One pedal. Several lamps. A safety-critical message.
Stop lamps tell following drivers that braking is commanded. A dark lamp, a lamp stuck on, or a lamp that is dim can each have a different cause. This unit teaches function first, then circuit testing.
By the end, you can…
Identify stop lamps and the center high-mounted stop lamp (CHMSL), distinguish a shared feed problem from a branch failure, and choose loaded measurements that isolate a fault.
How you show it
Finish four lessons, collect evidence in six virtual missions, pass the Challenger knowledge check, and complete supervised practical activities on approved equipment.
Which light means “I’m braking”?
Rear stop lamps communicate a braking command. Rear tail lamps identify the vehicle when exterior lighting is active; they are not the same function. Some lamp assemblies combine multiple functions and may use a dual-intensity bulb, multiple filaments, or independently controlled LEDs.
Outboard stop lamps
Usually illuminate when the braking system requests stop-lamp operation. Depending on vehicle design, the same rear lamp area can also support turn signaling; the wiring and control strategy vary.
CHMSL
The center high-mounted stop lamp adds a separate, elevated stop indication. It may have its own bulb, LED array, wiring branch, and return path.
Tail versus stop
A tail lamp may already be on before the driver touches the brake. Applying the brake requests a different stop-lamp function or intensity.
Actual behavior
Observe pedal released and applied, and any relevant ignition, transmission, or module conditions specified by service information. Never assume the lamp hardware tells you how its circuit is wired.
Trace the request from source to return.
In a simplified conventional circuit, a fused 12 V source feeds a normally open brake switch. Pressing the pedal closes the switch, applying voltage to three parallel lamp branches. Current passes through each lamp and returns to battery negative through the vehicle ground system.
Simplified teaching diagram — not a specific vehicle wiring diagram.
Shared feed failure
If the fuse is open or the simple switch fails open, all three branches lack the commanded supply.
One branch fails
An open left stop-lamp load can leave the right stop lamp and CHMSL operating normally. The left feed may still show nearly battery voltage despite zero left-branch current.
High resistance
Unwanted resistance in a loaded feed or return creates a voltage drop and reduces the lamp’s voltage. Measure across the suspect connection while the load is commanded on.
Parallel paths
In this model, each healthy lamp receives nearly the same applied voltage. A separate branch does not require the current from another branch to operate.
What does the pedal actually command?
A conventional brake switch may directly feed stop lamps or energize a relay. Many newer vehicles use a brake-pedal position sensor or switch as an input to one or more control modules. The module may command stop lamps using a protected transistor output or communicate with another module.
Conventional path
Battery → protection → pedal switch → lamp branches → ground. The switch may be normally open for the stop-lamp function, but verify the actual wiring.
Module-controlled path
Pedal input → control logic → protected lamp output(s). A test may require scan data, output commands, waveform checks, or manufacturer-defined functional tests.
Shared pedal input
The same brake request can also affect cruise control, shift interlock, stability systems, or regenerative braking logic. These functions do not necessarily share one circuit, and faults may affect them differently.
Combined stop/turn lamps
On some vehicles, a rear lamp performs both stop and turn signaling. A turn command can alter how that side illuminates while braking. Follow the vehicle-specific diagram instead of assuming separate rear stop and turn bulbs.
Use the symptom to choose the first test.
All stop lamps dark
Confirm the brake request. On a conventional system, compare voltage on both sides of the fuse and brake switch while the pedal is applied. On module-controlled systems, verify the appropriate input and output signals.
One stop lamp dark
Compare the dark branch’s feed and return against a working lamp. A feed reading alone does not prove a load or return can carry current.
Several lamps dim
Check voltage across the operating loads and measure their common feed and return drops while the brakes are applied. Inspect shared paths before replacing multiple lamps.
Lamps remain on
Verify whether a pedal request is stuck, the switch is incorrectly adjusted, or a module commands the output. Other vehicle-specific operating strategies may also apply.
Stop-Lamp Diagnostic Bay
Operate a simplified fused brake-switch circuit, select an instructor-supplied fault, place the virtual voltmeter, and compare the three stop lamps. Every mission requires a specific measurement, not just an identified fault name.
Pedal released · healthy reference
Six diagnostic missions
Set the requested condition, select the correct branch and test points, and inspect the reading before checking your setup.
0 of 6 missions complete
Prove you know the stop-lamp circuit.
Answer all eight questions. A score of 6/8 meets this unit’s knowledge-check target. Your best score is saved only on this device; you can review the explanations and retry.
Your best score: not attempted
Take it into the bay.
Use a service-information-approved procedure and an instructor-supervised vehicle or protected low-voltage trainer. These checkboxes are personal preparation notes, not instructor certification.
0 of 6 preparation tasks checked
Continue your lighting pathway.
Place this file next to the other Lighting Systems units when you assemble the complete subcourse. The unit itself works independently; previous/next files must be in the same folder for local links to work.