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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 85 (ENGINE CONTROLS - SELF-DIAGNOSTICS - 2.2L)DIAGNOSTIC TESTSDTC P1574: STOP LAMP SWITCH CIRCUITDIAGNOSTIC PROCEDURES
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Procedures

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
WARNING: This page is about a different car, the 2002 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.
  1. Turn ignition on, engine off. Using scan tool, monitor BRAKE SWITCH and CRUISE BRAKE SWITCH while fully depressing and releasing brake pedal several times. If cruise brake switch follows brake pedal command, but brake switch does not, go to next step. If brake switch follows brake pedal command, but cruise brake switch does not, go to step  9 . If brake switches follow brake pedal command, go to step  8 .
  2. If BRAKE SWITCH always indicates RELEASED, go to next step. If BRAKE SWITCH always indicates APPLIED, go to step  7 .
  3. If any stop lamps work properly, go to step  6 . If no stop lamps work properly, go to next step.
  4. Check BRAKE fuse (15-amp). Replace and retest if open. If fuse continues to open, check for short to ground in Orange wire. If fuse continues to open when brake pedal is applied, check for a short to ground in Light Blue wire or a shorted stop lamp bulb. Disconnect stop lamp switch. Connect a test light between stop lamp switch connector terminal "A" (Orange wire) and ground. If test light is on, go to next step. If test light is off, repair open in Orange wire between underhood fuse block and stop lamp switch.
  5. Connect a jumper wire between stop lamp switch connector terminals "A" (Orange wire) and "B" (Light Blue wire). Using scan tool, monitor BRAKE SWITCH. If BRAKE SWITCH indicates APPLIED, check stop lamp switch adjustment. See appropriate CRUISE CONTROL SYSTEMS article in ACCESSORIES & EQUIPMENT. If adjustment is okay, replace stop lamp switch. If BRAKE SWITCH does not indicate APPLIED, repair open in Light Blue wire between stop lamp switch and splice.
  6. Using scan tool, monitor BRAKE SWITCH in TRANS DATA table while depressing and releasing brake pedal. If BRAKE SWITCH follows brake pedal command, replace ECM. If BRAKE SWITCH does not follow brake pedal command, repair open in Light Blue wire between underhood fuse block and splice.
  7. If any stop lamps operate properly, repair open in Light Blue wire between ECM and splice. If no stop lamps operate properly, check Light Blue wire for short to voltage. If wire is okay, check stop lamp switch adjustment. See appropriate CRUISE CONTROL SYSTEMS article in ACCESSORIES & EQUIPMENT. If adjustment is okay, replace stop lamp switch.
  8. Using scan tool, closely monitor BRAKE SWITCH and CRUISE BRAKE SWITCH while slowly depressing brake pedal to full applied position. If both switch states on scan tool change together as brake pedal is applied, problem is intermittent. See DIAGNOSTIC AIDS  . If both switch states on scan tool do not change together as brake pedal is applied, adjust brake switches. See appropriate CRUISE CONTROL SYSTEMS article in ACCESSORIES & EQUIPMENT.
  9. If CRUISE BRAKE SWITCH always indicates APPLIED, go to next step. If CRUISE BRAKE SWITCH always indicates RELEASED, go to step  12 .
  10. Check BCM/ECM/CRUISE/ABS fuse (10-amp). Replace and retest if open. If fuse continues to open, check for short to ground on Pink wire or Purple wire. Remove lower left instrument panel cover. Disconnect cruise brake switch. Connect test light between cruise brake switch connector terminal "A" (Pink wire) and ground. If test light is on, go to next step. If test light is off, repair open in Pink wire between instrument panel fuse block and cruise brake switch.
  11. Connect jumper between cruise brake switch connector terminals "A" (Pink wire) and "B" (Purple wire). Using scan tool, monitor CRUISE BRAKE SWITCH. If CRUISE BRAKE SWITCH indicates RELEASED, check cruise brake switch adjustment. See appropriate CRUISE CONTROL SYSTEMS article in ACCESSORIES & EQUIPMENT. If adjustment is okay, replace cruise brake switch. If CRUISE BRAKE SWITCH does not indicate RELEASED, check Purple wire between cruise brake switch and ECM for open. If wire is okay, replace ECM.
  12. Disconnect cruise brake switch. Using scan tool, monitor CRUISE BRAKE SWITCH. If CRUISE BRAKE SWITCH indicates APPLIED, check cruise brake switch adjustment. See appropriate CRUISE CONTROL SYSTEMS article in ACCESSORIES & EQUIPMENT. If adjustment is okay, replace cruise brake switch. If CRUISE BRAKE SWITCH does not indicate APPLIED, repair short to voltage in Purple wire.
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.