Hi all,

I thought I'd share the fruit of some of my nerdy efforts.

Below is a schematic of the daytime running lights and auto headlights system in my 1999 Blazer. I did this so I could find a way to perhaps install an override circuit so that I could drive with only my parking lights such as when driving the last short distance to my hunting spots. BTW, amateur astronomers also like to drive with only their parking lights so to avoid blinding others using their telescopes.

I thought that others might find what I've worked out useful. I had to look at several pages of schematics in my 1999 service manual to piece it all together. I also simplified some of the circuit; e.g, the parking lights and fog lights.

Here are a few things I learned:

• The DRL (daytime running lights) circuit has current running in series though the the hi-beam filaments. This means that if one of your hi-beam filaments goes out, you will not have DRLs.
• The DRL function is made possible by two relays that that in their default setting (not energized) allow current to run the same direction, in parallel, through the hi-beam filaments to the ground also used by the lo-beams. When energized, the relays redirect the current so that it runs in series through the hi-beam filaments to a dedicated DRL ground.
• The body control module (BCM) takes input from the ambient light sensor, the state of the parking and headlight switch, the High-Beam/Flash to Pass switch, the fog light switch, and the state of the park brake warning switch to determine what relays to energize. One of the relays (the headlight power relay) is energized by the BCM providing 12V to the coil and the remainder are controlled by providing a ground for the coil.

Hope you find this useful.

P.S. The pinouts for the BCM have been added. C2 or C3 refer to connector 2 or 3, respectively.

 

Can you run the headlight ground through a switch?