Really Weird Rear Lights Issue

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jshectman

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Hey there, love this site. First post but been enjoying it on here a long time.

2015 Ram 1500 4x4 V6 Tradesman. Son just bought it used. Problem: In short, reverse light comes on/stays on instead of passenger rear running light when the headlights are turned on. The lower white light illuminates with the headlights instead of the upper red light. That's the long and short of it.

Mechanic checked all the switches, said they were fine. I put together this schematic below, but the gist is really what I wrote above. I'm not even sure what to check next. Anyone have any thoughts? I really appreciate it/thanks in advance.

Screenshot 2025-01-17 114345.png
 

mdc1990zr1

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I have seen this before. Sometimes it’s just the backup bulb in the running light socket and vice versa. It happens when changing bulbs and not paying attention to the socket it came out of. The sockets are usually the same, their placement is important
 
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jshectman

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I have seen this before. See one times it’s just the backup bulb in the running light socket and vice versa. It happens when changing bulbs and not paying attention to the socket it came out of. The sockets are usually the same, their placement is important
Thank you! I will try it - you mean swapping each wire that the sockets are attached to and/or maybe their position in the light box?
 

markabby

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Yeah original. I am hoping that's it - you mean each socket is plugged into the wrong wire, right?? Thank you!!!
each socket was plugged into the wrong hole of the tail light. i don't think the wires themselves were changed. I'm guessing some one had after market tail lights and before they traded it in, threw the OEM ones on and hastily snapped the sockets in.

just a guess
 
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jshectman

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each socket was plugged into the wrong hole of the tail light. i don't think the wires themselves were changed. I'm guessing some one had after market tail lights and before they traded it in, threw the OEM ones on and hastily snapped the sockets in.

just a guess
Crud, was hopeful that was it - but the bulbs are difference sizes and only fit into a large and a small hole...so not reversed at least in that manner.
 

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jshectman

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Sockets plugged into the wrong holes
Crud, was hopeful that was it - but the bulbs are difference sizes and only fit into a large and a small hole...so not reversed at least in that manner.

 

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mdc1990zr1

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I have seen this before. See one times it’s just the backup bulb in the running light socket and vice versa. It happens when changing bulbs and not paying attention to the socket it came out of. The sockets are usually the same, their placement is important
 

mdc1990zr1

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Crud, was hopeful that was it - but the bulbs are difference sizes and only fit into a large and a small hole...so not reversed at least in that manner.

Sorry, that sucks
Crud, was hopeful that was it - but the bulbs are difference sizes and only fit into a large and a small hole...so not reversed at least in that manner.

 
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markabby

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then i guess you have to start by pulling the fuse for the reverse light to see if it turns it off.

it might possibly be fuse #28 20amps (this is the trailer tow back up light fuse)

since the sockets are oriented correctly, you might have a simple short or, maybe just a blown fuse
 
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jshectman

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then i guess you have to start by pulling the fuse for the reverse light to see if it turns it off.

it might possibly be fuse #28 20amps (this is the trailer tow back up light fuse)

since the sockets are oriented correctly, you might have a simple short or, maybe just a blown fuse
Thank you so much. I wasn’t aware that blown fuses could cause this behavior of staying on rather than off. I’ll take a look. I know what to look for in a blown fuse. Any tips on what I’d look for in terms of a simple short?
 

markabby

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Thank you so much. I wasn’t aware that blown fuses could cause this behavior of staying on rather than off. I’ll take a look. I know what to look for in a blown fuse. Any tips on what I’d look for in terms of a simple short?
it could be a number of things....even a bad ground. you have to start at square one (the fuse) and go from there. While you're at it, you migh as well check other fuses too.

use this site to identify each fuse:

 

crash68

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then i guess you have to start by pulling the fuse for the reverse light to see if it turns it off.

it might possibly be fuse #28 20amps (this is the trailer tow back up light fuse)
The trailer lights are completely separate from the vehicle taillights. Each fuse for the trailer lights is just for that particular lighting circuit. The relays that control the trailer lights comes directly from the BCM.
 
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jshectman

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markabby

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The trailer lights are completely separate from the vehicle taillights. Each fuse for the trailer lights is just for that particular lighting circuit. The relays that control the trailer lights comes directly from the BCM.
the fuse diagram i gave him doesn't show a fuse directly related to the backup lights, only trailer backup lights.
i was hoping they were related, since i didn't see a fuse dedicated to reverse lights.

it could possibly be a bad relay as well.

also, my bad, i think i gave him the fuse location for a 2019, not a 2015.

ok...after much reading, the 2015 Ram does NOT have a fuse dedicated to the back up lights.

i found this: (who would have thought a simple back up light malfunctioning would be so complicated)

"The backup lights on a 2015 Ram 1500 are not directly controlled by a standalone fuse, but rather through the Body Control Module (BCM).

How the Backup Lights Work:
  1. Reverse Light Switch: When you shift into reverse, the reverse light switch on the transmission engages.

  2. BCM Signal: This switch sends a signal to the BCM, which then controls the backup lights.

  3. No Dedicated Fuse: Instead of a dedicated fuse, the BCM manages the power flow to the lights.









Troubleshooting Backup Light Issues:
  1. 1. Check the Reverse Light Switch:
    Ensure the switch is properly functioning and making contact when in reverse.

  2. 2. Inspect the BCM:
    If the switch is working, the issue might lie with the BCM or its wiring harness.

  3. 3. Scan for Fault Codes:
    Use a scan tool to check for any error codes related to the BCM or backup lights.

  4. 4. Check for Corrosion:
    Inspect the BCM connectors and wiring for any corrosion or damage.

  5. 5. Battery Reset:
    Try disconnecting the battery for a few minutes to reset the BCM, in case a software glitch is causing the problem.

If the issue persists, consider seeking assistance from a qualified mechanic or electrician to diagnose and repair the problem. "
 
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dpinvidic

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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.
 

Bob Hoops

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Hey there, love this site. First post but been enjoying it on here a long time.

2015 Ram 1500 4x4 V6 Tradesman. Son just bought it used. Problem: In short, reverse light comes on/stays on instead of passenger rear running light when the headlights are turned on. The lower white light illuminates with the headlights instead of the upper red light. That's the long and short of it.

Mechanic checked all the switches, said they were fine. I put together this schematic below, but the gist is really what I wrote above. I'm not even sure what to check next. Anyone have any thoughts? I really appreciate it/thanks in advance.

View attachment 558795
 

Bob Hoops

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I had a problem with my 2008 where the 4 pin trailer connector got full of mud and water and caused a weird problem. When you hit the brake pedal it would cause the front passenger side marker light to illuminate. Dealership cleaned the connector and had to replace the BCM because it shorted internally.
Then on my 2017 I had a similar problem when I hit the brakes it caused the front drivers side headlight to go out. First thing I did was check the trailer connector but it was clean. I bought this used and noticed the former owner had removed a bed cap before he traded it in. Come to find out they butchered the wires when they installed the cap by using non weather resistant barrel connectors on the tail light wiring and they were pinched and intermittently shorted to the frame. This caused an internal short of the BCM. This time I bought a used one on eBay, matched to my exact model, and replaced it myself. Not easy to to get at but worth saving the money.
 
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