DTCs.
  2. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC reset?

--

Go to Step 2

System OK