Daytime Running Lights have come to us by way of our neighbors to the north, Canada. They used the technology as a safety enhancement for many years prior to their migration south. The way GM goes about turning them on and off varies greatly from model to model and takes advantage of existing harnesses and switching as well as bulbs.

One of the more basic lighting routines used is on the Trailblazer Classic or Buick Park Ave where we take the high beams and through a network of resistors, change the circuitry from parallel to series, burning the high beams at half the intensity. Pretty darn clever huh? The way this is accomplished is by means of the BCM recognizing inputs such as ambient light and transmission gear position and controlling the relays accordingly. Not exactly new technology here, we do the same thing on some of our dual coolant fan set ups to go from low speed to high.

Confronted with this type of system and a customer concern of "Daytime Running Lights Inoperative", what would be a quick and easy way to diagnose the problem without even taking a tool out of your toolbox? Hmmm… How about turning the High Beams ON? Remember, the High Beams are supposed to be in series when the DRLs are activated, with only one path for electrons to flow, any break in the circuit will cause the whole system to go down. This can easily be caused by a blown out High Beam.

Still another DRL strategy that GM uses on the J body (Cavaliers, Sunfires) is to illuminate the low beam headlights at a reduced intensity. The DRL will operate when the ignition switch is in the RUN position, the gear selector is not in the PARK position and the parking brake is released. When these conditions have been met the DRL will illuminate.

The intensity is lowered by using an intentional drop in voltage on the ground side of the low beams. Here’s what we mean; When the criteria are met for DRL function, the BCM will ground the control side of the DRL relay and change the grounding path for the Low Beams. In this new ground path is a resistor of about three tenths of an ohm. Because we have put in this voltage drop, some intensity (brightness) will be lost.

Other systems like that on the T body trucks (Trailblazer, Bravada, Ranier, Envoy etc.) will use a Headlamp Driver Module or HDM.

The HDM is an electronic module that provides electrical power to the vehicle low beam headlamps. The HDM is controlled by a pulse width modulated (PWM) ground signal from the body control module (BCM). The BCM determines the desired lamp intensity using the ambient light sensor, multifunction switch and the park brake switch as inputs. Depending on input status there are 3 modes of operation the HDM will use: