1999 GMC Envoy: replaced both GE 3057 lamps in left rear light assembly. Lamps both light. However depressing brake pedal does not initiate brake light. The righ rear brake light does work. The high mount brake light does not. Replaced fuses wit new fuses. Circuit tests all indicate no circuit problems, lose connections, shorts or ground loops. Is the hign mount brake light standard type lamp or is it LED type?

ST/LP, Fuse 12, 25 Amp - feeds battery positive to brake switch ORN (orange) and out the WHT (white).

Test with a simple automotive incandescent test lamp to ground. Test lamp should light on both sides of the fuse and with contact to the switch ORN and to WHT while brake depressed.

Stop Lamps
The brake pedal position sensor is used to sense the action of the driver application of the brake pedal. The body control module (BCM) provides a 5-volt reference voltage and low reference to the brake pedal position sensor. The position sensor provides an analog voltage signal back to the BCM that will increase as the brake pedal is applied. When the brake is applied, the BCM will then apply battery voltage to the left and right stop/turn signal lamp supply voltage circuits and the center high mounted stop lamp (CHMSL) supply voltage circuit illuminating the stop lamps. At the same time the BCM applies battery voltage through the signal circuit to the transmission control module (TCM) and the engine control module (ECM) indicating that the brakes are applied.
Ground for the stop lamps is supplied as follows:
• G402 provides ground for the left rear stop/tail/turn signal lamps
• G403 provides ground for the right rear stop/tail/turn signal lamps
• G301 provides ground for the CHMSL
The stop lamps on this vehicle will not illuminate unless the ignition is in the accessory, run, or crank positions. When the ignition is in the OFF position the stop lamps will not illuminate when the brake pedal is applied.
Battery voltage from the LT TRN/SIG and RT TRN/SIG fuses located in the rear fuse block is also used by the body control module (BCM) for stop lamp operation.

Turn Signal Lamps
Battery voltage supplied to the body control module (BCM) from the fuses listed below is used for turn signal and hazard lamp operation.
• LT TRN/SIG fuse for the left turn signals
• CTSY/RF TRN fuse for the right front turn signal
• RR TRN/SIG fuse for the right rear turn signal
When the turn signal switch is place in either the LEFT or RIGHT position, ground from G300 is applied through the turn signal switch signal circuit to the BCM indicating the turn signal request. In response to this signal, the BCM applies a pulsating voltage to the front and rear turn signal lamps supply voltage circuits cycling the lamps ON and OFF. At the same time the BCM sends a GMLAN serial data message to the instrument panel cluster (IPC) to cycle the appropriate turn signal indicator ON and OFF.
Ground for the turn signal lamps is supplied as follows:
• G104 provides ground for the left and right front turn signal lamps
• G402 provides ground for the left rear stop/tail/turn signal lamps
• G403 provides ground for the right rear stop/tail/turn signal lamps

Do you have a factory scan tool (tech2) ? Your best bet , take it to the dealer .

Kevin , this is a very complex way of stoplamp control , I will post GM service Description and Operation of how they work an let you deside if you want to tackle this ! Do the center high mount brake lights work ?
The brake pedal position sensor is a resistive device used to sense the action of the driver's application of the brake pedal. The brake pedal position sensor provides an analog signal that will increase as the brake pedal is pressed.
The brake pedal position sensor receives both a 5-volt reference voltage and a low reference signal from the electronic brake control module (EBCM). When the brake pedal is pressed, a variable signal is applied from the brake pedal position sensor to the EBCM through the brake sensor signal circuit. The fuse block left rear supplies battery positive voltage to the bass relay switch through the BASS fuse. One side of the bass relay coil is permanently grounded at G402. The EBCM supplies voltage to the bass relay coil upon receiving the signal from the brake pedal position sensor. The switch contacts of the bas relay close and battery positive voltage is applied through the bas relay switch to the center high mounted stop lamp (CHMSL), and to the turn signal/hazard flasher module through the stop lamp switch signal circuit. The ground for the CHMSL is applied from G402. The turn signal/hazard flasher module then supplies voltage to the left and right stop lamps in the tail lamp assemblies that have ground at G401 and G402. For export vehicles the stop lamps receive battery positive voltage directly from the bass relay.

There could be a code stored in the EBCM - electronic brake control module - DTC B3903 Stop Lamp Relay Circuit
DTC C0278 Brake Pedal Position Sensor Not Calibrated
there are several other codes that pertane to the brake lights . you could try Replaceing the bas relay !
Fuse Block - Left Rear
In the passenger compartment, under the left side of the rear seat at carpet.

