Strange Trailer Light Problem - Dodge Cummins Diesel Forum
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post #1 of 32 (permalink) Old 04-29-2019, 11:13 AM Thread Starter
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Strange Trailer Light Problem

Hello to the forum. I have what I think is a strange issue with my 7 pin trailer plug. All lights work on the truck, it's just with the trailer connected. The left turn signal works but the left brake light does not. I have read about the relay issue in the tipm but all references that I have found say that neither the brake light or the turn signal on the given side doesn't work. I have searched for diagrams but can't seem to find one that shows how the brake light and the turn signal work together on the same wire. I'm thinking that maybe the relay in it's off state allows the brake light to function and when the turn signal is active them the FCM tells the relay to open and close making the light blink but I have found nothing to show that that is how it works. However if that is the case then it doesn't explain why only the brake light on the left isn't working. Any thoughts or experience out there?

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post #2 of 32 (permalink) Old 04-29-2019, 11:35 AM
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Here's a link to the Newbies Guide for your new truck. It contains a Factory Service Manual that may help your to troubleshoot your electrical issues.

https://www.cumminsforum.com/forum/3...you-truck.html

Have you tested all functions on the 7 pin plug with a test light to make sure everything is working on the truck? That would isolate the problem to the truck vs the trailer. Bad trailer grounds and corroded sockets, etc., can raise havoc with trailer lighting. I had a similar issue with my 05. Everything worked except the left turn signal signal on the trailer. After much troubleshooting I ended up having to replace the TIPM as it turned out to be a non-serviceable bad relay.

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post #3 of 32 (permalink) Old 04-29-2019, 02:54 PM Thread Starter
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Quote:
Originally Posted by MDSWA View Post
Here's a link to the Newbies Guide for your new truck. It contains a Factory Service Manual that may help your to troubleshoot your electrical issues.



Have you tested all functions on the 7 pin plug with a test light to make sure everything is working on the truck? That would isolate the problem to the truck vs the trailer. Bad trailer grounds and corroded sockets, etc., can raise havoc with trailer lighting. I had a similar issue with my 05. Everything worked except the left turn signal signal on the trailer. After much troubleshooting I ended up having to replace the TIPM as it turned out to be a non-serviceable bad relay.
Thanks! That link is worth money. What a wealth of info!
I have checked the plug. The problem is within the truck. I have also seen the problem when a different trailer was connected. I have seen youtube videos where people have changed out those soldered in relays. I do electronics so that is within my skill level to do. Again all the talk that I have read about that relay being the problem refers to both the brake light and the turn signal being out. It is only the brake light on the left that is out. They both work via the same wire and pin connector in the plug.
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post #4 of 32 (permalink) Old 08-20-2019, 09:07 AM Thread Starter
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So here is where I'm at with this now. Took me a while to get back to it. I have a 2003 Ram 2500 5.9 cummins. All rear lights on the truck work fine. Problem is trailer rear lights. Started as left turn signal was out but all brake lights worked. Go to the fuse box and you could hear the relay clicking with the right turn signal but no clicking with the left turn signal. I am a electronics/electrical tech so tearing apart the fuse box and determining the relay to be bad was simple enough. i changed all three relays that are soldered into the PCB. Re-coated the PCB with waterproofing and reassembled. Now both turn signals work but the right brake light does not. I'm pretty sure the brake lights work thru the same relays. Relays are energized continuous to energize brake lights. Relay is pulsed to make turn signal flash. This makes both these situations odd that each time I had one or the other not working. Thoughts?
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post #5 of 32 (permalink) Old 08-20-2019, 09:11 AM Thread Starter
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BTW: if you read the first post I described the original problem as opposite. That was the case at the time of posting then later it switched to problem just described without me doing a thing to it. Maybe that is a clue as to what the heck is going on.
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post #6 of 32 (permalink) Old 08-20-2019, 09:21 AM
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Of all of the trailer light circuit issues on the early 3rd gens and the IPM, I have not heard of this specific issue. Though, very few people attempt to actually service the IPM themselves. Is there any chance the relays you used to replace the OEM are different somehow that could cause this? I can't imagine but I'm just trying to think what is a constant and what is a variable in this situation.

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post #7 of 32 (permalink) Old 08-20-2019, 09:34 AM Thread Starter
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The relays are the same except the originals were rated at 20 amp and the new ones are 30 amp rated. Same brand and form factor. Look identical sitting side by side just a 30 in the part number instead of a 20.
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post #8 of 32 (permalink) Old 08-20-2019, 11:34 AM
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Quote:
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it's just with the trailer connected. The left turn signal works but the left brake light does not.

can't seem to find one that shows how the brake light and the turn signal work together on the same wire.
For the trailer tow turn signals there are TRAILER TOW LEFT & RIGHT TURN RELAY which are controlled by FCM.

On the other hand the trailer tow brake light circuit starts from Fuse 24 then to Brake Lamp Switch (TN//RD and WT/TN wires) then to tow connector. There is no relay or FCM involved.

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post #9 of 32 (permalink) Old 08-20-2019, 11:45 AM
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Quote:
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Now both turn signals work but the right brake light does not.
Are you talking about the right brake light on the truck?
There are the LEFT STOP LAMP & RIGHT STOP LAMP DRIVER outputs from FCM.

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post #10 of 32 (permalink) Old 08-20-2019, 11:51 AM
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Try look into the 03 FSM section 8 for wiring info.

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post #11 of 32 (permalink) Old 08-20-2019, 02:16 PM Thread Starter
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Quote:
Originally Posted by PWong View Post
For the trailer tow turn signals there are TRAILER TOW LEFT & RIGHT TURN RELAY which are controlled by FCM.

On the other hand the trailer tow brake light circuit starts from Fuse 24 then to Brake Lamp Switch (TN//RD and WT/TN wires) then to tow connector. There is no relay or FCM involved.
That is the electric brakes...
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post #12 of 32 (permalink) Old 08-20-2019, 02:24 PM Thread Starter
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Try look into the 03 FSM section 8 for wiring info.
8W-54-2 shows the right and left turn signal going through their individual relay then out to the 7 pin add on connector and the 4 pin connector. The relays are driven by the FCM. There is no wire for brake lights so they have to work through the green and yellow turn signal wires as they have for as long as I've been around. That would mean that the FCM has to drive those relays closed when the brakes are applied. However even looking at the pin out for the FCM socket there is nothing labeled as input from the brake switch. This is very peculiar for sure....
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