Common Symptoms
- Check engine light illuminated
- Cruise control inoperative or will not set
- Transmission may not shift out of park without pressing brake pedal harder than normal
- Brake lights remain on constantly or do not illuminate when brake pedal is pressed
- Torque converter clutch does not disengage properly when braking
- Harsh downshifts or delayed upshifts
Most Likely Root Causes
- Faulty brake light switch or brake pedal position switch
- Damaged or corroded wiring harness to brake switch
- Misadjusted brake pedal switch position or mounting bracket
- Corroded or loose electrical connector at brake switch
- Failed powertrain control module brake switch input circuit
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Component Location
The brake light switch is typically mounted on a bracket near the top of the brake pedal arm under the dashboard on the driver's side.
Manufacturer Notes
Some vehicles use dual brake switches where Switch B provides input specifically for transmission and cruise control functions separate from brake light operation.
Also diagnosing J1939 SPN/FMI codes?
Heavy-duty trucks often throw both OBD-II and J1939 faults simultaneously. AI Diesel Tech handles both protocols in a single guided session.
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