ASE G1 Auto Maintenance and Light RepairElectrical/Electronic SystemsHard
A vehicle's horn is inoperative. When the horn button is pressed, no sound is heard, and a test light connected to the horn's power wire at the horn itself illuminates brightly. A voltmeter connected between the horn's ground terminal and a known good chassis ground reads 12 volts when the horn button is pressed. What is the MOST likely cause?
- AAn open circuit in the horn's power supply wire.
- BA faulty horn unit (internal open).
- CAn open circuit in the horn's ground wire.
- DA faulty horn relay.
Show answer & explanationAnswer & explanation
Correct answer: C. An open circuit in the horn's ground wire.
If the test light illuminates brightly at the horn's power wire, power is reaching the horn. If the voltmeter reads 12V between the horn's ground terminal and a good chassis ground when the button is pressed, it means there is no good ground path from the horn through its ground wire to the chassis. The horn is receiving power but cannot complete its circuit to ground, thus it cannot operate. This indicates an open in the ground wire.
Why the other options are wrong
- A. If there was an open in the power wire, the test light at the horn would not illuminate.
- B. While a faulty horn unit is possible, the 12V reading at the ground terminal (with respect to chassis ground) specifically points to a lack of ground, not necessarily an internal open. If the horn had an internal open, it would still attempt to draw current and the ground path would ideally show 0V if it were good. The 12V reading is key here.
- D. If the horn relay were faulty and not engaging, power would not reach the horn, contradicting the test light illuminating.
Horn Circuit Diagnosis
Diagnosing an inoperative horn involves checking power, ground, and the horn unit itself.
- Power must reach the horn (checked with test light/voltmeter to chassis ground).
- Ground path must be complete (checked with voltmeter between horn ground and chassis ground, and for continuity).
- A 12V reading at a ground terminal (relative to chassis ground) indicates an open ground.
Memory trick: Power's there, ground's not where it should be.