Turn Signal Lamps
The LT T/SIG and RT T/SIG fuses located in the underhood fuse block supplies battery positive voltage to the body control module (BCM) for turn signal and hazard lamp operation. When the turn signal switch is place in either the LEFT or RIGHT position, ground from G202 is applied through the turn signal switch signal circuit to the BCM indicating the turn signal request. In response to this signal, the BCM applies a pulsating voltage to the front and rear turn signal lamps supply voltage circuits cycling the lamps ON and OFF. The BCM also sends a message via GMLAN to the instrument panel cluster (IPC) to cycle the turn signal indicator ON and OFF depending on the position of the turn signal switch.
Ground for the turn signal lamps is supplied as follows:
• G101 provides ground for the left front turn signal lamp
• G100 provides ground for the right front turn signal lamp
• G302 provides ground for the left and right rear turn signal lamps

The LT T/SIG and RT T/SIG fuses located in the underhood fuse block are also used by the body control module (BCM) for stop lamp operation.
The brake pedal position sensor is used to sense the action of the driver application of the brake pedal. The body control module (BCM) provides both, a 5-volt reference voltage and low reference to the brake pedal position sensor. The position sensor provides an analog voltage signal back to the BCM that will increase as the brake pedal is applied. When the brake pedal is applied, the BCM will then apply battery voltage to the left and right stop lamp supply voltage circuits and the center high mounted stop lamp (CHMSL) supply voltage circuit illuminating the stop lamps. At the same time the BCM signals the transmission control module (TCM) and the engine control module (ECM) that the brakes are applied. Ground for all stop lamps is provided at G302.
The stop lamps on this vehicle will not illuminate unless the ignition is in the accessory, run, or crank positions. When the ignition is in the OFF position the stop lamps will not illuminate when the brake pedal is applied.

Did you check the fuse's. ? Wouldn't think it's a ground because right an left tail lamp Assembly's use same ground ! Did you check for B+ voltage at the bulb. with a volt meter ? Power is supplied by the BCM . Could be the tail lamp assembly

First let's determine which brake lights are not working. Does the high mounted brake light work? Do both left and right brake light not work? If none of the brake light work, the problem could be the brake light switch at the brake pedal OR fuse #12 in the underhood fuse panel.
If only the bake lights at the corner lamp assembly don't work, check fuse #34 in the rear fuse panel.
The module your are referring to is the circuit board located in each of the corner lamp assemblies, check on website www.compnine.com for the part number of this item. I don't believe this is your problem though.

Hi,
check the brake light switch located in the brake pedal area. Normal procedure is to remove the two wires and connect a jumper' wire between them, check and see if the rear brake lights operate. Check the brake light switch and see if it is adjustable. If so, re-connect the removed wires, adjust the position of the switch untill the brake lights operate,when the brake pedal is depressed, then secure the brake light switch.

Rear Turn Signal, Brake & Parking Lights The taillamps, stoplamps and rear turn signals lamps are all incorporated into the rear taillamps. The taillamps are lit whenever the parking lamps or headlamps are in operation. The rear turn signal lamps flash with the front turn signal lamps whenever a turn is indicated with the turn signal/headlamp dimmer switch. The stoplamps light whenever the brake pedal is depressed and the stoplamp switch is closed.





The trim panel is likely held in place with retainers like this. Back out the screw to release tension on the pin legs and remove the fastener



The taillamp is retained by plastic wingnuts



Remove the plastic wing nuts from the taillamp housing



Remove the taillamp housing from the vehicle to service the light bulbs

1. Open the rear compartment (trunk).
   
2. If equipped, unhook the convenience net that stretches between the taillamps. It is attached to hooks molded into the plastic taillamp assembly retaining wingnuts. Loosen the rear compartment trim panel. It is retained by plastic pins with a screw in the center. Back out the screw to release any tension on the plastic pin legs, and pull the trim (thin carpet-like material) for access to the taillamps.
   
3. Remove the plastic taillamp assembly retaining wingnuts.
   
4. Remove the taillamp assembly from the vehicle and service the light bulbs as required.
   
5. Installation is the reverse of the removal process.
   
6. Test the lights for correct operation.
   

check the light they they can be on the wrongmay and check the light number i think there 3157 or 3057

Look up under dash where brake pedal pivots, you should see a switch that is activated by pedal arm.

The brake fuse #1 is located inside the truck below the steering column. I am assuming you have verified there is 12v power at the fuse & thru the brake lamp switch (orange wire power in & white wire power out, when pedal depressed) at all times. After the brake lamp switch , power goes (white wire) to the Turn signal/hazard lamp switch & out (dark green wire-rt brake lamp & yellow left brake lamp)