A vehicle's high-mount brake light (CHMSL) is inoperative, but the left and right brake lights are functioning correctly. The technician measures 0V at the power wire going into the CHMSL assembly when the brake pedal is pressed. What should the technician check NEXT?
- AThe ground circuit for the CHMSL.
- BThe brake light switch.
- CThe wiring between the brake light switch and the CHMSL.
- DThe CHMSL bulb filament.
Show answer & explanationAnswer & explanation
Correct answer: C. The wiring between the brake light switch and the CHMSL.
Since the left and right brake lights are working, the brake light switch and its power input are functioning correctly. The measurement of 0V at the CHMSL power wire when the brake pedal is pressed indicates that power is not reaching the CHMSL. This means there is an open circuit in the power wire path *after* the point where the left/right brake light circuits branch off, but *before* the CHMSL. Therefore, the next logical step is to check the wiring between the brake light switch (or the point where the CHMSL circuit branches off) and the CHMSL assembly.
Why the other options are wrong
- A. Checking the ground circuit for the CHMSL would be premature. There is no power reaching the light, so even a perfect ground wouldn't make it work. Ground is typically checked after confirming power.
- B. The brake light switch is working because the left and right brake lights illuminate. If the switch was faulty, none of the brake lights would work.
- D. Checking the bulb filament is premature. If 0V is present at the power wire to the assembly, the bulb cannot illuminate, regardless of its condition. Power must be restored first.
CHMSL Power Path Diagnosis
If the primary brake lights function but the CHMSL does not, and no power reaches the CHMSL, the fault lies in the dedicated power wiring or any specific components (like relays or fuses) in the CHMSL's circuit *after* the main brake light switch output.
- Primary brake lights work, CHMSL doesn't.
- No power at CHMSL assembly.
- Fault is in the CHMSL's unique power path.
- Wiring is a common culprit.
Memory trick: Main brake lights are on, but the high one's off. Find the broken wire that only feeds the high light.