Need help from the brain trust that is the "Beeb" - GMC Sierra 3500HD blinker issue

So my left rear brake light/blinker is not working. The dash tells me so and I have confirmed.

Troubleshooting so far:

• Checked bulb, replaced - Nope
• Checked fuse (all of them) - Nope
• Checked wiring - Nope
• Tried locating flasher unit and it seems there isn't one that I can find, the blinker "tick" noise is actually made by the speakers - Nope
• Exhausted my google and youtube Fu with nothing specific on my issue - Nope
• Owners manual - Nope

Its driving me nuts. The tail light has a total of 3 bulbs in each assembly. Upper blinker light (this works fine) blinker brake light combo in the middle (brake light works) lower parking light works.

The error does not appear with the hazards on but the bulb doesn't blink.

Much appreciate any other tips or ideas.

I have to think there is a still some sort of flasher relay I am missing but I just cannot source it.

 

blinker fluid...
#duh

 

Who uses blinkers? You must be in the Midwest

 

Will the 4 way flashers still trigger that bulb to light?

 

GMC.

Not BMW.

If it’s like the Dodge, all the lighting is controlled by the PCM. First step is verifying continuity on the wire to that lamp. Trace the wire through the harness (pain in the ass) or get the schematic and determine the pinout. If the wire is good, you’re probably looking at dealership diagnosis of the PCM.

 

Yep...work backwards from the bulb. Verify powers, grounds and controls at the socket. Verify powers, ground and controls at the BCM/PCM. If the BCM/PCM is triggering the correct control and you've got nothing at the socket there is a break in the wiring somewhere between those two points.

You'll almost certaintly need a wiring diagram to get much farther than that.

 

If the bulb never blinks, and we know you've changed the bulb already, see if the connector is dirty. I've seen that a few times. It's usually an intermittent problem, but if it's dirty enough, it will just stop.

 

My recommendation is go check the beeb at silveradosierra.com

 

Thats what I am thinking as well

In process of sourcing a wiring diagram is on my list for today's tasks.

Connector is clean, so it the harness connection back to where it plugs into a small transfer box

Thats where my google fu has failed, been on a few other forums and nothing

specific is popping up.

 

This might be a pain... I don't have a fault isolation manual but I would swap the leads back at your transfer box between left and right brake light assembly. Use your multimeter and check voltage at back of bulb connector. Do you have 12v? Check leads where bulb touches. Start truck and turn left blinker on? Check voltage at both?

 

What year?

depending on year, turn signal switch is a possibility.

VIN?

 

2015
1GT422CG7FF508133

 

Or you could follow the Broome Approach and sell it and buy a new one.

 

Sell it? Shit he normally just gives shit away

 

In those situations I always swap components to check if it works on the other side. Lamp or lamp assembly, etc. That way at least you know you’re working with components that are functional

 

Probably the under hood fuse box. Will pull a wiring diagram when I get to the shop and see what all is involved.

 

Check fuse F46dA, right side inst panel box first. If ok, check for codes.

 

A popped fuse is merely a symptom of the problem. If you blew a fuse, you still have work to find out the the fuse blew.