Hi all,

I swapped the broken filament bulb in the rear view mirror for an amber LED. It's the perfect illumination.

However, on installing it, I soldered it onto the wires from the old bulb, rather than free the wires from the terminal block and wire the new ones in; I didn't want to risk loosing the tightness of the clip.

Anyhow, so I know that the LED and resistor are wired in just as the old filament bulb was.

However I now notice that th LED is constantly illuminated. Doesn't matter if the ignition is on, or if the car is shut off and locked, it stays illuminated.



Would I be right in remembering that the old filament bulbs only light up when the key is in?

Is it drawing off some residual current in the system? Is this a problem? It's only a tiny LED so I can't imagine it having much of an affect on the battery.

 

It should definitely turn off if the car is locked. Have you checked a couple of minutes after locking the car? Still on? Some of the systems stay on for a couple of minutes after locking the car.

There should be no residual current in the system, certainly nothing more than a few hundreds of a second maybe. I would check the connections and make sure you're using the right size resistor.

 

Yeah, I had a sit in the car and waited to see if it went off with the MFD when it was locked, but it continued to be on, and remained so this morning.

 

So I just put a multimeter across the contacts, and it's drawing 21.5mA.

I imagine that whatever transistor supplies the current, that the filament bulbs would also receive the same 21.5mA, except in this instance it wouldn't be sufficient to illuminate the filament. My father says I have the option of putting in another resistor across the LED, to turn it off, however it will still draw the same 22mA regardless.

??

 

Hmmm. that's a pretty high draw to have on the battery constantly from just 1 LED. Try a regular bulb see if it goes off...if it stays on then your problem is unrelated to the LED. You could also try increasing the value of the resistor. These cars have a tendency of doing weird stuff when messing with the resistance of circuits.

 

I've put a 1kOhm resistor across the terminals that should result in the led turning off.

However after trying a couple resistors, the bulb stayed illuminated. I found an old filament bulb, and even that when wired in stayed illuminated when the car was all shut off.

So my father thinks that the reason is perhaps one of the control modules is faulty, or whatever it is that sends current to those wires. He says perhaps the wires came into contact with one another to blow it, however I can't see how that could have been the case.

Possibly explained why the bulb blew, if it was always illuminated. Just with this CLK I have never had the bulb working. Only on my last car did it work.

 

Well you wire a resistor on one of the legs of the LED, not across terminals. If you have the same issues with a regular bulb then it's probably unrelated to the LED. Check the wiring and make sure nothing is shorted out or have any other problems.

 

Yes, the first resistor is wired in series with the LED.

However in order to prevent the LED being lit, whilst the car is turned off, you can wire a resistor in parallel. It wouldn't stop the current draw, it would still pull 21.5mA, but it wouldn't illuminate the LED. When the car is then switched on, with a much larger current it'd overcome that and illuminate the LED as required.

But the draw would still be there, and I don't know whether 21.5mA is too much continual drain on the battery for what is a nicety.

Atm I've disconnected it, but left the wires there so I'm able to solder something in if the cause is established.

 

LEDs require very little current to illuminate. I have a "smart home" with computer control of all my lighting. When I switched over to LED bulbs, I had lots of problems. Some of the older lighting controls in combination with certain LED bulbs would allow enough current flow to make them flash. It looked like a haunted house! Swapping to a different bulb brand and updating some of my controls fixed that problem.

Understand that most lighting circuits in your car are not "hardwired". Instead, they are controlled by electronic circuits. The ambient light int he mirror is likely switch on and off electronically via circuitry in the OCP. An LED that draws very little current may get just enough from that circuit to illuminate.

 

Yes that is all well understood, until a normal filament bulb is put in (^) and even that continues to illuminate..?

 

OK, now that's strange. I'd say it's a problem in the OCP. It is always powered, and uses logic circuits to read CAN signals to turn its various components on and off. I know a "not uncommon" failure is when the fan on the humidity sensor continues to run after key-off. Its fix is to replace to OCP.

 

Hmm okay. I wonder whether a typical OCP failure results in other symptoms... At present I can't think of anything else which is not performing as expected. So for the sake of a non-essential, dim glow, I can't justify a replacement.

 

Too much thought going on here. There is enough power in the circuit to power the LED but not the filament bulb. Had the same issue when I changed all my interior bults to LED on my Jeep.......they did go off but only after about 5 hours ....lol. Probably feeding off the capacitors in the system. I just had to put one normal bulb in and hey presto, all was good. Had the same issue on other cars too. Funnily I have just changed all the halogens in my office to LED....on the bosses orders to save cash!......they were on faintly all the time now....switch on or off. Had to put one halogen back in to sort it!


.....as for a normal bulb staying lit now....well that's another issue!

 

There must be something faulty somewhere in the system for a regular bulb to stay lit.

These cars get weird when some of the components are disconnected or not communicating properly. For example, I had the steering wheel off the other day and turned the car to ACC...all lights turned on, including the headlights...reconnected the steering wheel controls and airbag and so on....everything back to normal. Why they would be connected...I have no idea.

I would get an OBD tool that can connect to individual components and read codes...see if any of them is having issues...Or simply ignore the stupid light. It doesn't do anything haha.

 

Haha it doesn't. I have been offered a follow up SDS scan in a few weeks time. I'll ask then if I haven't forgotten about the light by then, that is haha