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33 posts


#256042 11-Sep-2019 20:58
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I’m trying to connect an accessory to my Hazard Light switch that I want to turn on at the same time as the Hazard Lights.


The Hazard Light has switch connection has 4 leads two that appear to be ground another one that is +12V when the headlights are on to illuminate the switch and another (pink) that is +12V when the Hazard switch is off. This lead drops to 0V on pushing the switch.


I was thinking of using a NC Relay and running it from this pink lead as well as ground to energise the relay, the only problem is energising it in this manner switches the Hazard lights on as the coil in the relay is making a circuit the same as if the Hazard switch is pushed.




 Are there any clever ideas to get around this, adding diodes or similar?


Hope I’ve explained it well enough

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Uber Geek


  #2315260 11-Sep-2019 21:10
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Your relay needs one side of the coil connected to a battery feed with the other side of the coil via a diode (to stop the current back feeding) to the pink wire. If you've describe it properly that should work.



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648 posts

Ultimate Geek

  #2315261 11-Sep-2019 21:12
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I assume this is a late model car as it sounds like the switch earths an input to a ECU/BCM, rather than the older system that connects directly to the flasher unit and lights.


What make and model?


You could possibly cut the pink wire near the switch and connect the cathodes of a couple of 1N4007 diodes to the switch side. The anode of one diode goes to the pink wire that was cut. The anode of the other diode goes to the relay coil. The other side of the relay coil would go to a battery positive supply. The diodes would prevent the relay from feeding back into the ECU/BCM.




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  #2315264 11-Sep-2019 21:19
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Correct, It's a 2005 Nissan Van.


Yes I do believe it has a BCM module, it seems to be the only thing that makes sense that a 0V turns on the Hazards.

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  #2315265 11-Sep-2019 21:20
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Have you considered the effect on battery life as the relay would be energised the vast majority of the time?

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  #2315277 11-Sep-2019 22:08
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Have you considered the effect on battery life as the relay would be energised the vast majority of the time?

Not really although wouldn't think the coil would use much power?

Wdat about if I did it as k1w1k1d suggested, the battery positive could he run off an switched accessory feed so the coil wouldn't be energised when the vehicle hle was off.

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Ultimate Geek

  #2315355 12-Sep-2019 07:28
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Relay won't be on until the hazard switch is turned on, so no current flow through coil.

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  #2316582 12-Sep-2019 16:32
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Relay won't be on until the hazard switch is turned on, so no current flow through coil.





Thanks @k1w1k1d It worked perfectly what you suggested


Thanks to everyone else too :)


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Ultimate Geek

  #2316672 12-Sep-2019 17:38
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Glad I was able to help.



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