Question for those who know about automotive electronics, installed a light bar on my truck

Thread Starter

John appleseed

Joined May 20, 2020
86
I recently installed a light bar on my truck. The light bar has a relay that gets activated when i trigger my high beams. Sometimes when i start the truck i notice the light bar will be on even tho i hadnt triggered the relay. I believe this is because of a slight amount of voltage is leaking from my highbeam wire, enough to trigger the relay by its self. It only happends sometimes. I think a resistor should do the fix does anyone have any solutions or know what kind of resistor i might need? The relay is 12v. Also does anyone know why my highbeam wire might be leaking a small amount of voltage that is enough to trigger my relays coil?
 

Thread Starter

John appleseed

Joined May 20, 2020
86
Also i forgot to mention when it goes on by itself all i do To turn it off is flash my highbeams once and it turns the light bar off. Then stays off And works the way its supposed to.
 
What kind of truck? Daytime running lights? Error messages if a light is not working?

If you have daytime running lights I would check your grounds. Flashing your high beams may be enough to make the connection work again until you shut it off. Kind of like making a speaker start to work when you turn up the volume (if you've experienced that before).

If you have something that tells you a light is out that may be the culprit too, but that should make it more common then every now and again.

If you're getting enough power to turn on a relay now and again a resistor may not be the answer since it may not work at all then. Next time it happens measure the voltage from your high beams.
 

Thread Starter

John appleseed

Joined May 20, 2020
86
My headlight has three wires going to the bulb. I metered them to find out that one is high beam one is low beam one is ground. I flashed my highbeam with my leads to the wire to find out my high beam gives me 12v when triggered. I spliced into that wire and fed it to my relays coil. When my highbeams go off it sends 12v to my relay.
 

bwilliams60

Joined Nov 18, 2012
1,332
Provide a picture or rough schematic drawing of how you have this connected into the truck. Also provide year/make/model so we can look at our own diagrams. When you say the lights are on and should be off, how bright are they? LED or incandescent?
 
What kind of truck is it? Does it have daytime running lights? These are two very important pieces of information if you want any answers!! All headlights work more or less the same, but how they go from battery to headlight is a million and one different ways.
 

Thread Starter

John appleseed

Joined May 20, 2020
86
What kind of truck is it? Does it have daytime running lights? These are two very important pieces of information if you want any answers!! All headlights work more or less the same, but how they go from battery to headlight is a million and one different ways.
Oh sorry its a 2011 nissan frontier.
 

Thread Starter

John appleseed

Joined May 20, 2020
86
Provide a picture or rough schematic drawing of how you have this connected into the truck. Also provide year/make/model so we can look at our own diagrams. When you say the lights are on and should be off, how bright are they? LED or incandescent?
Its a 120watt led light bar but it is only activated through a relay. The relays coil gets power when my high beams are on. Unfortunately when i start the truck sometimes the relay is active only sometimes though
 

jpanhalt

Joined Jan 18, 2008
10,110
Did you use a latching relay (i.e., when it changes states, it stays in that state until changed ) or a relay that stayed in a new state only so long as it was energized?
 

Thread Starter

John appleseed

Joined May 20, 2020
86
Did you use a latching relay (i.e., when it changes states, it stays in that state until changed ) or a relay that stayed in a new state only so long as it was energized?
I used your typical automotive relay. When you energize the coil (flash highbeam trigger) the light bar will stay on untill u let go of the trigger. It is not a latching relay
 
The first thing that popped in my head... if you have daytime running lights and start the engine the lights come on. Since the lights are on if you have a weak ground connection some some power will bleed over to the high beam side and turn on the relay. It could be something as simple as a headlight going bad, or maybe worse wires in a harness corroding away and shorting together. I couldn't find a wiring diagram quickly for a Frontier, but it does look like Nissan generally uses relays to run the headlights. That pretty much eliminates most possibilities other than wires somehow touching between the fuse box and headlights.
 

Thread Starter

John appleseed

Joined May 20, 2020
86
The first thing that popped in my head... if you have daytime running lights and start the engine the lights come on. Since the lights are on if you have a weak ground connection some some power will bleed over to the high beam side and turn on the relay. It could be something as simple as a headlight going bad, or maybe worse wires in a harness corroding away and shorting together. I couldn't find a wiring diagram quickly for a Frontier, but it does look like Nissan generally uses relays to run the headlights. That pretty much eliminates most possibilities other than wires somehow touching between the fuse box and headlights.
But it only happends sometimes. Cant it be just a slight amount of voltage leaking through to my relay? After all the relay dosnt take much voltage to energize the coil. Also when i press my high beam it fixes the problem. Couldnt i put a resistor in line so the relay dosnt activate from the run off voltage?
 

LesJones

Joined Jan 8, 2017
2,687
I think you should do some tests while the lights are on when they should be off. With your meter negative (Black.) test lead connected DIRECTLY to the battery negative connection measure the voltage on both ends of the relay coil DIRECTLY on the relay. (You will probably have to extend the negative meter test lead.) This should be a start to understanding what is causing the problem.

Les.
 

jpanhalt

Joined Jan 18, 2008
10,110
The symptom reminds me of something is starting in an unknown state. Then as the computer boots, it goes through a check and when it finds that state/device, it tests it or turns it off which leads to your light bar turning on. That is obviously pretty generic and vague.

Let's assume for example an input to a certain pin of the MCU. During boot, it may assume the pin is in the same state then the motor was last stopped. If it is not, then the MCU cycles it. Is it the brake light? A/C fan? Back-up light? That list is probably pretty long. Whenever that device is cycled, it is activating your light bar switch.

It might be easier just to address the problem. That is, use a relay/electronic switch that ensures the light bar is off and stays off for a certain period (3 seconds?) whenever the starter is engaged.
 

drc_567

Joined Dec 29, 2008
875
The activation of the truck starter motor initiates a relatively large current pulse through the battery cable that connects to the starter motor. If the lightbar relay activation wire is positioned adjacent or close to the starter-battery cable, it may be that the inductive coupling of the two wires is sufficient to turn the lightbar relay on. This would explain why turning on the lightbar switch returns normal operation.
 
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