When I Step On My Brakes My Dash Lights Dim Or My Parking Lights Turn On, WHY?

Posted: November 19, 2010 in How to:, Technical, Trouble Shooting
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I have been fielding this question more and more lately. I experienced this problem first hand before I worked here at V-LEDS. I diagnosed the cause of the problem and then I was able to come up with a solution to fix it. Lets find out the cause first.

My experience was stumbled upon after the car I worked on left the shop. The customer called back a short time later and stated that he noticed something weird when he was parking in his garage. When he stepped on the brakes he noticed that the white LEDs that were installed up front were lighting up at the same time. I did not know how to explain this, this was my first time using any products from V-LEDS. I then contacted V-LEDs and asked if anyone had seen or heard of this happening before. At the time they had not. (this was about 3 years ago) So I asked the customer to bring his car back to the shop and leave it with me so I could try to fix the problem. I figured that the source of this problem was the brake lights, because it happened when the brakes were pressed. This particular car, a 2007 Shelby GT 500 uses 3 pair of lights for the tail-lights/brake lights. I used the best of best from V-LEDS, and had installed 3 pair of the 3157_92_R LEDs. Up front I had installed the 3157_60_SMT_WA1_6K. What I noticed was this:  when the parking lights are in the off position the white LEDs on the switchbacks would come on at about half power when the brake lights were on. I removed the taillights and grabbed my DMM (digital multi meter) electrical tester and got to work. The condensed edition of what I found is this: With the Parking Lights off and the Brakes lights on about 5 volts would show up on the parking light circuit. I reinstalled the original filament bulbs and the problem went away. So I concluded that the LEDs were causing the issues. I didn’t think much more at the time other than that it needed to get fixed so the customer could have his car back. I had plenty of electrical components in my shop and went straight for the diodes. I figured i could use a 2 amp diode to keep the +voltage from back feeding into the cars parking light circuit. I installed 6 diodes, 1 for each brake/parking light and it fixed the problem.

This all happened a few years back. Now that it is my full time job to help customers of V-LEDS find solutions for problems that can occur from replacing filament bulbs with LEDs I have seen other variations of this same problem. These include, when the brake lights are on: the dash lights dim, the navigation or radio display dims, and the fog lights come on. Some of you just want to know how to fix it, but others are interested in WHY it is happening in the first place. So I would like to take the time to explain why. Here goes…

There is an electrical component on a circuit board inside the LED bulbs. This component is responsible for the output brightness of the LEDs on the bulb. This means that the LEDs are being run at half power when the parking lights are on, and full power when the brake lights are on. This is how our LEDs differ from a filament bulb. In filament bulb there are 2 separate filaments, a low filament and a high filament. They are not connected internally and they both operate at full 12 volts. Each of the 2 filaments are of different wattage. This is how a dual intensity filament bulb works.

In the pictures below there is a diagram showing how to install the diode inline on the parking light circuit and an illustration that explains how a filament bulb works compared to an LED bulb.It also shows how the voltage back-feed happens.

This diagram shows how to install the diode inline to fix the problem

This illustration shows how a filament bulb works VS an LED replacement bulb.

If you have any more questions about this or if you are experiencing similar problems feel free to contact me via email here:  tech@v-leds.com, and I can give you a hand.

Thanks for taking the time to read through this article, I hope it helped you out.

James, the tech@v-leds.com

Comments
  1. Vikesh Keeshan says:

    Hey! Nice work on the lights.

    Now, I have the same problem happening with my instrument panel and was wondering where you had gotten the 2A Diode. I saw that RadioShack had both a 1A and a 3A but no 2A. Would either of those work?

    Please let me know!
    –Vikesh

  2. Hef Lozano says:

    Hi, this is very helpful. Let me start by explaining my vehicle. It is a 2004 chevy suburban and I installed LEDs for the front parking/turn lights, tail/ brake lights, and drl. It first only happened with the front turn signals. When I would use a turn signal the parking lights would flash as well. Then I changed the tail lights to different LEDs and now when I step on brakes the parking lights turn on as well. Now for the question. Should I put a diode for the parking lights front and back? Any input would help thanks

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