GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2005 🢒 TopKick C-Series (Crew) C6/C7/C8 🢒 Body 🢒 Lighting Systems 🢒 Stop Lamps Inoperative

Test Description

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

  1. Listen for an audible click when the center high mounted stop lamp (CHMSL) relay operates. Repeat as necessary.

    2.

  2. Tests for voltage at the control side of the CHMSL relay.

    3.

  3. Verifies the ground to the CHMSL relay.

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. Place the turn signal switch in the OFF position.
  2. Press down the brake pedal and release.

Do the stop lamps operate properly?

Go to Testing for Intermittent Conditions and Poor Connections

Go to Step 3

3

Do the turn signals operate properly?

Go to Step 4

Go to Turn Signal Lamps and/or Indicators Inoperative

4

Press down the brake pedal and release.

Does the center high mounted stop lamp (CHMSL) relay turn ON and OFF with the brake pedals position?

Go to Step 7

Go to Step 5

5

  1. Turn OFF the ignition.
  2. Disconnect the CHMSL relay. Refer to Relay Replacement in Wiring Systems.
  3. Turn ON the ignition, with the engine OFF.
  4. Probe the CHMSL relay control side stop lamp supply voltage circuit with a test lamp that is connected to a good ground.
  5. Press down the brake pedal.

Does the test lamp illuminate?

Go to Step 6

Go to Step 10

6

  1. Turn OFF the ignition.
  2. Connect a test lamp between the CHMSL relay control side supply voltage circuit and the CHMSL relay control side ground circuit.
  3. Turn ON the ignition, with the engine OFF.
  4. Press down the brake pedal and release.

Does the test lamp turn ON and OFF with the brake pedals position?

Go to Step 12

Go to Step 17

7

Probe the CHMSL relay switch side battery positive voltage circuit with a test lamp that is connected to a good ground.

Does the test lamp illuminate?

Go to Step 8

Go to Step 15

8

Connect a 15-amp fused jumper between the CHMSL relay switch side battery positive voltage circuit and the stop sw brake priority circuit.

Do both of the stop lamps illuminate?

Go to Step 12

Go to Step 9

9
  1. Turn OFF the ignition.
  2. Disconnect the turn signal/multifunction switch.
  3. Connect a test lamp between the stop sw brake priority circuit of the turn signal/multifunction switch and ground.
  4. Turn ON the ignition, with the engine OFF.

Does the test lamp illuminate?

Go to Step 14

Go to Step 16

10

Test the stop lamp switch supply voltage circuit for a high resistance, an open or a short to ground.

Did you find and correct the condition?

Go to Step 21

Go to Step 11

11

Test the battery positive voltage circuit of the stop lamp switch for the following:

    • An open
    • A high resistance
    • A short to ground

Did you find and correct the condition?

Go to Step 21

Go to Step 13

12

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

Did you find and correct the condition?

Go to Step 21

Go to Step 18

13

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

Did you find and correct the condition?

Go to Step 21

Go to Step 19

14

Inspect for poor connections at the harness connector of the turn signal/multifunction switch. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs.

Did you find and correct the condition?

Go to Step 21

Go to Step 20

15

Repair the open circuit in the CHMSL relay switch side battery positive voltage circuit. Refer to Circuit Testing and Wiring Repairs.

Did you complete the repair?

Go to Step 21

--

16

Repair the open circuit in the stop sw brake priority circuit. Refer to Circuit Testing and Wiring Repairs.

Did you complete the repair?

Go to Step 21

--

17

Repair the open or a short to battery positive voltage in the CHMSL relay control side ground circuit. Refer to Circuit Testing and Wiring Repairs.

Did you find and correct the condition?

Go to Step 21

--

18

Replace the CHMSL relay. Refer to Relay Replacement.

Did you complete the replacement?

Go to Step 21

--

19

Replace the stop lamp switch. Refer to Stop Lamp Switch Replacement.

Did you complete the replacement?

Go to Step 21

--

20

Replace the turn signal/multifunction switch. Refer to Turn Signal Multifunction Switch Replacement.

Did you complete the replacement?

Go to Step 21

--

21

Operate the system in order to verify the repair.

Did you correct the condition?

System OK

Go to Step 2