only 3 brake light working

[batty1214]

SO, in the last two days I have been moved and I took over the gas shop at work.( I take care of the pickups and other "small" equipment)
Anyway, I have been fighting with a 2005 gmc yukon xl for a few days trying to get the brake lights to work. I have done the simple stuff..... fuses, light bulbs,and checked grounds, but I still dont have anything. I replaced the wiring spliter at the rear of the vehicle, and still don't have anything. arghhhhh..... I am hoping that one of you guys know what the **** is happening

[leolkfrm]

does it have one of those harness adapters on it because of the third brake light, to be able to hook up a trailer harness?

[batty1214]

the third brake light wire runs across the top of the cab, and into the light..... it shares the harness with the rear window defrost, it does have the factory plug or the trailer lights at the back.... and im getting power for the brakes there

[crabtruck]

You checked Both fuses, VEH-CHMSL 10A and STOP-FUSE 15A ?

If you really have metered the hot at the output of stop fuse,
then next check it's hot through the underhood-fuse-block
(think LtBlu at in/E7 and out/F4).

Next stop is rear-lamp-junction block, inC1-B and outC4-A&D.
Since both rear lamps out, I'm thinking it's open between splice blocks.

Don't think it's trailer wire related since the blunt cut set
runs off hi-stop fuse.

[Sneakysportsman]

brake switch at the pedal, first sign, third light works only.

[batty1214]

I wondered about the switch at the pedal...... but, it is working enough to work the third brake light. and yeah, I checked both fuses, and I replaced the splice block today and nothing happened. so Im kinda stuck there...

[crabtruck]

Don't replace - test.

1) it's hot at supply side stop fuse
2) it's hot at load side stop fuse

3) it's hot at underhood-fuse-block E7
4) it's hot at underhood-fuse-block F4

5) it's hot at rear-lamp-junction block C1-B
6a) it's hot at rear-lamp-junction block C4-A
6b) it's hot at rear-lamp-junction block C4-D

7a) it's hot at rear-lamp-bulb right
7b) it's hot at rear-lamp-bulb left

Somewhere along that path you are losing it.
Just a matter of checking with voltmeter
while someone steps on brake pedal
(unless you have a brick, useful for pedal AND windshield)

I'm guessing open will be found #2 to #5
Remember this has nothing to do with trailer wires if stock.
Not like the old styles, now everything gets its own spagetti.

[Sneakysportsman]

In the late 97 and up, gm had a recall on brake light switches, and the third light was the only light working when the tail lights failed. I had a 97 that was the third number on the lis of failures. And as always I tested and checked everything. Went down to the parts house and got a $20 switch and was on my way.

A trailer connector will stop only one side from working not both.

[batty1214]

I was in the process of checking power when quiting time came yesterday.... darn the luck. so monday, I'll check the other stuff with my meter......I have checked both fuses with the meter and have power on both sides. but, that isn't checking that I am getting power from the switch. any clue on how to check to see if the switch is bad?

[leolkfrm]

jump the connector to the switch!