# Trouble converting 12VDC incandescent to LED: lack of resistance

I'm converting two, independent, incandescent bulbs (OEM 212-2) fixtures to use LEDs for automotive map reading. Each incandescent bulb (12-volt/system) measures 1.5 ohms with a meter; however, there is no resistance across the LED [replacement] bulbs. The LEDs will not light up with their respective switches (however they do come on properly when a door is opened).

When I press the switch to "on" with no bulb installed, I see 4.55-volts DC at the socket connectors. This is not enough voltage to light the LED bulb. However, when I reinstall the incandescent bulb and switch it "on", it lights and I see 12.2-volts across the lighted bulb. It appears the circuit (computer?) tests for resistance initially with 4.55-volts, then ups the voltage to 12.2-volts if the resistance is proper (perhaps a method to detect if bulb is good/proper?).

Does anyone have any suggestions regarding how to make this LED work? I'm hesitant to install any resistor, but the reality is that's exectly what's there now albeit in a "controlled-burn" setting. My concern is simply installing a parallel resistor would generate similar heat and merely burn it.

Thanks for the opportunity to ask this question and for any help/suggestions. -Brian

• What LEDs are you using? Depending on the LED ratings, you will need a current-limiting resistor in series with each LED, selected by R = (Vin - Vf)/If where R is the resistance you need, Vin is the DC input voltage, Vf is the forward voltage drop of the LED (should be specified in the datasheet) and If is the current required by the LED (also should be in the datasheet). I'm not sure about the computer testing for resistance. Apr 17, 2015 at 15:46
• How long is the measurement phase at 4.55V? Apr 17, 2015 at 16:48
• Thanks. The problem here is the computer/module determining when to supply proper (12VDC) voltage. Basically, it needs a certain amount of resistance which it gets normally from 1.5 ohms from OEM 212-2 incandescent bulbs. Putting a resistor in series won't matter because the LED bulb itself offers no resistance across the receptacle's contacts. So when these LEDs are inserted, the module senses no resistance/no bulb. There are LED companies that offer "error cancelling" bulbs (i.e., with resistance). My project is simply: make the bulbs I already have work or determine why not. Apr 19, 2015 at 22:49
• Check the factory wiring diagram to make sure that this is the case or not. Apr 20, 2015 at 0:17

I would tackle this problem in two phases.

1) Find out the minimum resistance needed to tell the computer that the bulb is installed. You can do this by connecting the original bulb with various resistors in series with the bulb. Take that resistance and divide the value by two so as to have sufficient safety margin.

2) Design a circuit that has that resistor installed while 12V is not available. That could be a small 12 Vdc relay with the resistor connected to the Normally Closed contact or it might be able to be a J-FET, depending upon the resistor value.

The idea is that the resistor tells the computer that the bulb is installed and working so that the full 12 Vdc is supplied. When that full voltage is available, the LED consumes enough current to keep the computer happy.

We can give you better guidance once you determine that minimum resistance value.

• Dwayne - The bulb measures 1.5 ohms. I'd posit this is the minimum, no? Apr 19, 2015 at 22:52
• Dwayne: 1) OEM bulbs measure 1.5 ohms which suggests a minimum. 2) Tricky because unfortunately, 4.55VDC is almost ALWAYS flowing to the bulb socket. By this I mean, if the module senses [seemingly almost any] resistance, it cuts the power to 0VDC while the switch is "off." However, if no resistance is detected, 4.55VDC flows continuously regardless switch on/off. I prove this by installing 2 map lights with LEDs and 100 ohm 10W resistors. Turn them both on (light properly) and the dome light (s/b off) will glow faintly when an LED is installed (no resistance). Yes, crazy. Apr 19, 2015 at 23:05

Firstly, to everyone, thank you for taking the time to respond and for giving me the opportunity to learn from you all.

