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DRL Flashing Between Off, Low, and High Beam on My Peterbilt

The DRL Input Wire (4th Wire) Is the Most Likely Cause Here Is How to Address It

If your headlights are flashing or switching between modes on their own when the daytime running lights kick in, there is a good chance the fix is simpler than you think.

The Likely Cause: The 4th Red Wire

Many JW Speaker commercial headlights include an extra red wire on the harness — separate from the standard low beam and high beam wires. This wire is for the DRL (Daytime Running Light) or Front Position function.

If this wire gets connected to the wrong circuit on your Peterbilt — or connected to a circuit that sends a signal the headlight's electronics weren't designed to handle — the light can start cycling or flashing between modes unpredictably. It's confusing to look at, but it's a wiring issue, not a failed light.

The Fix Most People Need: Just Cap That Wire

If you connected the 4th red wire and that's when the problem started, here is what to do:

  1. Disconnect the 4th red wire from whatever circuit it's connected to
  2. Cap the end of the wire with a connector cap
  3. Test the headlights

That's usually all it takes. Your low beam, high beam, and all standard functions will work fine without the 4th wire connected. On most commercial trucks, the DRL function runs through the low beam circuit anyway — you don't need the 4th wire to have working daytime running lights.

Still Cycling After Capping the Wire?

If the problem continues after disconnecting the 4th wire, check two things:

  • Are you using the right product for your truck? Confirm the JW Speaker headlight and harness you have are the correct fit for your specific Peterbilt model and year.
  • Does your truck use a PWM DRL signal? Some trucks output a PWM (pulsing) DRL signal that can confuse the LED driver. This is less common but worth flagging when you contact support.

If neither of those resolves it, reach out and we can work through it.

Have your truck year, model, headlight part number, and a description of how the wiring is currently set up ready when you call or write — it helps us get to the answer faster.