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On a whim, I decided to replace the fairly dull incandescent bulbs in my 2011 Ford Taurus with LED retrofit bulbs. This was working fine until I got to dome light over the rear seats. The dome light has three bulbs, 2 of the bulbs have individual switches and sit in rotating mounts on the outside of the unit. The third turns on only when a door is opened, and sits in the center of the unit.

When I replaced all three incandescent bulbs with LED replacements, I found that the outside 2 lights stayed on continuously, were not affected by the switches that were supposed to control them, and stayed on after the car was turned off (the car will cut all power to the interior after a set period of time when the car is locked, and this is the only time that they would turn off).

I know that some cars require a resistor to be soldered inline to LED bulbs in order to function correctly, so I measured the original bulb at 6-ohms and sourced 2 5.1-ohm resistors to solder inline with the outside bulbs. This did not fix the problem.

But wait, there's more. In the spirit of testing, I put the incandescent bulbs back into the 2 outside sockets, and they were now staying on (the LED in the middle was still functioning correctly). I had thought the unit may be defective, until I put the LED's back into the outside sockets and put an incandescent bulb in the center socket, and the unit functioned correctly. The LED bulbs were now turning on and off, and even dimming.

Realizing now that the LED bulb in the center socket was the culprit, I soldered in a 5.1-ohm resistor on the ground wire from the center socket — this did not solve the problem and the outside lights went back to being constantly on. So, I soldered in a couple more 22-ohm resistors I had laying around, topping out at a measured 51-ohms. This still had no effect on the outside bulbs, though it did take the incandescent bulb from 12.2v to a rather dim 2.8v and the LED from 12.2v to 9.8v.

To summarize (TLDR):

  • When center bulb is incandescent, outside bulbs work fine.
  • When center bulb is LED, outside bulbs always on (don't respond to switches/turn off with car).
  • Center bulb functions as intended whether LED or incandescent.
  • Adding resistors (as specified above) did not change this.
  • These are the bulbs that were used, they are listed as being canbus friendly and are supposed to not give errors.

Dome light top2 Dome light top3 PCB4

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    \$\begingroup\$ Sounds like you're putting your resistors in series with the LEDs when they should be in parallel. The aim is to decrease the resistance so it looks like a filament lamp, not increase it and make the problem worse. \$\endgroup\$ – Finbarr Jun 30 '17 at 9:06
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    \$\begingroup\$ And your resistors are way too low a value. 5.1 Ohms across 12V will dissipate over 28W and they will live a short but very hot life. What is the wattage of the filament lamps you're replacing? \$\endgroup\$ – Finbarr Jun 30 '17 at 9:11
  • \$\begingroup\$ Thanks for your quick response. I don't really understand the physics behind parallel vs. series resistance (I will have to do some reading), but I will put the resistor in parallel with the bulb and see if that changes anything. I also don't understand the heat dissipation issue (my electrical knowledge is very basic), what sort range of resistor would you suggest? \$\endgroup\$ – Greg W Jun 30 '17 at 9:15
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    \$\begingroup\$ Then, in principle, the lowest value of resistor you will need will be 12V*12V/5W = 28.8 Ohms. That will, however, dissipate 5W in heat so you need to make sure that can safely happen. In practice, however, the sensing circuits probably won't need to see a resistance that low and you'll be able to use a higher value that won't get so hot. What that value is will be down to trial and error (I'd suggest maybe starting at around 220 Ohms and working down until it fixes the problem but maybe someone on an auto forum has done this work already). \$\endgroup\$ – Finbarr Jun 30 '17 at 9:37
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    \$\begingroup\$ @Lundin Those lamps are designed to directly replace 12V filament lamps in vehicle applications and have whatever current control they need built in. They don't require additional resistors. \$\endgroup\$ – Finbarr Jun 30 '17 at 13:21
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The cold resistance of a light bulb is usually far lower than it's resistance when running. If you can find the wattage of the bulb, you can calculate the current from P/V = I and calculate the resistance needed from V/I=R

V here is 13.8 volts, P is power in watts, I is current in amps, and R is resistance in ohms

Kinda defeats the power savings of the LED lamps, and watch out that the heat from the resistor won't slowly discolor or melt the plastic. But with the existing electronics, you have to "fool it" into thinking that the bulb is ok because it's drawing the expected current.

Option: You COULD replace the lamp with an LED that draws the same current. I advise if you do this, to invest in some very good sunglasses. :)

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OP has solved this by adding a large resistance in series with the center bulb.

This works because the circuit was designed to enable the other lights in case the center door light is on or maybe burned out . U1 is a microchip pic microcontroller tied into the blue wire, likely configured as a ADC. With the led bulb, the lower resistance and leakage current would make the microcontroller think the node it is measuring has a voltage or current that exceeds it's preset limit, and causes the other two to turn on, until keyed or relayed power is cut. This is a fairly simple comparator setup.

There seems to be a voltage divider at r5 that could be the sense node.

Other options would be for op to cut the trace for the sensing, remove r5 or q3, or replace the microcontroller.

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