GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2004 🢒 TopKick C-Series (Conventional) C4/C5 🢒 Brakes 🢒 Air Antilock Brake System 🢒 Status Lamps Illuminated - LFT, FRT, and SEN

Circuit Description

As the wheel spins, the wheel speed sensor produces an AC signal. The electronic brake control module (EBCM) uses the frequency of the AC signal to calculate the wheel speed.

Conditions for Running the Diagnostic Fault
    • The ignition is ON.
    • The vehicle speed is greater than 26 km/h (16 mph).

Conditions for Setting the Diagnostic Fault
    • Open wheel speed sensor or wheel speed sensor wiring.
    • Shorted wheel speed sensor or wheel speed sensor wiring.
    • Wheel speed signal not present or does not conform to design criteria.

Action Taken When the Diagnostic Fault Sets
    • The ABS indicator lamp turns ON.
    • The ABS operates using the input from the other front wheel speed sensor.
    • The EBCM illuminates the FRT, LFT, and SEN LEDs.

Conditions for Clearing the Diagnostic Fault
    • Repair the condition responsible for illuminating the status lamps.
    • Perform the Status Lamps Reset Procedure.

Diagnostic Aids

Thoroughly inspect connections or circuitry that may cause an intermittent malfunction. Refer to Testing for Intermittent Conditions and Poor Connections , Connector Repairs , Testing for Electrical Intermittents and Wiring Repairs in Wiring Systems.

If the customer's concern is that the ABS indicator is on only during humid conditions such as rain, snow or vehicle wash, thoroughly inspect the wheel speed sensor circuits for signs of water intrusion. Use the following procedure in order to help isolate the problem area:

  1. Spray the suspected area with a 5 percent salt water solution: 10 grams (2 teaspoons) of salt to 200 ml (12 oz) of water.
  2. Operate the vehicle at a speed greater than 19 km/h (12 mph) for at least 30 seconds.

Repair or replace the suspect harness if the DTC sets.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. This step tests for the correct bearing play. Excessive bearing play can cause wheel speed sensor air gap to exceed specification. Wheel speed sensor air gap must be reinspected after a repair is made to correct excessive bearing play.

    2.

  2. Measure the resistance value of the wheel speed sensor in order to determine if the sensor has a valid resistance value.

    3.

  3. Ensures the wheel speed sensor is generating a valid AC voltage output.

Step

Action

Values

Yes

No

Schematic Reference: Air Antilock Brake System Schematics

Connector End View Reference: Air Brake System Component Views

1

Did you perform the ABS Diagnostic System Check?

--

Go to Step 2

Go to Diagnostic System Check - Air ABS

2
  1. Operate the vehicle at a speed greater than the specified value.
  2. Observe the LEDs on the EBCM.

Do the LEDs on the EBCM indicate a current wheel speed sensor fault?

26 km/h (16 mph)

Go to Step 3

Go to Diagnostic Aids

3

  1. Raise and support the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
  2. Inspect the wheel bearings for looseness or excessive wear and verify that the wheel bearing play is within specifications. Refer to Wheel Bearing Adjustment or Front Wheel Hub, Bearing, and Seal Replacement in Front Suspension or Wheel Bearing Wear - Rear Drive Axle or Rear Axle Hub, Bearing, Cup, and/or Seal Replacement in Rear Drive Axle.

Did you find and correct the condition?

--

Go to Step 12

Go to Step 4

4

Inspect the wheel speed sensor in order to ensure the sensor is installed and retained properly for proper air gap. Refer to Front Wheel Speed Sensor Replacement or Rear Wheel Speed Sensor Replacement .

Did you find and correct the condition?

--

Go to Step 12

Go to Step 5

5

  1. Disconnect the wheel speed sensor harness connector.
  2. Use a DMM in order to measure the resistance across the wheel speed sensor.

Does the resistance measure within the specified range?

1500-2500 ohms

Go to Step 6

Go to Step 8

6

  1. Spin the wheel by hand as fast as possible.
  2. Measure the AC voltage across the wheel speed sensor.

Does the AC voltage measure greater than the specified value?

800 mV

Go to Step 7

Go to Step 8

7

Inspect for poor connections at the harness connector of the wheel speed sensor. 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 12

Go to Step 9

8

Replace the wheel speed sensor. Refer to Front Wheel Speed Sensor Replacement or Rear Wheel Speed Sensor Replacement .

Did you complete the replacement?

--

Go to Step 12

--

9

Test the wheel speed sensor circuits for the following:

    • An open
    • A short to ground
    • A short to voltage
    • Shorted together

Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 12

Go to Step  10

10

Inspect for poor connections at the harness connector of the EBCM. 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 12

Go to Step 11

11

Important: Following EBCM replacement, perform the EBCM self-configuration procedure.

Replace the EBCM. Refer to Electronic Brake Control Module Replacement .

Did you complete the replacement?

--

Go to Step 12

--

12
  1. Perform the magnetic reset procedure. Refer to Status Lamp Resetting .
  2. Operate the vehicle within the Conditions for Running the Diagnostic Fault.

Do the same red LEDs illuminate?

--

Go to Step 3

Go to Step 13

13

Do any red LEDs illuminate?

--

Go to Diagnostic System Check - Air ABS

System OK