Hello. I want to install two led strips in my car for a footwell ligth. Do you think they can be connected to the dome light connector? One possible issue is that led has two wires but the connector - three. Of course i will measure the voltage but want to know if it’s technically possible. My dome light illuminate when open a door and turn off after about 30 seconds. It can be turned off and on permanently by pressing buttons. My car has no specific fuse for this light, probably body computer command it
The three wires used in the existing light are what enables the switch to select between always-on or controlled by the car’s computer. You can identify which you want to use by examining the switch contacts, or by testing the switch inputs. Or you can make the connection at the existing light contacts (the switched output) in which case the dome light select switch will also select for the footwell lights.
1 Like
Hi All
Be aware that some of these interior/door lamps, wipers etc actually make the ground or chassis connection to operate. Much depends on the car type and model.
Which means the 12V is supplied (via the appropriate fuse) at all times (sometimes only while the ignition is on, depends on function) and the circuit is completed by making the chassis connection.
A bit of sleuthing before acting is recommended.
Cheers Bob
Hey @Ron280737,
With some careful testing before hand, this should be possible. As @Jeff105671 has pointed out the 3 wires are likely intended to be ground, 12V and door controller 12V so it sounds like you want to get power to the LEDs from the door controller 12V line.
Let us know how it goes! 
Not what I said 
The three wires will be ground, 12v and door controller, but the door controller will not necessarily be 12V (as mentioned above). A 12V common that is switched to either of two grounds (direct, or controller) is becoming more usual, perhaps because it is more convenient for the controller to switch to ground rather than 12V. Whatever, OP needs to check carefully, as you note.
Whoops! Thanks for catching that.
Hopefully, OP does a better job checking those wires than I did.
Hi All
As Jeff just pointed out
this could be the case. Used to be common with windscreen wipers where the “auto park” system was simply a switch that switched off a connection to ground. When the wipers were switched on that switch was in parallel to the park switch and override it thus maintaining the ground connection for the ON duration. Very simple and it worked.
I have some electrical circuits for my vehicle (2008 Peugeot 307) which is annotated in French. Would you believe the front wipers seem to switch 12V under some sort of electronic control and the rear wiper uses the switched ground system. But I suppose being French it could be anything.
One diagram shows all the lamps in the vehicle. Without being sure of the actual function they all seem to have a permanent ground connection as if the 12V was switched. A bit hard to tell as the wires seem to go into boxes with a picture of an IC on them which I think signifies some mysterious electronic (Computer???) control.
I guess what I am doing is reinforcing the need to check this individual case as it could easily be either way around.
Cheers Bob
Checking should be easy enough.
With the dome light on you should measure 12V WRT chassis on one side of the lamp only.
With the lamps off if you measure 0V on both sides of the lamp you are switching 12V. If you measure 12V on both sides of the lamp you are switching the ground connection.
All measurements WRT chassis or ground.
1 Like
Thanks for everything you pointed out. I will take the measures but prefer to wait until the parts arrive. As you can see, the area of the upholstery around the dome light became unglued so i need to fix it once everything is removed. I read that may one wire is 12v, the other is normal ground and the third is ground to the body computer, responsible for smoothly turn on and off the lights. So will check if it’s true
Hi. I take the measurements today. Middle pin is 12v, left and right are both ground. So, red wire of the led strip in the middle pin and black wire on the right grounding means led strip lights all the time. Moving black wire to the left grounding means it works with the ambient light and goes off when ambient goes off. But with one Small detail - doesn’t goes of completely and lights just a little bit. Not much but enough to drain the battery. So this project will not come to live 
It’s not a fabric defect of the led strip because I installed already the same in the trunk, wired from trunk light connector which has 2 wires - the same as the led strip
1 Like
Hey @Robert93820,
Interesting! Have you measured the circuit while this is happening?
I wonder if this is something you could fix with a voltage comparator circuit to only allow power to the LED strip under certain conditions.
I don’t know a lot about car electronics so hopefully someone more knowledgable has some advice for you.
Hi Ron
This might be normal and you might find that it will go out after a minute or two, you will have to find a way to check this. If you look closely you might find they ate flickering rapidly.
I have a vehicle of 2008 vintage and fitted LEDs to the stop lamps. When the interior lights have been on after a door has been operated and the door closes the interior lamps stay on for a short period then after they go out the LEDs remain flickering for a time. I believe that is the computer system sending a signal to check the integrity of all the light globes or something like that. Checking something anyway.
As the cold resistance of the incandescent lamps is very low and very little current is available these do not come on so to us mere mortals it appears as if nothing is happening. The LEDs will come on dimly with very little current so you see them as on (dimly). I just ignore it.
Think about it a bit. If the LEDs were going to drain the battery The incandescent lamp would do so also only you just don’t see it as being on. The LEDs are effectively connected in parallel with the lamp but because they are much more sensitive you see them
Cheers Bob
1 Like
I will check at the weekend if it goes off completely or staying on for a longer period. I have only led bulbs inside the car and they goes completely off, as expected
Edit- probably i need original lights like those, with 3 wires. Mines for the back seat area are identical - 3 wires
Hi Ron
I just went out to check my stop light LED operation and made a bit of a discovery.
