Question about GMC Sierra
Try this possibilities to locate the fault.
First get your voltmeter or 12v test lamp out. Check the 30 amp Brake (#2) maxi fuse in the under hood junction box.Fuse should have 12v at both sides at all times.
If okay proceed.
Go to the brake light switch. The orange wire should have 12v at all times. The white wire should have 12v when stepping on the brake.
If okay, proceed.
Next thing is the brake lamp relay in the underhood fuse box. Swap the relay with one that looks just like it and recheck the brake light operation. If the relay is no good replace it.
If the relay is swapped and still not working, see if you can feel the relay clicking when someone else steps on the brake pedal.
batt is power from the maxi fuse 12v all times
sls is the white wire from the brake switch 12v with brake applied.
gnd is body ground 0v all times
mfs goes to the multi function switch 12v with relay activated
the circuit is simple—power from the brake light switch white wires turns on the relay which then sends power back to the multifunction switch for distribution to the rear lights.
if you jump terminals batt and mfs the brake lights should come on.
If you have no power at terminal sls with the brake applied then you have a wiring fault white wire between the brake light switch and the relay
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Posted on Aug 17, 2010
SOURCE: tail lights not working. fuses
do the front marker/park lights work if not you could have a bad light switch on the tail light side of the switch.you need to check the brown wire at the back of the truck in the wiring harness as this is the tail light wire that feeds the bulbs.if you have power there then you will have to check for power at the socket,if no power at the socket but were you check the wire then there is a problem between the two that you will have to check out.if there is no power when checking the wire then you will have to check for lose of power between the switch and rear wiring.make sure the fuse is good by using a test light and not by looking as sometimes they look good but don`t work.
Posted on Jul 11, 2011
Testimonial: "Thanks a million!!! problem solved, just a simple loose connection at switched saved me $$$$ from taking it to the shop. thanks for the info!!"
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