Dome light 2005 Colorado

[delcecchi]

Question in Equipment forum. Guy has 2005 Chevy Colorado. Dome Light melted. Installed LEDs to replace Halogen bulb. Now instead of on and off it is bright and dim. When it should be off there is still 6.9 volts.

Is this a feature or is something wrong? Information appreciated. Feel free to join the "electrical circuit guru" thread.

[Jeremy airjer W]

The BCM supplies power and ground to the dome lamp. There should be 12 volts on the orange wire (not sure if it is supplied all the time) and ground on the white wire (again not sure if it is grounded all the time). The BCM I'd either supplying voltage when the dome light is requested or supplying ground when the dome light is requested.

The BCM may be seeing something wrong with the circuit because of the low draw of the LED lamp. Have you tried a normal bulb to see if it works properly.

Or let us know what side still has power on it. Pull the bulb and see if there is 12 volts on the positive side and see if there is a ground.

The other thought is that this may be a dimming system where the dome light slowly dims out until it turns off completely. If this is the case than the LED may be interfering with the process.

[CAMAN]

No it's not a dimming system, normal bulbs function properly. LEDs are just more sensitive and are able to stay lit at lower voltages.

Just checked with everything disconnected and light OFF; 2volts between the white and orange, 0volts between white and ground, 12 volts between orange and ground.

Light ON; 12 volts between white and orange, 0 between white and ground, 12 between orange and ground.

Now I'm he11a confused.....

[LMITOUT]

Did the previous light have a switch right on the housing to turn the light on?

[Macgyver55]

Judging by your readings, there is voltage supplied to the light all the time and the circuit is completed by grounding it. There would have to be a small ground leakage somewhere in the circuit, maybe a defective door switch, dimmer switch or control module. You could try unplugging the individual door switches and dome light switch one at a time and check voltages to see if your readings are different. If they stay the same, the BCM could be the culprit.

[CAMAN]

Did the previous light have a switch right on the housing to turn the light on?

No, I'm using the same housing, just built an LED array to fit inside.

[CAMAN]

Thanks Macgyver, I will check when I get a chance.

[Jeremy airjer W]

The BCM is the sole controller of the dome light. The door switches just supply signal to the BCM. The BCM then supplies power and ground to the dome light. The switches do not supply power or ground to the dome light.

[Macgyver55]

I'm sure your are probably right! I am not real familiar with the Colorado pickups and I have no access to schematics anymore so I'm just taking a shot in the dark! Since my second reply I have suspected that the BCM is being fooled by the lower voltage of the LED or that it is bad. Aside from that, if a door switch is not completely open or closed and not giving the BCM the expected signal, it seems feasible that the "magical module" could be fooled into doing something wrong too. I've opened modules up and stared into them more than a few times....never could see what was wrong even when I knew it was bad!

[LMITOUT]

No, I'm using the same housing, just built an LED array to fit inside.

OK. I thought that it might have had a switch right on the dome light so you could turn on the light without opening a door or turning all the interior lights on with the main light/dimmer switch and would explain the constant 12V on the 3rd wire. With no switch there I don't understand why there are three wires (specifically constant 12V)...one switched, one constant, and a ground...unless it's there for a different overhead lighting option where there is a dome light switch?

Jeremy is spot-on about the BCM. It only switches the ground to complete the circuit so the "hot" can come from a common bus in the truck. I have modified the headlight wiring in my pickup so that when the high beams are on so are the low beams and fogs by just installing a few simple jumpers and diodes in the fuse box. Really lights up the night.

[CAMAN]

LMIT there are only two wires, orange and white, the ground I mentioned was the truck body.

And I've been thinking of doing the same as you with my headlights, I hate how high beams seem dimmer than low beams.

[LMITOUT]

Oh, OK. That makes more sense.

[hydro]

CAMAN, I have enough knowledge here to be dangerous, so bear with me.

Since the BCM is controlling the dome light I am guessing it contains a solid state switch to operate the connection between the lamp socket and ground (white wire back to the BCM). That solid state switch will require a voltage to turn on and off, and may well have a normal leakage current through it. Consider now that a halogen lamp is basically a resistor which glows when current flows through it, and an LED is more like a valve, either on or off depending on the voltage applied across it, and it requires a series resistance to limit current when the circuit is completed. If the BCM is not switching internally but operating a relay to switch this, there is a high resistance short somewhere that you would have to find, but let’s not consider that now.

With a halogen bulb, the normal current leakage through a solid state switch would not be enough to make the lamp glow and everything would work fine. An LED, however, will glow when forward biased, or when about 0.2V (if I remember right) is present across it. Now, you measured voltage present on the orange wire (+12V from battery) which is enough to forward bias that LED within the battery/LED/BCM circuit with the solid state switch off. Once biased on, the LED is like a valve and whatever current available will flow. The leakage current through the BCM switch will be enough to make the LED glow slightly. I guess that is what you are seeing. To take care of the glow, you would need to add a resistance parallel to the LED to bleed off that current and prevent that voltage from forward biasing the LED when the solid state switch is off. Determining the proper resistance and wattage of that resistor is the trick to success.

There are two solutions that I can see. First, you can buy an LED dome light (tmart.com) for about $10.00 and that should take care of the problem if it is designed to work with your vehicle’s BCM. Second, you can research the BCM/dome LED circuit further and design a series/parallel array that will address the residual current flow issue.

[delcecchi]

Actually the forward voltage on a white led is more like 2 volts more or less depending on manufacturer and ratings.

I find it hard to imagine that a solid state switch would have leakage in the milliamp range, unless the car guys did it on purpose. Stick a multimeter on the ground wire with the bulb disconnected and the light "off" and see what the resistance is.

[Surface Tension]

Put the halogen light back in and test for voltage across the contacts when the dome light is off.

If no voltage then that would tell me the BCM is working properly.

Then you can decide if continuing with the leds is something you want to do.

I'd prolly put it back to original and go fishing.

Other then that, being leds are specific to polarity could this throw a wrench

as far as the BMC.

[hydro]

Even at a 2V forward voltage, there is 12V to divide up in the BCM/LED circuit so it is possible to make that LED glow. Again, this is just a guess on my part.

If you want to measure the current in the off state, you would need to put a resistor across the dome light poles, measure the voltage, and then calculate from there.

[CAMAN]

I wouldn't go back to the Halogen if you paid me, it was just way too dim and I couldn't find anything in the back seat. I have until Memorial Day to wait for my fishing to start, so I have time play with these things.

The LEDs staying dim when off was just a minor inconvenience and I was just asking to see if there was some quick circuit or device I could install at the light that would prevent it.

I'm not sure I'd like to install a resistor as that would still draw a current and eventually kill the battery if the truck sat for a while. I will play with a couple standard diodes and see if it fixes it, if that doesn't provide a fix I'll probably just leave it unless somebody has other ideas.

[Scott K]

Does it have a dome light over ride switch on the headlamp switch? That should shut the power off to the domw lights.

[CAMAN]

Yes it does but does the same as shutting a door.