GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 GMC C-Series Medium Duty Truck B7 🢒 Brakes 🢒 Air Antilock Brake System 🢒 Green VLT Status Lamp Inoperative

Circuit Description

The green VLT LED remains illuminated when the ignition is ON. The VLT LED flashes when system voltage is 9-11 volts or greater than 17 volts. The VLT LED turns OFF when voltage to the EBCM is less than 9  volts.

Conditions for Running the Diagnostic Fault

The ignition is ON.

Conditions for Setting the Diagnostic Fault

The EBCM detects low or high system voltage.

Action Taken When the Diagnostic Fault Sets
    • The ABS indicator turns ON.
    • The ABS is disabled.

Conditions for Clearing the Diagnostic Fault

Repair the condition responsible for setting the diagnostic fault.

Diagnostic Aids

Thoroughly inspect connections or circuitry that may cause an intermittent malfunction. Refer to the following:

    •  Testing for Electrical Intermittents
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs
    •  Connector Repairs

Test Description

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

  1. The green VLT LED flashes if the system voltage is 9-11 volts or greater than 17 volts.

    2.

  2. This step tests for adequate power to the EBCM. Voltage of 9-11 volts may cause the green VLT LED to flash.

    3.

  3. This step tests for adequate EBCM ground.

    4.

  4. Total lack of voltage on all of the ignition 3 voltage circuits may be due to a malfunction in the ABS relay circuitry.

Step

Action

Yes

No

Schematic Reference: Air Antilock Brake System Schematics

1

Did you perform the ABS Diagnostic System Check?

Go to Step 2

Go to Diagnostic System Check - Air ABS

2

Turn ON the ignition.

Does the green VLT LED remain ON steady?

Go to Diagnostic Aids

Go to Step 3

3

Ensure the charging system is operating normally. Refer to Charging System Test and Symptoms - Engine Electrical in Engine Electrical.

Did you find and correct the condition?

Go to Step 17

Go to Step 4

4

  1. Turn OFF the ignition.
  2. Disconnect from EBCM, the harness connector containing the ignition 3 voltage circuits and the ground circuits of the EBCM.
  3. Turn ON the ignition.
  4. Use a test lamp connected to a good ground in order to probe all 3 of the ignition 3 voltage circuits at the EBCM harness connector, one at a time.

Does the test lamp illuminate when each ignition 3 voltage circuit is probed?

Go to Step 6

Go to Step 5

5

Test the ignition 3 voltage circuits that did not illuminate the test lamp for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 17

Go to Step 7

6

Use a test lamp connected to battery positive voltage in order to probe all 3 of the ground circuits at the EBCM harness connector, one at a time.

Does the test lamp illuminate when each ground circuit is probed?

Go to Step 13

Go to Step 12

7

  1. Turn OFF the ignition.
  2. Disconnect the ABS relay harness connector.
  3. Turn ON the ignition.
  4. Connect a test lamp between the battery positive voltage circuit of the ABS relay and a good ground.

Does the test lamp illuminate?

Go to Step 8

Go to Step 10

8

At the harness connector of the ABS relay, connect a test lamp between the ignition 3 voltage circuit , which supplies voltage to the ABS relay coil, and a good ground.

Does the test lamp illuminate?

Go to Step 9

Go to Step 11

9

Connect a test lamp between battery positive voltage and the ground circuit of the ABS relay coil.

Does the test lamp illuminate?

Go to Step 15

Go to Step 12

10

Repair the open in the battery positive voltage circuit. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 17

--

11

Repair the open in the ignition 3 voltage circuit. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 17

--

12

Repair the open in the ground circuits that did not illuminate the test lamp. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair(s)?

Go to Step 17

--

13

Inspect for poor connections at the harness connectors 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 17

Go to Step 14

14

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 17

--

15

Inspect for poor connections at the harness connectors of the ABS relay. 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 17

Go to Step 16

16

Replace the ABS relay.

Did you complete the replacement?

Go to Step 17

--

17
  1. Perform the magnetic reset procedure. Refer to Status Lamp Resetting .
  2. Operate the vehicle within the Conditions for Running the Diagnostic Fault.

Does the green VLT LED remain ON steady?

System OK

Go to Step 3