Custom DRL circuit (automotive) strange behavior; fix my circuit?

Thread Starter


Joined Jun 28, 2020
Hi. First time post. I've practically given up despite being so close.
I've tried building a relay series to control DRLs on a vehicle that did not come with them factory.
This circuit turns off the appropriate DRL when the turn signal comes on, and turns off both with hazard or headlights.

The problem I am having is that when I turn the signal on in either direction, it's fine. If I flip the signal to the other direction, both left and right turn signals will run. If I turn the signal off and flip it back on in the same direction it will work properly. Again, switch to the other direction and both sides will run.

It makes no damn sense and has probably already confuse me anyone reading this. I may have to make a video.

I know it's definitely my DRL circuit because when I disconnect it the signals work fine.

Note: the combination flasher on this vehicle uses N.O. relay operation, so the turn signal stalk actually removes power to activate the turn signal circuit. There is some resistance in the signal stalk causing a 9v instead of the expected 12v, but that only affects the relay sensitivity and I have switched to 9v relays for this purpose. I am using the "sense" wires direct from the signal stalk and not post-flasher otherwise the DRLs would just flash opposite the turn signals.

Right now I just don't understand where this weird feedback is coming from that causes both turn signals to run unless I select the same signal direction twice. Like, if I do a tap-then-lock on the signal stalk .. no problem.

Any ideas.. Any additional information needed to help me with this?



Joined Jul 16, 2008
I looked and read this over and over trying to understand what you are tryng to do. I still may not see the light.
Something seems to be awry with your diagram. You say you want the lights to turn off if hazard lights are on or headlights are on. Just to make sure this is correct, the hazard and headlight relay need to be normally closed. So the lights have power to them all the time, not neccessarily on. When relay headlight or relay hazard turns on, then the lights have no power. All seems to work well until you energize either left or right relay. I bet even headlight or harzard relay would cause the same weirdness. Here is what i think is happening and its easy to fix. Once you energize a relay, it hangs and wont release. The reason is, each relay needs a diode, say a 1n4007 which is my coice. The diode goes across the coil winding on the relay. the line on the diode (cathode) on the side where 12 volts or 9 come connect to the coil. not the actuator. To the coil. see attachment. It will work after that. Just add a diode to each relay. Its standard practice when building a circuit using relays. It just wont work right until you do. Makes sense now. You had to quickly bump the turn signal to get it to release the relay.


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