Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisMedium
A technician is diagnosing a vehicle where the horn does not work. The fuse for the horn circuit is good, and the horn relay clicks when the horn button is pressed. Using a digital multimeter, the technician measures 12.5V at the output terminal of the horn relay when the horn button is pressed, but the horn itself does not sound. Which of the following is the MOST likely cause of the inoperative horn?
- AA faulty horn switch.
- BAn open circuit between the horn relay output and the horn.
- CA short to ground in the horn circuit.
- DA faulty horn relay.
Show answer & explanationAnswer & explanation
Correct answer: B. An open circuit between the horn relay output and the horn.
Since the fuse is good, the relay clicks, and 12.5V is measured at the relay's output, it means power is correctly leaving the relay. If the horn still doesn't sound, the most likely issue is an open circuit (break in the wire) between the relay and the horn, or a faulty horn itself, preventing power from reaching or activating it.
Why the other options are wrong
- A. The horn switch is working, as the relay clicks when pressed.
- C. A short to ground would likely blow the fuse or prevent the relay from clicking, which is not the case here.
- D. The relay is working, as it clicks and outputs 12.5V.
Open Circuit Diagnosis
Locating a break in an electrical path that prevents current flow, often diagnosed by verifying voltage at one point but not at a subsequent point in the circuit.
- No current flow occurs in an open circuit.
- Voltage may be present up to the point of the open.
- Can be caused by broken wires, loose connections, or failed components.
Memory trick: Relay's Power, Horn's Silence: An Open Road to the Horn.