Small lamps. Big visibility job.
Position and tail lamps make the vehicle’s presence and outline more apparent in low light. They do not replace headlamps, stop lamps, or turn signals, and their exact operation varies with vehicle design and market.
By the end, you can…
Identify common position-related lamps, separate a shared-circuit failure from a single-branch failure, predict loaded voltage readings, and justify a diagnostic decision.
How to show it
Complete four lessons, solve five virtual missions, pass the knowledge check, and perform a supervised check on a vehicle or low-voltage trainer.
Parking, position, tail, marker: related, not identical.
Exterior lights are categorized by where they are fitted, what they communicate, and the control strategy used. A single lamp assembly can perform multiple functions, but that does not mean every function uses the same circuit or brightness.
Front position / parking
Helps other road users locate the vehicle’s front in reduced light. It often operates when a parking/position lamp command is active. A daytime running lamp (DRL) has a different purpose and may use different hardware or brightness.
Rear tail
Marks the vehicle’s rear presence. It is not the same as the brighter stop-lamp function, even when both functions share one housing or light source.
Side marker
Helps identify the vehicle’s side and length. Position, color, and activation details vary by region and vehicle.
License-plate lamp
Illuminates the rear registration plate. It commonly operates with exterior position/tail lighting but may have its own protected branch or module output.
A shared command can feed several independent branches.
In our simplified model, one 12 V source supplies a switch, a common protective device, and three parallel resistive loads: a front position lamp, a rear tail lamp, and a license-plate lamp. The three branches return through one small shared ground resistance.
TRAINING CIRCUIT · not an OEM schematic
The front load is modeled as 12 Ω; the rear and license-plate loads are each 24 Ω; healthy common return is 0.2 Ω. Each load has its own branch, so a single open load need not disable its neighbors.
Shared failure
If the common supply, switch, fuse, or shared return opens, multiple lamps can fail together. Use the diagram to identify components common to every failed lamp.
Branch-specific failure
If one lamp opens, that branch stops drawing current. The other branches may remain on. Voltage at an open lamp’s feed can still read near battery voltage.
“Lights ON” is a request, not always a direct connection.
Older systems may place a manual switch directly in the lamp supply or relay control circuit. Newer systems may send switch and ambient-light inputs to a body control module (BCM), which commands and monitors one or more lighting outputs. Automatic mode, ignition status, reduced-light conditions, and region-specific programming can affect operation.
Incandescent
A heated filament emits light; a combined tail/stop assembly can use separate filaments or bulbs. A cold filament’s resistance differs from its hot operating resistance.
LED / module-driven
An LED assembly may contain electronics to regulate current and control intensity. Simple resistance checks and fixed-resistor calculations do not describe every module-driven lamp.
Use symptoms to choose your first measurement.
Step 1 — Verify the complaint. Record ignition and switch mode, ambient conditions, and which lamps illuminate. Distinguish DRL, parking, tail, stop, and turn functions.
Step 2 — Establish the circuit. Read the correct wiring diagram and control description. Identify common supplies, module outputs, branch connections, splices, and returns.
Step 3 — Measure with the circuit loaded when appropriate. Compare battery voltage, the failed lamp feed to battery negative, the lamp return to battery negative, and voltage directly across the lamp. A voltage reading alone does not prove the branch can carry rated load current.
Step 4 — Isolate, correct, verify. Use readings to distinguish an open load, high-resistance feed or return, and a shared command issue. Follow the relevant repair procedure; then confirm every required lamp function.
Position-Light Diagnostic Bay
Command the lamps, select a fault, choose a branch, and place a virtual meter. Observe which lamps are on and compare the selected lamp’s feed, return, voltage, and current. Solve all five missions to finish the lab.
SHARED SWITCH → THREE PARALLEL BRANCHES → SHARED GROUND
The bulbs are a visual training aid. Voltage and current values are calculated from the simplified resistor network, not actual vehicle LED-driver behavior.
The Challenger’s five missions
Prove you know the circuit.
Answer eight questions. A score of 6/8 meets this unit’s knowledge-check target. Review explanations after submitting; your best score is saved locally.
Take it into the bay.
Complete these activities on an instructor-approved 12 V trainer or vehicle using the correct service procedure. These checkboxes are personal preparation notes, not instructor certification.