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.

Step 1: The Flasher.

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.

Step 2: Inside the Flasher

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.

Step 3: Remove the Resistor

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

Step 4: Variable Resistor.

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.

Step 5: Connections.

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!*

Step 6: Schematics

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.



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41 Discussions


7 days ago

I stand corrected about my previous reply, this instructable modifies the resistor value feeding the internal oscillator, that's one way to do it.

The formula is in the datasheet
1/(R1*C1*1.5), eg in the OP's case: 1/(100000*0.0000047*1.5)=1.42hz
Or to make things easier and not get lost in zeroes, use kΩ (6.8k-510k) and uF (<47uF):

Automotive blinkers should be between 1-2hz (60-120 times per min), 1.42hz will yield ~85hz
If using LEDs, the shunt will measure a too low of a load and the internal comparator with a Vref of 81mV will trigger the failure mode (freq x 2.2, so 3.1hz/132 blinks per min)

To fix this, R1 would need to be 200-220k when C1 is 4.7uF (210k will give a nearly perfect 1.5hz/90 times per min)
My Toyota blinker oddly have 90k9/3.3uF...which will blink quite fast at 2.2hz...

Keep in mind that the duty cycle is also slightly different in that mode, the ON time is 40% as opposed to 50%.

So the other option would be modifying the shunt appropriately or lose the lamp failure detection and use a voltage divider to feed a constant >81mV to pin 7 to fool the IC (ratio of 100:1 should work and provide 120mV, haven't tested this yet)


Question 5 months ago on Step 5

How many amps can the secondary contacts on this relay take? I have a 9 amp circuit which I need to flash fairly slowly!


4 years ago on Introduction

I think, the only right way to do this mod, is to change the resistance of the shunt resistor (the thick L shaped piece of wire) wich needs to have a higher resistance. I would proberbly work if the wire is replaced with a 0.5 - 1 Ohm 5W resistor. This would give enough voltage over the resistor so the chip would think that the load is higher :-)

This is a great little modification and I was lucky to find it! The overall advantage is that you adjust the blink speed while utilising low power draw LED's, unlike other solutions which include adding ballast resistors to create the same amount of power load!

Not all modern Flasher relays are like this as I discovered, I have a brand new one which is basically a relay, 2 transistors and a few resistors inside to make it work, nothing else! However as luck would have it the one on my vehicle (an old vw camper) I had replaced with a unit which pretty much matches the design of yours, same chip driving it too.

I have found a couple of inherent problems though, not sure if you may have experienced this and found a solution....

1) while you can adjust the blink rate of your flashers, in the case of the 4 way hazard flashers it slows them down a fair amount too. I had to try and adjust the variable resistor (1M ohm as thats what I had to use at the time) so that hazards were just about flashing 60 blink/minute(maybe less) and that meant the indicators were about 120 blink/minute, which is just about on the limit of uk requirements, something worth noting and I am not sure if there is a way round this.

2) One other problem I have been having which seems odd is power leakage. When the indicators go 'off' during the blink cycle they are actually illuminated really dim, not something that was happening before I added the POT and it seems to happen when you slow the speed down its more apparent, any idea on this?


7 years ago on Introduction

Hello! michaelsteinbach

This is a nice instructable!..^^
I just wanna ask i have 3 pin flasher for LED which i bought from ebay I also replace my 4 halogen signal light bulb 10W each to LED and now my problem is when I signal to left or right the 4 LED'S lights up together. What's wrong?..Can you help me?..please..

Thank you very much...


7 years ago on Introduction

Good one ! I drilled a couple of small holes in the top of the unit and installed a trimpot, and glued it to the top. Works a treat. Cheers.

2 replies

Glad this worked for you.
Wish I could do it on my current car ('09 Mazda 6) but the flash rate is controlled by the BCM and doesn't seem to be programmable.

'Tis one of the bummers of the modern car. Would cost you a fortune to be able to programme these things. I am still driving a 1998 TD Isuzu twin cab, so don't have these problems. Will have to come into this century in a few years I suspect.....


8 years ago on Introduction

Thanks..... It work perfect in an OPTRA chevrolet... I buyed the 500k knob potentiometer and added a 2' long cable and installed in my dash.. to give it a strobe effect once in a while...
I calculated that in my car it was needed about 250k resistance to make it flash "normally"


8 years ago on Introduction

I have a 2000 Dodge 3500 4x4 diesel and I replaced my tail lights with LED types and of course because of the lighter load on the circuit it would flash rapidly. I went in and found the resistor on pins 4&5 and lifted one leg and then installed a PC mount mini 100k variable from Radio Shack p/n 271-284 cost $1.49 and tuned in the desired slower flash rate. In some cases you might need to replace the original resistor with a 200k and then add the 100k variable and then adjust, On my flasher the electrolytic cap is a 3.3uf as opposed the the 4.7uf in the schematics. Easy mod if you know how to solder, use a magnifier and have a steady hand.


8 years ago on Introduction

really? car flasher "relays" (not sure if it called a relay) have IC's in them? i didn't know that...


8 years ago on Step 6

OOPS!  You've highlighted the CAPACITOR, but the note says RESISTOR.  It's probably clear enough for most of us, but it COULD confuse the noobies!

Just sayin'…

Joe L

P.S. - Great "ible", I'll be doing this soon!


10 years ago on Step 4

Could you just use the equation for the frequency on page 4 of the datasheet for the IC and the value of the capacitor to determine what resistance you'll need for a given flash frequency? Or is that not going to work since the load of the indicator lamps is different?
Or, could you at least use that formula to determine what the initial flashing frequency was?

Either way, I'd still do it your way, using a potentiometer, because you can set it exactly how you like it.

1 reply

Reply 9 years ago on Step 4

Yes, you can calculate it and set the flash speed to your liking but I like the ability to adjust the speed easily with the pot.


9 years ago on Step 4

What would be needed to make the tail/turn signals sequential? I would like to add a 3 stage turn signal for each cycle.

1 reply

Reply 9 years ago on Step 4

It would be a completely different circuit I think. If your tails are separate from your turns, you could use some relay logic to take the signal from the blinker module and latch each light in turn then reset. You would have to add a timer to reset it after a couple second of non-blinking and if you do this with taillights that double at blinkers, the relays would cause the sequence to run on both sides if you tapped the brakes several times.

Mr. Rig It

11 years ago on Introduction

Excellent, I need to build an entire turn signal flashing unit for my atv. I can use this. Would you happen to have the schematics for such a unit? Good ible

3 replies
jongscxMr. Rig It

Reply 11 years ago on Introduction

are your re-building one from scratch or are you building a new one without a pre-existing signal? I'm just saying because if it's to replace the light-bulb with LEDs, but you have the signalling wiring in place, then this would work. If you're adding a turn signal to an ATV without one, then it would probably be better to use purpose-built components (I'm thinking a 555 based blinker would be simplest) than adapting car components to fit LEDs to fit IN an ATV

Mr. Rig Itjongscx

Reply 11 years ago on Introduction

It would be one from scratch. If it gets really complilcated then I think I will shy away from building one. But if it straight forward I could probably do it. I just need a simple plan.

dude if there's no real worry about flash speed (FLED aren't far of anyway) use a set of flashing LEDs these are LEDs with a tiny flash circuit inside then you switch them on like normal so make a grid of flash LEDs for the light shape hook up to a DPDT switch or possibly a triple throw trpile pole... simple stuff basically same switch but no complicated blink mechanisms or circuits.