Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsLighting Systems Diagnosis and RepairHard
A vehicle with an integrated turn signal/parking light bulb (1157 type) on the front right side exhibits the following: the parking light works, but the turn signal does not flash (it remains off). When the hazard lights are activated, the right front bulb also remains off. The left side functions normally. Which of the following is the most likely cause?
- AA corroded ground connection for the right front bulb
- BAn open circuit in the turn signal power wire to the right front bulb
- CA faulty turn signal flasher relay
- DA burnt-out parking light filament in the right front bulb
Show answer & explanationAnswer & explanation
Correct answer: B. An open circuit in the turn signal power wire to the right front bulb
The 1157 bulb has two filaments and two power contacts, sharing a common ground. Since the parking light filament works, power is reaching one contact and the ground is good. The fact that the turn signal filament does not illuminate at all (even with hazards) indicates that power is not reaching the turn signal contact for that specific bulb, pointing to an open circuit in the turn signal's dedicated power wire to that bulb.
Why the other options are wrong
- A. If the ground connection were corroded, both the parking light and the turn signal (which share that ground) would be affected, likely causing dimness or no operation for both, which contradicts the working parking light.
- C. A faulty flasher relay would affect both turn signals or cause a rapid flash, not just one side remaining off while the parking light works.
- D. The question states the parking light works, so its filament is not burnt out.
Dual-Filament Bulb Wiring
Bulbs like the 1157 have two separate power inputs for two filaments (e.g., parking light and turn signal) but typically share a single ground connection.
- Each filament has its own power circuit.
- Common ground for both filaments.
- Allows for independent control and multiple functions from one bulb.
Memory trick: Dual power, single ground, fault unwound!