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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Does the third brake light work? Try wiggling the turn signal switch while applying the brakes and have someone check to see if the brake lights come on or flicker.If they do the turn signal switch/multifunction switch is bad and needs replaced.
Check the relays....
Check For Poor GROUND ,Just Behind the Driverside Rear WHEEL ON the OUTSIDE or BOTTOM Of Frame RAIL. It is Common Forthis Ground to Corrode upp And Interupt the Rear Lights in 1 of Many Ways your Problem is 1 of the ways and
SOURCE: headlight replacement 1998 GMC Sierra
there are two rods that slide down from the top. you have to turn to get them out of clip then pull up. once both are out then headlight can be removed.
SOURCE: 1998 GMC Sierra- runs rough then stalls when
had this problem on my neighbors 98 chevy truck with the same engine !!
it turnd out too be the fuel pressure regulator,you have too pull the upper intake too replace it!!
SOURCE: wiring diagram for 1996 gmc sierra brake asembly
The brake light switch is at fault. Get a new one and replace it. The location is at the brake pedal area mounted on a bracket with a plunger stud that rest on the brake pedal's arm the pad is attached to
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