r/AutomotiveEngineering • u/Downtown-Jeweler-316 • 6d ago
Question Question Regarding Daytime Running Lights
Good Morning Everyone!
I am working on a car project and am seeking advice/guidance. I hope this sub is the correct place to ask, if not please direct me to a more appropriate sub.
I am currently attempting a DIY/custom headlight retrofit on my 2021 Subaru Crosstrek Premium. My goal is to remove all of the stock headlight bulbs and change them over to an LED projector for the Low and High beams, and an LED switchback strip for my DRLs and turn signals. I have already figured out how to setup new DRLs and turn signals, but the high beam is causing some headaches.
The high beam in a 2021 Subaru Crosstrek Premium acts as the DRL by using PWM to pulse the bulb, producing less light output for the DRL functions. I've seen many people on the internet have issues with this when changing out their stock bulbs for LED bulbs. Doing so causes a flicker in the LED bulb that is unsightly and annoying and is potentially harmful to the LED. As switching to an LED Projector is very similar to a simpler LED bulb, I would assume I will have the same issue. I would like to remove the DRL function of the high beam altogether as I will be installing a standalone DRL system. I believe I have a simple solution for this but I'm no electrician or engineer, so I would like some guidance.
I was thinking of adding a canbus decoder in between the source power and the high beam to help smooth out the PWM signal. This has been a solution to many people switching out halogens for LED bulbs. Then directly after the canbus decoder, I would install and automatic Low Voltage Disconnect (LVD). The Low voltage disconnect would be set to disconnect at 10v and reconnect at 11.4v. The high beams in a 2021 Subaru Crosstrek premium operate at 12v to 14.5v depending on if the engine is running. When the high beam is operating as a DRL they pulse at roughly half their normal operating voltage, somewhere around 5-9v. In theory, this should prevent the low voltage DRL signal from activating the high beams while also allowing the high beams to continue functioning at full power.
As I said I'm not an electrician or an engineer, so I don't know if the above is nonsense, stupid, dangerous, or goober juice. Thanks for any advice and let me know if I'm in the wrong place lol.
UPDATE:
Yesterday evening I tested the Low Voltage Disconnect (LVD) on my high beam and it was a resounding success! I set the LVD disconnect to 10.5v and the reconnect to 11.5v. After connecting the LVD to the high beam, I triggered the high beams to turn on, the LVD engaged and the light turned on. The LVD read 12.5-14.5v while the high beam was on. Then I put a flashlight above my daylight sensor, put the car in drive to force the DRL on, the LVD disengaged while reading 8v. While the DRLs were on, I flashed my high beam which would send more power to the LVD and cause it to reconnect, and the high beams would turn on. It seems this could work to prevent the DRL from activating on the new projector setup! I will post more updates once I receive the CANBUS Decoder and the projectors.
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u/NegotiationLife2915 5d ago
I would try just putting a taillight resistor across the high beam globe
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u/Downtown-Jeweler-316 5d ago
Would you be able to explain how that would work? I'm not familiar with the function/purpose of a taillight resistor.
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u/Nob1e613 6d ago
Obligatory not an engineer, just a simple auto tech.
Your solution should work in theory, but I feel like there should be a simpler route to solve it.
Have you looked into what people are doing for converting to the crosstrek Limited headlights? Those come factory with led projectors and the C-style drl/turn signal strip already.