I am having an issue in my car where the LED light I put in will glow very dim even when the car is off and there should be no power to it. The same thing is happening to the third brake light on the back of the car, it will glow very faintly when it’s supposed to be off but work normal under full power.
I assume there is some sort of voltage leakage that is causing this to happen. I know diodes require just about 0.7 volts to conduct so I assume LEDs are the same way and there is at least 0.7v being supplied to them when they are “off”. I also know LEDs change their brightness with the change if current and so I was thinking a diode in series would drop the voltage enough to have the LED turn completely off. Does anyone have any advice or ideas? This is a 12VDC system with an alternator that can add transients and boost the voltage to over 14V, but even with the car not running this happens.
Helicopter12:
I am having an issue in my car where the LED light I put in will glow very dim even when the car is off and there should be no power to it. The same thing is happening to the third brake light on the back of the car, it will glow very faintly when it’s supposed to be off but work normal under full power.
Was that happening to the "third brake light" before you added your "LED"? Or is that something that showed up after?
Helicopter12:
I assume there is some sort of voltage leakage that is causing this to happen. I know diodes require just about 0.7 volts to conduct so I assume LEDs are the same way and there is at least 0.7v being supplied to them when they are “off”.
You have the right idea, about how LEDs behave like diodes, but the forward voltage on an LED is a bit higher. For example here is a short list of Forward Voltage values, typical for different kinds of LED:
IR: 1.1V
Red: 1.7V
Super Bright Red: 2.1V
Green: 2.3V
Super Bright Green: 3.3V
Blue LED: 3.0V to 3.4V
Yes, they are a type of diode [though rarely, if ever, used in that way], but no, their Forward Voltage is nothing like "0.7V".
Helicopter12:
I also know LEDs change their brightness with the change if current and so I was thinking a diode in series would drop the voltage enough to have the LED turn completely off.
Even if that works, is it going to fix your brake light, too? I would be more interested [even worried], in what's going on with that brake light -- doesn't sound normal. But, I /we need an answer to my first question [above], to even begin to ponder those particulars. In fact, we need a LOT more information, as other's have already indicated.
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The interior lamp worked with the incandescence bulb from the factory, but switching to the LED it did not. Clearly this is because there is a 7V across it even when the doors are both closed (seem image below). The tail light remains untouched and I never noticed it in the past so I can't say for certain if it is a new issue or an old issue. From the factor I believe the third brake light is a LED. The schematics below seem super straight forward and so what I believe might be happening is one of the door switches is bad. The light should be floating with both of the doors closed, and then connected to ground when one of the doors open, thus completing the circuit. I am going to take a few more measurements later today and double check there is a solid ground and 12V on the other pin.
Interror Light glowing slightly showing 7v with both doors closed:
There might be a parasitic and moderately high resistance path to ground through the A11 pin on the "Alarm Unit (M99T2)" One could test this by removing or disconnecting that unit.
I checked the door switch connections and they were fine, but just to be sure I hit them with some electrical cleaner. I measured the voltage across the leads of the door switch and got 4.7V, when it should be 0V, see image below;
Door Switch Reading:
Light Reading with Door Switch Unplugged:
Light Resistance to Ground:
I found this video which is the exact issue I am having with my tail light and also the interior light. This seems to be a common problem and this guy is fixing it by putting a resistor is parallel with the light.
My original though to fix the interior light was also a parallel resistor of 1k - 10k ohm, or two series diodes. Which do you guys see fit and why?
I do not see any monitoring circuit in place so it seems like there is a voltage leak somewhere. I am still between using diodes or a parallel resistor.
