Diagnostic Instructions
| • | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. |
| • | Review Strategy Based Diagnosis for an overview of the diagnostic approach. |
| • | Diagnostic Procedure Instructions provides an overview of each diagnostic category. |
Diagnostic Fault Information
Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance |
|---|---|---|---|---|
STOP LAMP Fuse B+ | 1 | 1 | -- | -- |
VEH CHMSL Fuse B+ | 1 | 1 | 1 | -- |
VEH STOP Fuse B+ | 1 | 1 | 1 | -- |
Stop Lamp Switch Signal | 1 | 1 | 1 | -- |
Tail Lamp Circuit Board Ground | -- | 2 | -- | -- |
CHMSL Ground | -- | 2 | -- | -- |
1. Stop Lamp Switch Malfunction 2. Stop Lamp Malfunction | ||||
Circuit/System Description
The stop lamp switch is used to sense the action of the driver application of the brake pedal. The Turn Signal/Hazard Flasher Module provides a low reference signal to the stop lamp switch, and a B+ voltage is supplied by the STOP LAMP fuse. When the stop lamp switch contacts close indicating the brakes have been applied, battery voltage is applied to the VEH CHMSL fuse and the VEH STOP fuse, which then applies the voltage to the center high mounted stop lamp (CHMSL) and the Tail Lamp Circuit Boards illuminating both of the components.
Reference Information
Schematic Reference
Connector End View Reference
Description and Operation
Exterior Lighting Systems Description and Operation
Electrical Information Reference
Scan Tool Reference
Control Module References for scan tool information
Circuit/System Verification
Press and release the brake pedal while observing brake lamp operation.
| ⇒ | If all of the brake lamps are always on or inoperative, refer to the Stop Lamp Switch Malfunction. |
| ⇒ | If 1 or 2 of the brake lamps function normally, refer to the Stop Lamp Malfunction. |
Circuit/System Testing
Stop Lamp Switch Malfunction
- Ignition OFF, disconnect the harness connector at the stop lamp switch.
- Test for less than 5.0 ohms between the low reference circuit terminal A and ground.
- Verify that a test lap illuminates between the B+ circuit terminal B and ground.
- Install a 25A fused jumper wire between the stop lamp control circuit terminal A and the B+ circuit terminal B. Verify the stop lamps illuminate.
- If all circuits test normal, replace the stop lamp switch.
| ⇒ | If greater than the specified range, test the low reference circuit for an open/high resistance. |
| ⇒ | If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal, and the B+ circuit fuse is open, test the stop lamp control circuit terminal A for a short to ground. |
| ⇒ | If the stop lamps are not illuminated, test the stop lamp control circuit for a short to ground or an open/high resistance. |
Stop Lamp Malfunction
- Ignition OFF, disconnect the harness connector at the inoperative stop lamp.
- Test for less than 5 ohms between the listed ground circuit terminal and ground.
- Connect a test lamp between the listed stop lamp control circuit terminals below and ground circuit terminal.
- Press and release the brake pedal. The test lamp should turn ON and OFF when changing between the commanded states.
- If all circuits test normal, test or replace the inoperative stop lamp.
| • | Center High Mounted Stop Lamp terminal B |
| • | Tail Lamp Circuit Board terminal E |
| ⇒ | If greater than the specified range, test the ground circuit for an open/high resistance. |
| • | Center High Mounted Stop Lamp terminal A to B |
| • | Tail Lamp Circuit Board terminal D to E - Chevy, Isuzu |
| • | Tail Lamp Circuit Board terminal A to E - GMC |
| ⇒ | If the test lamp is always ON, test the control circuit for a short to voltage. |
| ⇒ | If the test lamp is always OFF, test the control circuit for a short to ground or an open/high resistance. |
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
