Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsLighting Systems Diagnosis and RepairHard
A vehicle's high-beam indicator light on the dashboard illuminates, but the high-beam headlights themselves do not turn on. The low-beam headlights function normally. The technician verifies that the high-beam fuse is good and there is battery voltage present at the high-beam relay's control side (pin 86). What should the technician check NEXT?
- AGround circuit for the high-beam relay's control side (pin 85).
- BVoltage at the high-beam relay's load side (pin 30).
- CContinuity of the high-beam bulbs' filaments.
- DVoltage at the high-beam switch output.
Show answer & explanationAnswer & explanation
Correct answer: A. Ground circuit for the high-beam relay's control side (pin 85).
Since the high-beam indicator light works and there's voltage at pin 86 (control side power), the relay's control circuit is receiving power. For the relay to activate, its control coil needs a complete circuit, which means it also needs a good ground at pin 85. Checking this ground is the next logical step to determine why the relay isn't clicking or switching power.
Why the other options are wrong
- B. Pin 30 (load side power input) should have constant battery voltage. While important, the problem lies in the relay activation (control side), not its power supply. The indicator working suggests the switch and fuse are good, so the control side is the focus.
- C. If the bulbs were bad, the relay would still activate (click) and send power, which could be verified by checking voltage at the bulb socket. This check is premature as the relay's function is in question.
- D. The high-beam indicator light illuminates, which typically means the high-beam switch has successfully sent a signal (power or ground) to activate the high-beam circuit, including the relay control. Checking the switch output directly is less efficient than checking the relay's ground, as the indicator already confirms the switch is at least partially functioning for the control circuit.
Relay Control Circuit Diagnosis
Diagnosing a relay control circuit involves verifying power and ground inputs to the relay's control coil (pins 86 and 85, or similar) to ensure the relay is receiving the signal to activate.
- Relays have two circuits: control (low current) and load (high current).
- Control circuit needs power and ground to energize the coil.
- If the control circuit is complete, the relay 'clicks' and switches the load.
Memory trick: Relay's stuck? Check its ground luck!