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How to Mod your blinker relay for LED turn signals
Some of you guys had a discussion going about using LED replacement bulbs and the problems with the blinker relay going into 'hyper-blink' because of the reduced current drain. The standard fix for this is to add resistors to each turn signal cirruit where you install an LED to increase the current back to the expected range, which slows the blink rate back to normal.
It just bugged me that the resistor fix means we can't have the benefit of less current drain on the system by using LED's, so I decided to check out the blinker relay and see what could be done.
Here it is, the blinker relay. It's installed in the under-dash fuse box and is a mother-trucker to remove. The dealer must have a special tool for this. You have to pry two plastic spring clips apart on opposite sides of the relay at the same time you pull it out of the socket. It takes three hands and a lot of cursing to get it out, but here's what it looks like.
You can see where I broke one of the tabs on the relay that fits into the plastic clips built into the fuse box. No big deal, the socket has enough gripping power to hold it secure.
Here's what it looks like inside. One relay, one integrated circuit and a few misc parts. The important part is the shiny silver rectangular wire in front of the relay - that's the current shunt that senses how much current is being used by the turn signal lamps. If it senses too little current it blinks really fast to let you know you have a bulb burned out.
I tested the circuit and found that currents above 3.5 amps will blink slow, less than that will blink fast. Hmmmm. How do we fool it into thinking the current is higher than it really is? Since the current shunt is just a low-ohm resistor in series with the bulb circuit, if we change it's resistance we can change the slow/fast current set point.
After some experimenting with parts in my junk box, here's what I did:
I have replaced the current shunt with a 0.1 ohm 3 watt resistor. Now the relay will blink slow for any current above about .8 amps.
But that may still not be low enough for you guys with an all-LED system. An all-LED car may only have about .3-.5 amps on each turn signal circuit. For you guys, I would suggest you replace the current shunt with a 3-amp 50volt diode, which you can get at an electronics store (at Radio Shack here ) for cheap. The diode will have a white stripe near one end, that side needs to be on the right as viewed in my photo above.
The diode works because it has a .7 volt drop across it no matter how much current runs through it, and the .7 volts is enough to make the relay circuit always blink slow.
If you have an all-LED system and want slow blink AND sensing of a burned out or loose bulb, you can do this:
Remove the stock current shunt wire and temporarily solder a 30 foot piece of 22 guage wire in it's place (yes, 30 feet, bear with me). The 30-foot piece of 22 gauge wire is equivelent to a .5 ohm resistor. With it soldered into the relay board, (but not put back in the housing), plug the relay back into the fuse box and turn on the blinkers. They will blink slow.
Now start shortening the wire by 2 foot increments until they start to blink fast. When they do, you have crossed the hyperblink threshold. Go back and add a few more feet of wire to make them blink slow again. At this point you should be able to get a slow blink with all the bulbs installed, and a fast blink if you remove one of the front or rear turn signals.
When you're done the wire will probably be too long to stuff inside the relay box, so you have two choices: drill holes in the relay housing and leave the wire outside the housing (you can coil it up) or replace it with a resistor of the proper resistance. Every foot of wire is equal to .016 ohms. So if you ended up with 9 feet of wire you could replace it with a .15 ohm resistor (actually .144, but you'll never find that size resistor).
Please note: Do not do any of these mods if you still have regular incandescant bulbs in your turn signal circuit. The resistors or diodes I suggest above may overheat and possibly cause a fire.
LEDs DO burn out from time to time (poor manufacturing tolerances), but in most cases you'd still want this option enabled to determine if the bulb came loose.
That's what I thought. But you could alter the flow through an off the shelf potentiometer to get some kind of reading, and doing some calcs you could find the required resistance.
You'd need a 1 ohm ceramic pot capable of handling several amps. I found one at Digikey for $21, but you're not likely to find one at a local shop, they're pretty unusual.
12-18-2003, 08:38 AM
How to Mod your blinker relay for LED turn signals
haha