These lamps are on very dimly while ever there is some activity like the ignition on and the car running or any interior lights on (ignition or no ignition). This is not much of an issue during the day as they are so dim you have got to know they ate on to notice and at night the tail light overshadows this dim bit of light. The only one you would notice is the 3rd light in the centre of the tail gate top. Nobody has so far and I still pass rego check.
What it has shown though is my theory of the computer(s) doing a pre shut down check as the cause of this is blown out of the water. It is still possible that the Can Bus is doing a continuous check of the lamp status and only stops when it shuts down.
You say you have all LEDs inside the car. It is quite possible if these were a factory fit they might be “Can Bus compatible” which I think involves a parallel resistor (built in to the lamp???) to make the LED appear as an incandescent globe with the associated low cold resistance.
The LEDs I have fitted ARE NOT Can Bus compatible and neither would be your LED strip so would look like a relatively high resistance.
This has got me interested enough to find out just what this signal is that causes the LEDs to flicker like that. I have a tablet oscilloscope, very portable, so in the next few days I will go out and have a look at this. Will let you know.
Cheers Bob
They are not fabric, I installed them because prefer white light. Two ot them already replaced less than year after fitting because of flickering. They may be canbus but canbus doesn’t check interior lights inside. Probably because are not important. For example, brake lights are attached to canbus line
Hi Ron
Just a theory. I am not familiar with Can Bus finer details. But I have just learned something for to-day and will file ti away in the grey matter for future.
Cheers Bob
Hi @Ron280737
Another thing to look out with if you’re using those type of LED replacement bulbs is that unlike a standard halogen bulb they are polarised and will only work one way.
Hi Ron
Just did a bit of a search and came across this page where there are a number of LED Festoon laps and all the ones I checked the detail for are Can Bus compatible. Quite a few look just like your pic.
Which means they will behave just like an incandescent lamp. So the signal I was referring to could well be there and causing your observation (won’t call it a problem yet).
Jaycar seem to only stock 1 festoon lamp. It is also 31mm and Can Bus compatible.
Cheers Bob
1 Like
Hi Ron
Have not had a chance to look at the Can Bus in my car yet. Might have to wait until Monday.
But I still think that the Can Bus signal is what is causing the strip to stay on. Can you see if the LEDs are flickering. I think you could prove this by connecting a “Can Bus compatible” led (festoon or other) in place of the LED strip and see if that goes out. I think it will.
I can see that your situation where the foot well strip is always on, albeit dimly could be a bit off while driving at night. My situation being stop lamps is not so drastic. I can’t see them. And at night they are overshadowed by the tail lamps and during the day they are behind the red plastic and you have to know about this and go looking to notice them being on.
If it proves to be caused by the Can Bus you might have to find a suitable fitting and install a Can Bus compatible festoon lamp in place of the LRD strip. Something to consider. Use the lowest wattage footwell lamp as you can as the compatible lamps look like an incandescent device and you will be using 2 in parallel.I think you could use just as much currents incandescent but I could be wrong here and this is done with some other trickery so you still get the benefit of lower LED current. Don’t know about this but in early days a resistor had to be fitted across the LED to emulate incandescent. Particularly with Bi-Metal controlled turn indicator relay. These days with electronic control anything could happen. My car has a little switch in the trailer connector to tell the system a trailer is hooked up. This disables the reverse parking sensor and allows for the extra lamps.
Cheers Bob
Add on
A quick google turned up the following in the questions bit.
What is a CANbus compatible LED bulb?
For vehicles with a CAN Bus system, installing LED CAN Bus compatible bulbs can prevent the “bulb-out” warning signal since these bulbs have built-in resistors that simulate the expected working power of a halogen bulb.
Why do LED lights flicker with CANbus?
The CANbus actively sends voltage in varying amounts to the headlights when off, the voltage is too small to be turned into visible light by the tungsten filament in conventional bulbs. The problem arises because LED’s see this small voltage and instantly turn on and off, creating the flicker/flashing effect.
These answers refer to head lamps mostly but I believe it applies to all. I did ask my dealer about it some years ago and advised the only thing that can be done is use compatible LEDs or in my case I can ignore it.
I will look further later and see what else I can find.
Made some checks today. Uploading pictires. First, led strip attached and working:
Second, led strip not completely goes off. It’s daylight and cannot see the white dots properly, sorry about that:
Third picture, comparison between not connected led on the left and not completely goes off led on the right . Again apologize for the daylight but dots are visible:
Also, take measurements of the battery voltage. In both cases - not completely goes off led and not attached led, it shows 12.62v. So probably, two led strips won’t ruin my battery. Checked other leds inside my car and they goes completely off
Hi Ron
You apparently have not read my posts above
A signal on the Can Bus would be causing this.
The other LEDs like the pic you posted are “Can Bus compatible” and the LED strips are not.
You did say earlier that the dome light is not subject to Can Bus scanning. How do you know that for sure. I think that because the strips remain on you could be mistaken here. I and many others experience the same “problem”.
I thought that my last post with the copied extracts from Google might indicate that many others have similar “problems” and the diagnosis is always that it is caused by a Can Bus signal.
But if you are not going to take any notice of my posts and try my suggestion of trying a compatible LED in place of the strip I will not bother to post anything.
Please advise.
Cheers Bob
