One thing that has perplexed many drivers over the years is why, when activating your high beams on your vehicle, the low beam shuts off in the process. Wouldn’t it make sense if both beams remained active, if just for the sake of lending some extra lighting to illuminate the road and your surroundings? There are a couple of reasons why vehicles were designed this way, but it boils down to either the type of bulb used or the electrical setup flipping between low and high beams but not powering both simultaneously.
Believe it or not, there’s a pretty easy way to modify your pickup to keep the low beams and fog lights active when using high beams. Here, we’ll go over an “all lights mod” where fog lights stay on with high beams and all lights, and outline how to make low beams stay on with high beams on your Chevy Silverado, as well as other GMC pickups. All it takes is a diode, some basic tools, and a modest comfort level with basic wiring skills.
Why Does My Truck Shut Off the Low Beams Anyway, and What is a Diode?
As we touched on above, most vehicles alternate between high and low beams because of either bulb limitations (single filament, vs dual filament), or because of how the headlight relay is designed. Since dual filament bulbs would mean only one headlight bulb is present, using both filaments would cause the bulb to overheat and burn out rapidly. Bad news for those folks that own vehicles with one headlight bulb: this mod might not work for you.
For vehicles with more than one headlight bulb, the idea is to have low beams illuminate the road in one set of circumstances when speeds are reduced and the area has ambient lighting; high beams kick in when there is less lighting around, and the vehicle is traveling at a speed where low beams are not bringing potential hazards into focus quickly enough. (Some old-time drivers would call this “out-driving your headlights,” and it’s discouraged.) There’s also the tricky notion of traffic laws, and in many jurisdictions in the United States, it’s not legal to drive around with more than four lights illuminated on the front of the vehicle.
For those cars and trucks that have multiple headlight bulbs, the easiest way to modify the wiring to allow all the headlights to remain on by essentially “deactivating” the requirement of the vehicle to disable low beam and fog lamps when the high beam headlight is selected. For most Chevy Silverado and GMC pickups, when you have either the low beam or daytime running lights active and then flip over to use the high beams, the other bulbs are deactivated via an electrical relay. This modification will trick the vehicle into keeping the bulbs active.
First, you’ll need a set of diodes; you can either find these pretty cheaply online, or some local hardware or electronic stores may carry them. Look for part numbers starting with “1N” followed by 4001, 4002, 4003, and 4004; most of these should work for vehicle use. Additionally, having a set of relay pliers will help spare your fingers from a bit of soreness, and make the job much easier.
To add some context to the modification steps below on what a diode is: you will essentially be connecting a wire (diode) that will help channel power from your low beam relay, over to the high beam and fog light relays, in order for them to remain powered and active when using the high beams. The diode will allow voltage to travel from the low beam relay to the high beam or fog lamp relay, but voltage is not intended to travel back upstream on the diode and back into the low beam relay.
The following directions were created according to a Chevy Silverado pickup; you will need to reference a wiring diagram for your own vehicle to proceed from this point. Complete the following steps with the vehicle turned off.
How to Set Up the All-Lights Mod:
Identify where the high beam relay, low beam relay, and fog light relay are located – this can either be in the fuse box under the hood on the driver’s side, or in the BCM near the driver’s side footwell, just under the dash. You’ll need to know which harness or relay contains the wiring for the high beam and fog lamps, and/or the daytime running lamps. The diode will eventually be plugged in and run between these two relays, and they are typically very close together.
Look at your diode; there should be a triangle shape with a line or stripe on one side, and this line is called a bottle cap. One longer arm of the diode is most likely a brighter silver color, with the brighter side being closest to the bottle cap or stripe on the diode. The brighter arm that branches off of the side with the stripe will be the end that plugs into the high beam relay. You can wrap the diode around the pin, or just push it in.
Place the diode arm that is darker and further from the stripe, into the relay for the low beam headlight. Place the other, brighter end of the diode arm (this should be the side that is closest to the stripe or bottle cap) into the high beam relay. Same as before, you can wrap the diode around the pin, or just squeeze it into the relay.
If done correctly, this will enable your low-beam headlights to stay lit while using your high-beam headlights. If you have fog lamps installed and wish for them to stay on when the high beams are on, you will repeat this process, but instead will stick the dark side of the diode into the relay for the fog light. The brighter, bottle cap side will again plug into the high beam relay. If you are running both low beam and fog lamp diodes, the space might get crowded around the relay; don’t be shocked if you have to really work to get the second diode to fit.
If you are successful with this mod, it will help with nighttime visibility when driving your Chevy Silverado, or other GMC truck. Having an All-Lights mod, and having both low-beam and high beam headlights active can be helpful in extremely low visibility situations where the extra candle power is appreciated.