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Possible to rewire brake switch so that it works when ignition is off?


timmerk

Question

Is it possible to rewire brake switch so that it works when ignition is off on a 2008 GMC Sierra? I see the switch has 4 pins/contacts, but I couldn't find anything about the brake switch in the service manual (maybe I just missed it?). I'm guessing there is not a CANBUS transceiver inside the switch because of the large size of contacts. Does anyone know the pinouts? I'm guessing one will be an ACC or ignition power supply, and that I could just change it to an always on supply. Any suggestions?

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For my Viair compressor switch, I used that black box next to your left foot when driving. It's the power supply for one half of the instrument cluster - I found an always-hot pin, and used that to power the relay.

 

Could probably do the same with the brake switch - just need to know which wire is 12v, cut it, and wire in the hot one from that box. Would have to see what, if any, odd issues arise in the instrument cluster - the brake lights use quite a bit more juice than a relay does. Might be better to tap into the fuse box. They used to make piggyback connectors for the old ATO/ATC fuses - not sure if they make anything like that for these modern mini fuses.

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They always have to take a simple thing and make it complicated. 4 wires in a friggin brake switch ... :nonod:

 

Now that 12v reference may throw a wrench into the mix. Cruise doesn't matter when your parked. Not sure why it needs a 12v reference, or what will happen if it's not there when the key is off.

 

I would try moving just that one wire (pin B) to 12v direct, making sure it's fused at some point, and see what happens. Switch shouldn't care where the 12v comes from. It's just the reference line that's the unknown to me. Maybe it throws a body code if it's below say 11v or something, indicating a wiring issue.

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I finally found the pinouts in the manual:

 

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I suppose I could swap the Ignition supply with an always-on one, but I'm afraid of the power draw while off since the stop light switch has a bunch of circuitry in it for some reason.

 

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post-47500-0-87537500-1468812990_thumb.png

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