Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisMedium
A technician is diagnosing a vehicle with an intermittent issue where the horn occasionally does not sound. The horn fuse is good, and the horn relay clicks when the horn button is pressed. When testing the horn circuit directly at the horn, the technician measures 10.5 volts across the horn terminals when activated. The battery voltage is 12.6 volts. What is the most likely cause of the intermittent horn operation?
- AExcessive resistance in the horn circuit power supply or ground
- BA faulty horn relay coil
- CA short to ground in the horn wiring
- DA faulty horn switch
Show answer & explanationAnswer & explanation
Correct answer: A. Excessive resistance in the horn circuit power supply or ground
A voltage drop of 2.1 volts (12.6V - 10.5V) indicates excessive resistance in the horn's power or ground circuit, preventing the horn from receiving sufficient voltage to operate reliably. The horn relay clicking suggests the switch and relay coil are functional.
Why the other options are wrong
- B. The horn relay clicks, suggesting its coil is energizing properly.
- C. A short to ground would likely blow the fuse or cause continuous horn operation, neither of which is indicated.
- D. The horn relay clicks, indicating the switch is likely sending a signal to activate the relay.
Voltage Drop Diagnosis
Measuring the voltage difference across a component or circuit segment to identify excessive resistance, which can cause components to operate poorly or not at all.
- Indicates resistance in the circuit path
- Measured with a multimeter while the circuit is active (under load)
- Excessive drop (typically > 0.2V for power/ground, > 0.5V for entire circuit) indicates a problem
Memory trick: Resistance ROBS voltage from the component.