Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsLighting Systems Diagnosis and RepairHard
A vehicle's daytime running lights (DRLs) are inoperative, but the high beams, low beams, and parking lights all function correctly. The DRL module is integrated into the vehicle's Body Control Module (BCM). A scan tool shows no diagnostic trouble codes (DTCs) related to the DRL system. When the DRLs should be on, a voltage reading at the DRL bulb socket shows 0V. What is the MOST likely cause?
- AAn internal fault within the BCM specifically affecting the DRL output circuit.
- BA faulty DRL bulb.
- CAn open circuit in the ground wire for the DRLs.
- DA faulty DRL relay.
Show answer & explanationAnswer & explanation
Correct answer: A. An internal fault within the BCM specifically affecting the DRL output circuit.
Since the BCM controls the DRLs and there's 0V at the bulb socket when DRLs should be on, power is not being supplied. Given that other headlight functions work, and there are no DTCs (which might indicate a sensor or input issue to the BCM), an internal fault within the BCM's DRL output circuit is the most plausible explanation for the BCM failing to provide power.
Why the other options are wrong
- B. If the DRL bulb was faulty, there would still be voltage present at the bulb socket when the DRLs should be on.
- C. If the ground wire was open, a voltmeter connected from the power wire at the bulb socket to chassis ground *would* show voltage. Since 0V is measured, the power feed is missing.
- D. Many modern DRL systems are directly controlled by the BCM without a separate DRL relay. Even if there was one, the BCM would control it, and the lack of voltage at the bulb suggests the problem is upstream of the bulb, likely the BCM's output.
BCM Output Circuit Fault
An internal fault within a Body Control Module (BCM) can cause specific output circuits to fail (e.g., DRLs, interior lights) even if other functions and inputs are working, and DTCs may not always be present.
- BCMs control many vehicle electrical functions.
- Internal component failure (e.g., transistor, driver) can affect single circuits.
- Lack of DTCs can make BCM diagnosis challenging; requires checking outputs directly.
Memory trick: BCM's quiet, DRLs riot!