Resistor? Help me! (12v auto problem)

ACMarina · #1 02-27-2006, 07:01 PM

Okay, I've purchased an LED 3rd Brake Light (CHMSL) or whatever you'd like to call it for my car. It's from Europe, and apparently the wiring system is different there. I had to rewire my taillights to get them to work and everything...

ANYWAY..

I'm down to the 3rd brake light. Since it's LED, it draws less current than the bulb version, right?? Well, apparently the 3rd brake light circuit also takes care of the BOO (brake/on/off) switch, as well as the cruise control. Basically, if I put the LEDs in there, the cruise stops working, and the brake lights won't come on and turn off properly. In the olden days you'd just plug in a no-load relay for it, but everything is a computer nowadays and you can't do that (there's no relay). So I'm guessing what I need to do is wire in a resistor somehow, right?? How do I figure out how much resistance I need?? And then would I be better off wiring it in myself, or buying a load equilizer to clip in that would do it for me??

Help me!!

IsaacHayes · #2 02-27-2006, 07:31 PM

Huh? Just tap into a wire going to the taillight bulbs.

Say, left side tail light wire harness, tap into that, run the wires to the led 3rd light.

Don't mess with the brake light switch or anything like that. Sounds like you're making it more difficult than it needs to be.

ACMarina · #3 02-28-2006, 07:14 AM

The 3rd brake light is on a seperate circuit, apparently. If I disconnect it, the same thing happens (lose cruise control and brake lights don't turn off) so I have to do someting with the wires that are already there. I'm thinking that I measure the current through the normal bulb assembly somehow, then measure through the LEDs, and then add a resistor to make them equal out. Right, or am I completely off??

scott.cr · #4 02-28-2006, 09:03 AM

What is the make/model/year of the car?

Many new cars have bulb sockets with addresses that communicate to the PCM... so you get a "bulb out" warning on the dash, etc etc etc. On a car like that, the fix might not be as simple as a resistor. You might have to dig up the old socket and reuse it somehow.

Most 2001+ cars sold in North America have a "K Line" where various vehicle stats are communicated... usuall this wire is light blue. If you have a light blue third wire coming from your sockets, well then there's your answer.

ACMarina · #5 02-28-2006, 09:42 AM

It's a Ford/Focus/2003. Just two wires, 12v, and it goes from the socket back to the Lighting Control Module, seperate from the PCM. You know how the LED replacement "Bulbs" draw so little current that the car's turn signals blink fast, indicating a burned-out bulb?? I'm thinking it's the same thing here..

Diesel_Bomber · #6 03-05-2006, 08:27 PM

Dang I'm bored tonight. If you haven't gotten it figured out AC, here's what I'd do:

Measure the resistance of the stock incandescent light bulb, and use a resistor with the same ohm rating connected in parallel with the CHMSL. The LED CHMSL won't reduce the resistance by enough or draw enough extra power to make a difference. Of course you'll want to get a resistor with enough heat dissipation capability, and mount it where it won't melt anything. Good luck!

Cheers.

Morelite · #7 03-05-2006, 08:50 PM

I don't know what kind of socket your bulb or led replacement plugs into, but the Led is polarity sensitive and it is possible to plug it in the wrong way on some sockets. A third brake light should not need a resister, but the bulb or led has to be in place to pass current through the socket so the computer circuit knows that the circuit is closed / working.