Automotive Electronic Flasher Rate Modification.

For anyone who has added LED bulbs to their vehicles turn signals or brake lights.
Since the LED bulbs use less Amps than normal bulbs, the flasher unit thinks there is a bulb burnt out and doubles the flash rate. This instructable will show you how to alter the flasher module in your car for variable flash speeds.

Note: This is for electronic relay type flashers only. This will not work on thermal flashers.

Here is the flasher I modified. Since I am using the flasher in my car, I just pulled this one out of another '97 Cougar at the junkyard.
There are two tabs holding the circuit board in, just pry them loose with a small screwdriver.

The inside of the flasher is pretty simple. it consists of an open frame relay, a couple resistors, a capacitor, a shunt, and the controller IC. I looked up the part number to find the schematic.

For this particular flasher, the 100k Ohm resistor connected to pins 4 and 5 will be removed and replaced with a 500k Ohm variable resistor.

Simply desolder the resistor fromt he board. Careful not to overheat and burn out the IC.

Here, the new variable resistor is connected. There are three pins on the variable resistor. The two on the ends connect to the 500k Ohm resistance. The center pin connects to the wiper. Connect two wires, one to the wiper and another to one of the side pins.

Once you set your flash rate, you can measure the resistance on the variable resistor and replace it with a fixed resistor if you want. I left it variable because I plan to continue to add LED bulbs to my vehicle.

If you wish to test the unit outside your vehicle, connect everything as follows:
B = Battery +
E = Ground
L = Lights (This is a positive output. Connect the other lamp wire to ground.)

*These connections may vary for different vehicles!*

Here is the original schematic for my flasher unit and the altered version with the variable resistor.
I have added the part values that are in my unit to this schematic.