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Ask GM Tech (Cam) Your Own Question
GM Tech (Cam)
GM Tech (Cam), GMC Mechanic
Category: GM
Satisfied Customers: 5787
Experience:  GM Grand Master Technician 2007. 14 years experience.
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Brake lights aren't working third light is JA: Sometimes

Customer Question

Brake lights aren't working third light is
JA: Sometimes things that you think will be really complicated end up being easy to fix with GM. The GM Mechanic I'm going to connect you with knows all the tricks and shortcuts. Tell me a bit more about what's going on so he can help you best.
Customer: When applying brakes rear lights don't come on third brake light does come on
JA: Are you hoping to fix this yourself?
Customer: Yes
JA: Great! What have you tried so far?
Customer: Changed head lamp switch nothing changed. looked at brake switch but when pedal is pushed third light works so assuming the switch is ok
JA: Is there anything else the GM Mechanic should be aware of about your GM?
Customer: Not at this time
JA: OK. Got it. I'm sending you to a secure page on JustAnswer so you can place the $5 fully-refundable deposit now. While you're filling out that form, I'll tell the GM Mechanic about your situation and then connect you two.
Submitted: 1 year ago.
Category: GM
Expert:  Jared D replied 1 year ago.
Hi,What year and model is your truck?
Customer: replied 1 year ago.
2005 gmc yukon
Customer: replied 1 year ago.
Are you still there?
Expert:  Jared D replied 1 year ago.
Sorry, I'm looking over the schematic for the brake light system. Do you have a multimeter to check for voltage at the connector?
Customer: replied 1 year ago.
I do
Customer: replied 1 year ago.
My question if the center brake light works would the brake switch be bad? I can also check the junction box (tail lamp circuit board if you think
Expert:  Jared D replied 1 year ago.
Sory, for some reason GM lists the lighting connector 1-50 e-i. They can't just label the diagram "brake lights".
Expert:  Jared D replied 1 year ago.
Ok, found it. So the 3rd brake light has its own circuit. According to the diagram the best place to look for a break in the circuit is starting at the instrument panel fuse box. Do you have 12 volts at the 15 amp VEH STOP fuse with the brake pressed?
Expert:  Jared D replied 1 year ago.
From there you should check for 12 volts on the light blue wire at the rear lamp junction block. It is a small electrical connector block under the rear bumper, mounted on the frame. You should see 12 volts with the brake pressed.
Customer: replied 1 year ago.
When checking the stop fuse should I be checking across the two fuse plugs
Customer: replied 1 year ago.
I'm not getting 12 volts more like 2 volts
Expert:  Jared D replied 1 year ago.
The fuse should have a tiny metal nub on it where you can touch the positive side of your probe. The negative side of the probe goes to any ground.
Expert:  Jared D replied 1 year ago.
2 volts? That's a little strange. Try pulling the fuse out and checking each side of the fuse socket. One side will show 0 volts, the other will should show 12 volts with the brake pressed.
Customer: replied 1 year ago.
No it's 12 volts
Expert:  Jared D replied 1 year ago.
Ok. With the fuse installed can you get 12 volts at both sides of the fuse?
Customer: replied 1 year ago.
Yes
Customer: replied 1 year ago.
I'm going to check the light blue wire
Expert:  Jared D replied 1 year ago.
Ok. Good. Now you need to move to the rear junction block. It's underneath the car in the rear. There is a light blue wire coming from that fuse you just checked to that junction block. Then from that junction block it runs to each of the tail lights.
Customer: replied 1 year ago.
On the junction book there are 4 different plugs which one should I be checking? It looks like there are blue on at least 2 of the plugs
Customer: replied 1 year ago.
1 Plug is black, 1 plug is blue, 1 plug is brown, the last is gray
Expert:  Jared D replied 1 year ago.
C1- it's one of the connectors on the side. The one closest to the side with a taper. You can see that the whole thing slides into the metal bracket, the connector you want is on the side that slides in first.
Expert:  Jared D replied 1 year ago.
The blue one.
Customer: replied 1 year ago.
There are 2 light blue wires on the blue plug C1. One is toward the outside and the other is next to it.
Customer: replied 1 year ago.
Looks like the one inside is the right one but I'm only getting are 8 volts
Customer: replied 1 year ago.
Could the junction box be bad
Expert:  Jared D replied 1 year ago.
The blue wire should run straight up to the dash from the blue connector. If you are only getting 8 volts then there is a break in that wire as it runs up the frame to the fuse box.
Customer: replied 1 year ago.
What fix would you suggest if the wire is broken
Expert:  Jared D replied 1 year ago.
I would start at the back of the back of the fuse box and work my way forward. It's most likely pinched or worn through somewhere in the cab before it goes out of the car. However, I have seen rodent and one time dog damage to wiring outside of the cab as well.
Expert:  Jared D replied 1 year ago.
When you were testing for volts at the blue connector, you had the connector unplugged from the junction block right?