Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Daytime Running Lamps
Daytime running lamps (DRL) are controlled by the BCM based on input from the high beam headlamp circuit 1200, the low beam headlamp circuit 1201, the parking brake circuit 1134 and ignition switch state.
When DRL operation is desired, the body control module (BCM) switches circuit 592 to ground. This action energizes the DRL relay. When energized, the DRL relay operates high beam headlamps or low beam headlamps (later models) as a series circuit, one-half of battery voltage across each high beam headlamp.
When DRL operation is not desired, the BCM does not provide ground for the DRL relay. When not energized, as when high beam headlamps or low beam headlamps (later models) are desired, the DRL relay operates the high beam headlamps or low beam headlamps (later models) as a parallel circuit, full battery voltage across each headlamp.
For DRL operation, the parking brake must be fully released, the ignition switch must be in the RUN position and headlamp switch must be in the Off position. Under normal operating conditions, the feedback voltage to the BCM measured at circuit 1200 is (see table below):
| Headlamp | Circuit 1200 or Circuit 1201 (later models) Voltage |
|---|---|
| DRL ON | 6 to 8 Volts |
| High beam or low beam (later models) ON | Less than 1 Volt |
| High beam or low beam (later models) OFF | Battery Voltage |
NO RELATED
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.