GM daytime running lights don’t work

Diagnose and fix GM daytime running lights

GM uses a few different daytime running light schematics, but most use a series wiring method that divides battery voltage in half through series wiring. Here’s how that GM daytime running lights system works.

How GM daytime running lights work

Thirty seconds after you start your vehicle with the headlights off, the body control module (BCM) checks the voltage on the ambient light sensor. For the DRL system to work, the IGN must be in the RUN position, the parking brake must not be activated and the transmission must not be in PARK. If those conditions are met, and the BCM determines there’s daylight, it provides ground to the control coil on the DRL relay. The DRL relay contacts close which provides battery power from the EXT LTS fuse through the right HIGH beam headlight and then through the left HIGH beam headlight before reaching ground. By routing the power in series through both HIGH beam headlights, each light burns at half brightness. GM and many other car makers choose the HIGH beam headlights as the DRL source to avoid burning out the filaments in the low beams.

NOTE: Both HIGH beam bulbs must have good filaments for the DRL system to work

Wiring diagram daytime running lights

Test daytime running light circuit

Check the EXT LTS fuse. With a friend in the vehicle, start the engine during daylight hours with parking brake OFF and transmission in drive. Remove the connector from the right high beam bulb and check for battery voltage on the orange wire. If you see battery voltage, you’ve confirmed the fuse is good and the wiring to the right beam is good. Next, backprobe the bulb connector on the left high beam (do not remove the connector). Check for voltage on the pink wire. If you see about 6 volts, the system is operating so far. If you see battery voltage, suspect an open in the dark blue wire, a faulty DRL relay or a bad ground connection.

Diagnose the DRL relay

With a friend in the vehicle, start the engine during

19116058 GM DRL relay

daylight hours with parking brake OFF and transmission in drive, remove the DRL relay from the socket and use your voltmeter to check for battery voltage on one terminal in the relay socket and partial voltage in another terminal. Then check for good ground on the other two terminals.

If you don’t see good ground on the DRL relay control coil, suspect a bad ambient light sensor or bad body control module.

©, 2017 Rick Muscoplat