Really Weird Rear Lights Issue

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jshectman

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My bet is a bad ground to that light assembly. A poor ground can act really wierd depending on how much current is drawn. Taillight filament is fairly low current ( not too bright), but the brake light is high current. Don't bother with fuses. However, pay attention to the brightness and how it changes when you apply brakes.
Another test of the BCM is to remove the sockets and check if the socke gets a ground from the tail light housing or if there is a ground wire coming into the socket. This will help you trace the source of the ground. Get a jumper lead (with clips) and provide a good bround to the sockets/assembly.
Your chart (which is very classy by the way) indicates you are testing with the socket both installed & removed from the housing.
This is hugely helpful - thank you! I will try this and check back when I can get down there to try it on my son's truck with him. Very much appreciated.
 

rickjkline

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I am just wondering if some previous owner got in to the BCM via Alpha OBD and changed something incorrectly with the tail light system?
My 2 cents worth....
 

Marshall

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Pull transmission, :rolleyes:


BTW check number of wires going to each light socket, back up should be 2 I presume, tail /brake light would be 3 or 4. then look for bad spots on the wiring harness along the frame to rear end, or someone butchering / mods.
Bo the brake , signal lights work as they should? Nice chart but hard to follow what's happening.
 
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Pull transmission, :rolleyes:


BTW check number of wires going to each light socket, back up should be 2 I presume, tail /brake light would be 3 or 4. then look for bad spots on the wiring harness along the frame to rear end, or someone butchering / mods.
Bo the brake , signal lights work as they should? Nice chart but hard to follow what's happening.
The brake lights work. The passenger tail light (same bulb as the brake) does not come on with the headlights but rather the reverse light comes on/stays in with the headlights instead.
 

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Wonder if someone was in the wiring harness doing a repair, and when putting it together, and mixed wire colors, ie color blind.

If everything else is correct, I would be tempered to snip wires on that light near the socket and switch them , and if it works , just tape it up, use solder, shrink tube and good tape, or water proof connectors.
I presume you have pulled the bulbs and checked the sockets make sure the connectors look good,

I think mine are LED, but not 100% sure on that , never had them out (2014 sport)
Now I am going to have a look , back up is likely just regular bulb.
 
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The brake lights work. The passenger tail light (same bulb as the brake) does not come on with the headlights but rather the reverse light comes on/stays in with the headlights instead.

You can verify the wiring colors and pinouts by looking up the connectors here:


I'd also look at the trailer tow wiring back there, make sure that hasn't been "improved" by the prev ownere
 
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Wonder if someone was in the wiring harness doing a repair, and when putting it together, and mixed wire colors, ie color blind.

If everything else is correct, I would be tempered to snip wires on that light near the socket and switch them , and if it works , just tape it up, use solder, shrink tube and good tape, or water proof connectors.
I presume you have pulled the bulbs and checked the sockets make sure the connectors look good,

I think mine are LED, but not 100% sure on that , never had them out (2014 sport)
Now I am going to have a look , back up is likely just regular bulb.
 
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Hey thank you!

First, Yeah the sockets are OK. You are suggesting to try switching the one single wire responsible for running lights (the one that turns the reverse light "on" at present instead of the tail light with the headlights), right?

Second, I wouldn't mind throwing some $ at the problem and installing LEDs but if the wires are switched I don't think that would solve it.
 
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You can verify the wiring colors and pinouts by looking up the connectors here:


I'd also look at the trailer tow wiring back there, make sure that hasn't been "improved" by the prev ownere
That schematic is hugely helpful! Ishould be able to see what colors are back there and if they're correct or not I think...?
 

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Hey thank you!

First, Yeah the sockets are OK. You are suggesting to try switching the one single wire responsible for running lights (the one that turns the reverse light "on" at present instead of the tail light with the headlights), right?

Second, I wouldn't mind throwing some $ at the problem and installing LEDs but if the wires are switched I don't think that would solve it.
Yes if everything else checks out , I would chop the two wires and switch them, after going over the wiring and looking for mods where you can get at.
you could do it with jumper wires as well with out cutting , if you pulled the bulbs and probed the sockets.
 
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Yes if everything else checks out , I would chop the two wires and switch them, after going over the wiring and looking for mods where you can get at.
you could do it with jumper wires as well with out cutting , if you pulled the bulbs and probed the sockets.
I am tracking that - I like that idea of testing with jumper wires, thank you so much! I will try it soon and report back what I find out. Hopefully I can get this solved with my son the next time I can get down there. Much appreaciated.
 

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Maybe I missed it, but what happens when you put the truck in reverse? You said the passenger side reverse light comes on with the headlights, but does it come on also when you put it in reverse?
 
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Maybe I missed it, but what happens when you put the truck in reverse? You said the passenger side reverse light comes on with the headlights, but does it come on also when you put it in reverse?
I'll double check with my son. I think when the headlights are off, the reverse light works as expected (comes on when put in reverse).
 

62Blazer

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I'll double check with my son. I think when the headlights are off, the reverse light works as expected (comes on when put in reverse).
Depending on what happens, it would make a difference in where to look for an issue.
If somebody got the wires mixed up or the sockets, I would expect the reverse light to come on as the headlight bulb.
 

Marshall

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If it also still works as well in reverse,
Then I don’t know WTH is going on
I was thinking that in rev, a red tail light would come on if wires where switched
Good Luck
 
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