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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> External Pages >> Different car >> Section 208 (Automatic Transmission - ALLISON - Diagnosis) >> Diagnostic Tests >> DTC P0703: Brake Switch Circuit >> Diagnostic Procedure

Diagnostic Procedure

WARNING: This page is about a different car, the 2003 GMC Sierra, 2003 GMC Cab & Chassis Sierra, 2003 Chevrolet Silverado, and 2003 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
  1. Did you perform the diagnostic system check? If so, go to next step. If not, see DIAGNOSTIC SYSTEM CHECK  under SELF-DIAGNOSTIC SYSTEM.
  2. This DTC indicates that the TCM did not see the proper input signal for service brake status during an acceleration or deceleration cycle. This may indicate an open or short in the TCC brake switch/cruise control release circuit or a faulty stoplamp switch. Go to next step.
  3. Install the scan tool. Turn ignition on, engine off. Record the DTC failure record data. Apply and release the service brake. Does the scan tool indicate that the brake switch is toggling off and on? If so, see DIAGNOSTIC AIDS . If not, go to next step.
  4. Using DVOM, backprobe TCM connector C1 terminals No. 5 and 7. Apply and release the service brake. There should be no voltage reading at terminal No. 7 when the brake is applied. With the brake released, there should be 12 volts. Was voltage reading okay? If so, go to step  9. If not, go to next step.
  5. Using DVOM, probe stoplamp switch terminal "C" at the switch. Supply a ground connection at the other lead. Apply and release the service brake. Is the switch turning voltage on and off? If so, go to step  7. If not, go to next step.
  6. Measure voltage at stoplamp switch terminal "D". Is voltage available? If so, go to step  8. If not, see appropriate ANTI-LOCK article in BRAKES.
  7. Repair the TCC brake switch/cruise control wiring harness. Is the repair complete? If so, go to step  10.
  8. Replace the stoplamp switch. See STOPLAMP SWITCH  under REMOVAL & INSTALLATION. Is the repair or replacement complete? If so, go to step  10.
  9. Replace the TCM. See TRANSMISSION CONTROL MODULE  under REMOVAL & INSTALLATION. Is the replacement complete? If so, go to next step.
    CAUTION: In most cases, the TCM is not at fault. Investigate thoroughly before replacing the TCM.
  10. Perform the following procedure to verify the repair:
    • Clear the DTC.
    • Operate the vehicle under normal driving conditions.

    Did the DTC return? If so, go to step  1. If not, system is okay.

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