GM Service Manual Online
For 1990-2009 cars only

Backup Lamps Always On RPO M74, MX4, and MTW

Test Description

The number below refers to the step number on the diagnostic table.

  1. Listen for an audible click when the reverse relay operates. Repeat as necessary.

Step

Action

Yes

No

Schematic Reference: Exterior Lights Schematics

Connector End View Reference: Master Electrical Component List

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

  1. Block the wheels and set the parking brake.
  2. Turn ON the ignition, with the engine OFF.
  3. Place the gear selector in and out of the reverse position.

Do the backup lamps operate properly?

Go to Testing for Intermittent Conditions and Poor Connections

Go to Step 3

3

  1. Turn ON the ignition, with the engine OFF.
  2. Move the gear selector in and out of the reverse gear position.

Does the reverse lamp relay turn ON and OFF with the gear selector position?

Go to Step 4

Go to Step 5

4

  1. Turn OFF the ignition.
  2. Disconnect the reverse relay. Refer to Relay Replacement .
  3. Turn ON the ignition, with the engine OFF.

Do the backup lamps turn OFF?

Go to Step 5

Go to Step 7

5

  1. Turn OFF the ignition.
  2. Connect a test lamp between the reverse relay control side supply voltage circuit and the reverse lamp relay control circuit.
  3. Turn ON the ignition, with the engine OFF.
  4. Move the gear selector in and out of reverse gear position.

Does the test lamp turn ON and OFF in with the gear selector position?

Go to Step 8

Go to Step 6

6

Test the reverse lamp relay control circuit for a short ground. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 12

Go to Step 9

7

Test the backup lamp supply voltage circuit for a short to battery positive voltage. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 12

Go to Step 8

8

Inspect for poor connections at the reverse lamp relay. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 12

Go to Step 10

9

Inspect for poor connections at the harness connector of the transmission control module (TCM). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 12

Go to Step 11

10

Replace the reverse relay. Refer to Relay Replacement .

Did you complete the replacement?

Go to Step 12

--

11

Replace the TCM. Refer to Control Module References for replacement, setup, and programming.

Did you complete the replacement?

Go to Step 12

--

12

Operate the system in order to verify the repair.

Did you correct the condition?

System OK

Go to Step 2

Backup Lamps Always On RPO MWF, MWW, and MWX

Step

Action

Yes

No

Schematic Reference: Exterior Lights Schematics

Connector End View Reference: Master Electrical Component List

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

  1. Block the wheels and set the parking brake.
  2. Turn ON the ignition, with the engine OFF.
  3. Place the gear selector in and out of the reverse position.

Do the backup lamps operate properly?

Go to Testing for Intermittent Conditions and Poor Connections

Go to Step 3

3

  1. Turn OFF the ignition switch.
  2. Disconnect the backup lamp switch or the neutral start backup switch.
  3. Turn ON the ignition switch, with the engine OFF.

Are the backup lamps illuminated?

Go to Step 5

Go to Step 4

4

Inspect for poor connections at the harness connector of the backup lamp switch. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 7

Go to Step 6

5

Repair the short to battery positive voltage in the backup lamp supply voltage circuit. Refer to Circuit Testing and Wiring Repairs .

Did you complete the repair?

Go to Step 7

--

6

Replace the inoperative backup lamp switch. Refer to Park/Neutral Position and Backup Lamp Switch Replacement or Backup Lamp Switch Replacement .

Did you complete the replacement?

Go to Step 7

--

7

Operate the system in order to verify the repair.

Did you correct the condition?

System OK

Go to Step 3