Crazy Center Dome Light - Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum



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  #1  
Old 05-09-2019, 07:20 PM
dmtaurus dmtaurus is offline
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2006 Chevy TrailBlazer LS
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Crazy Center Dome Light

I have a 2006 short wheel-base Trailblazer, and it has a crazy problem.

My son noticed while walking through the garage, when it was dark, that the center dome light was dimly lit. I did a quick check by pressing both center dome lenses to activate the "reading" setting of the switches and the bulbs went from dim to normal brightness. I pressed the lenses again and both the bulbs went back to dim. Then, I closed the door and the front dome light and tailgate dome light went out, like normal. But the center dome pair will stay on, but still at a lesser brightness. I am not sure why this is happening. I did a quick review of the wiring diagram, that is found on this site, and found that the lights are connected to the same wire harness, so nothing jumped out to me as to why this is happening.

About a year ago I replaced the dome bulbs with LED equivalents, yet have never had a problem until now.

Any ideas?
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  #2  
Old 05-10-2019, 09:31 AM
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TB-Al TB-Al is offline
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LEDs are not the same as incandescent bulbs. BIG difference. An incandescent is a voltage device where the power is dependent on the voltage while an LED is a current device where the voltage is constant and the power is dependent on the current allowed through. The computer that was designed for incandescents wasn't designed for LEDs. To allow the lights to be dimmed the computer creates a square wave A/C signal where, to get full bright, full voltage is applied and to dim the light the voltage is pulsed on and off with shorter and shorter intervals.

The computer is not seeing enough current to completely shut off the power. But the LED will show a little light with very little power.

And, yes, you've always had this problem. You've just never noticed it before.
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Old 05-10-2019, 09:49 PM
dmtaurus dmtaurus is offline
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What you have written makes sense. These particular LED's are of the CAN bus style. Does that mean a resistor is included in the LED design to load the circuit so that the module sees a sufficient load and doesn't allow voltage thru to illuminate the bulb?
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Old 05-11-2019, 08:48 AM
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I had never heard of a "CAN bus LED" before. So I looked it up and it seems to be one company's way to get more money for the same thing other companies sell. Their advertising uses half-truths to make these seem better and more trouble free. What they are talking about in their ads is the very real problem of hyperflash when you replace brake/parking/running lights or any other light that is monitored by the BCM computer for failure. When an incandescent turn signal (brake) filament light fails it doesn't pass current which the computer senses and changes the turn signal flash rate from normal to fast. i.e. hyperflash. This is corrected by putting a resistor across the LED to pass just as much current as the incandescent bulb did. This means the resistor gets just as hot as the incandescent bulb filament did. LEDs don't like heat so sometimes the resistor is mounted separately. People bolt them to plastic parts and are then surprised when the plastic melts.

Anyhoo, back to your issue. The computer doesn't monitor interior lights because you would see if they failed and they aren't a safety hazard when they do. So you don't need a resistor to fool the computer. But an LED (Light Emitting Diode ) is a diode. A semi-conductor. It passes no current in the reverse direction until you destroy it. In the forward direction it passes no current until you get to about 1.5 volts. Then it passes as much current as needed to keep the voltage no higher than about 1.5 volts. Put 12 volts across one and it goes BOOM instantly. So EVERY LED replacement of an incandescent requires a resistor. The resistor is either internal to the socket or in the external wiring. Companies that sell LEDs intended for automotive use all put a resistor in the socket. If you want to know more about that resistor see https://www.sparkfun.com/tutorials/219 Oh, and if you find a vehicle with LED lighting, even changing the color of the LEDs is not as easy as it sounds because the voltage across the LED is different for different colors so if the resistor is not matched to the new color the result could be a blown computer.

So your problem is that the computer is supplying very short 12V pulses which is more than enough to illuminate an LED. The LED is a very fast device and, unlike an incandescent, illuminates fully as soon as the voltage appears. An incandescent bulb has to heat up first before it starts coming out. The way the computer generates the pulse is to pass a very small amount of current which comes around and turns it off again. Since the pulse is so short an incandescent never gets hot enough to emit light so you see nothing. The LED emits light immediately but, because the pulse is so short, your eye interprets that as a very dim light even though it is full intensity.

So how do you fix it? You don't. The amount of power is trivial and will not discharge the battery significantly. Some cars will stop the computer from supplying any power to lighting some time after the vehicle is shut off while others don't bother. Compared to the power being consumed by the computers even in a sleep state, the lighting load is trivial. But this could also help explain why you could let a 1950 Ford sit for 6 month but when you turned the key it would fire right up while you let any modern vehicle sit 6 months and the battery will be totally dead.
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Old 05-12-2019, 09:48 AM
dmtaurus dmtaurus is offline
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Consider this though; the center dome and the front dome and the cargo light, according to what I see in the TB wiring schematic, are on the same circuit. Given that fact, all bulbs in the front dome, the center dome, and the cargo light should be lit. But they aren't; only the center dome bulbs are lit, and that is what is perplexing to me.
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Old 05-12-2019, 10:29 AM
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Are they all the same LED? You will never see an incandescent and add a capacitor across the LED (after the resistor) and you wouldn't see it either.
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Old 05-12-2019, 12:59 PM
dmtaurus dmtaurus is offline
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The front and center dome bulbs are the same model number and CAN bus. The cargo light is factory different than the factory dome bulbs, but the cargo light LED is of the CAN bus style. These LEDs I bought from SuperBrightLEDS.com.
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Old 05-13-2019, 07:48 AM
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Again, calling these LEDs "CAN Bus" is just a marketing scam. I have never heard of any vehicle giving a warning alarm for a burnt out interior light. Checking superbrightleds web site shows they use the same part number for both the "CAN bus" and regular bulbs. So, evidently, they don't bother keeping up the pretense for stocking them.

So why does one glow and another doesn't? A slight difference in the wire going to each? A little more capacitance or inductance in one wire vs. the other? We are taking about a very short pulse of voltage. I don't know. But the bottom line is it is nothing to be concerned about. If you are still worried you could just change all of them. I see a full set of 14 LED bulbs on EBay for $6.95 delivered. They would be expected to arrive in less than a week from New Jersey.
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Old 05-22-2019, 01:08 PM
Rgulia Rgulia is offline
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It's Fine!

I replaced my dome lights with LED's over 2 years ago. They have the same faint glow. Not once have I had a battery issue or otherwise. Another form suggested changing the cargo light back to incandescent, as this would quickly suck up the phantom voltage, darkening the dome lights, but
I never had a problem with it, so I have not tried.
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