Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsLighting Systems Diagnosis and RepairMedium
A vehicle's third brake light (center high mount stop light - CHMSL) is inoperative, but the left and right brake lights work correctly. The technician confirms there is no voltage at the CHMSL bulb socket when the brake pedal is pressed. What is the MOST likely cause?
- AA defective CHMSL bulb.
- BAn open circuit in the ground wire for the CHMSL.
- CAn open circuit in the power wire specifically for the CHMSL.
- DA faulty brake light switch.
Show answer & explanationAnswer & explanation
Correct answer: C. An open circuit in the power wire specifically for the CHMSL.
Since the left and right brake lights work, the brake light switch is functioning. The absence of voltage at the CHMSL socket, while the other brake lights are working, points to an open circuit in the power wire feeding only the CHMSL. A bad bulb would still show voltage at the socket.
Why the other options are wrong
- A. If the bulb were defective, voltage would still be present at the socket when the brakes are applied. The problem states no voltage is present.
- B. An open ground would prevent the light from working, but a voltmeter connected across the bulb socket (power to ground) would still read voltage if the power side is good. Since no voltage is detected, the power side is the issue.
- D. A faulty brake light switch would prevent ALL brake lights (left, right, and CHMSL) from working, which is not the case here.
CHMSL Power Circuit Fault
The CHMSL (third brake light) often has a dedicated power supply circuit, or a specific branch from the main brake light circuit. An open in this path can cause only the CHMSL to fail.
- Required on all passenger vehicles since 1986.
- Often powered separately or branched off main brake light circuit.
- Diagnosis involves checking voltage at the bulb socket and tracing the specific power wire.
Memory trick: Third light's gone? Power's not on!