What LEDs? (From Amazon) Cutequeen White 42mm(1.72") 8-SMD 12V Festoon Dome Light LED Bulbs. Unfortunately, I don't have any technical specifications at this point. I do know there is zero (measurable) resistance across the bulbs vs. 1.5 ohms in the OEM 212-2 incandescent bulbs.

Also note, I used these same LEDs for the exact same application in a similar but older 1996 vehicle, and they work without exception...wonderfully. It's this newer (2004) vehicle's electronics that seems to monitor and/or regulate things (now) that's added much more complexity to an otherwise simple task. I confirmed this with a mechanic I spoke with on Friday who stated (from schematics) there is a "module" that also serves the instrument cluster as well as these lights. It uses "drivers" to supply the requisite 12VDC when switched "on" and there is resistance vis a vis the incandescent bulb (1.5 ohms).

As for the measurement phase, as best I understand it, it's constant while there is no resistance. In fact, I think this is precisely why the LED lights glow faintly while the ignition is on. Oddly enough, once you open the glove box, which has a tiny, single incandescent bulb, then the map lights will work independently on/off normally just like with the incandescent. However, using LEDs in both the map lights and the dome light--while all in the off position--and with the glove box closed, all three lights faintly glow due to the constant 4.55 volts reaching them. So while this isn't enough to light them, it is enough to make them glow. So the measurement phase seems indefinite while there is no resistance like with the LEDs installed or the quick connect plug is disconnected (that serves the map lights).

For what it's worth, here are some other things I've observed: If I put a LED bulb in the dome light but leave the two incandescents in the map lights, everything seems to work like normal. However, if I put LEDs in the the two map lights and an incandescent in the dome, only the dome light works when I turn on all interior lights via the dimmer control [position] switch yet the dome and two map lights will all light when a door is opened. But with doors closed, if the glove box door is opened (which automatically engages the tiny light inside the glove box), both map lights will work like normal with LEDs installed; however, if you swap the dome light to an LED as well, it will glow faintly in this situation.

So, prior to reading your input and responses to my question, I [guessed at] soldering in two 100 ohm 10-watt wire-wound Cermet resistors (one resistor soldered across both map light bulb receptacle contacts...no resistor for the dome light) . While these did get pretty warm over a constant 5+ minutes while left "on" for testig, they worked perfectly in what I hoped would happen. Basically, these resistors triggered the module into sensing adequate resistance and hence supplying the proper 12 VDC to properly light both the map lights and the dome when selected. Everything seems to work properly with perhaps one exception: the dome light glows very lightly when both map lights are on (which probably could be remedied with another similar resistor setup...but I only had two).

Friday, I ordered some similar 150 ohm resistors to further reduce heat buildup in the circuit. Unfortunately I do not know anything about the "module's" specifications regarding min/max resistance to determining when it supplies [full] 12VDC. I did disconnect the quick coupler and measured the two positive power leads and one ground supplying the map lights. Unplugged and with ignition on, one of the positive leads measures 4.55VDC continuously and the other measures 4.9VDC also continuously. In speaking with the mechanic, he seemed to think one of those (perhaps the 4.9VDC) is one that supplies the 12VDC when proper resistance is sensed and lighting is switched on (i.e., door is opened, remote unlocked, map light(s) switched on, or all-interior-lights switch set to on (all three lights are controlled by on/off by a specific position using the same switch as the instrument dimmer switch...many cars do this). Note, however, the dimmer switch does not impact the intensity of the interior map or dome lights whatsoever--again common in most if not all vehicles.

Conclusively, this is all the additional information I have since posting this question and reading your responses--which I'm still digesting. At this point, I'd prefer to generate as little heat as possible with two installed resistors (note: this is why I switched to LEDs to begin with) but also I'm concerned with risk of overheating the resistors if they're left on indefinitely accidentally (although my guess is after so long, the module or computer will attempt to disable the circuit?...guessing here).

Ultimately, I'd like to build a better mousetrap from here. I just wanted to get some response back (and thanks) to those responding so quickly.

• Are you sure the LED resistance is zero vs open/infinite? Dec 13, 2019 at 21:19