Dodge Cummins Diesel Forum banner

1 - 20 of 32 Posts

·
Registered
Joined
·
21 Posts
Discussion Starter #1
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?

2003 2500 Quad Cab with the mighty 5.9 Cummins.
 

·
Registered
Joined
·
1,378 Posts
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/3rd-gen-powertrain/1909713-newbies-guide-your-new-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.
 

·
Registered
Joined
·
21 Posts
Discussion Starter #3
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.
 

·
Registered
Joined
·
21 Posts
Discussion Starter #4
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?
 

·
Registered
Joined
·
21 Posts
Discussion Starter #5
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.
 

·
Registered
Joined
·
577 Posts
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.
 

·
Registered
Joined
·
21 Posts
Discussion Starter #7
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.
 

·
Registered
Joined
·
5,355 Posts
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. :wink2:
 

·
Registered
Joined
·
5,355 Posts
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.:wink2:
 

·
Registered
Joined
·
5,355 Posts
Try look into the 03 FSM section 8 for wiring info.
 

·
Registered
Joined
·
21 Posts
Discussion Starter #11
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. :wink2:
That is the electric brakes...
 

·
Registered
Joined
·
21 Posts
Discussion Starter #12
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....
 

·
Registered
Joined
·
21 Posts
Discussion Starter #13
Are you talking about the right brake light on the truck?
There are the LEFT STOP LAMP & RIGHT STOP LAMP DRIVER outputs from FCM.:wink2:
Not the truck, the trailer. Appreciate the reply but I did state this....
 

·
Registered
Joined
·
5,355 Posts
Can you have turn signal light and brake light at the same time?
If yes then turn signal light and brake light cannot share the same circuit.
FCM controls the left and right brake lights but not the high center brake light.

FCM controls the turn signal lights on the truck and trailer separately.
Two relays is used to control the trailer.
I expect the same on the brake lights.
There is no specific trailer brake light info from diagrams so I think the trailer brake light should be hooked up to the high center brake light circuit which is coming from the brake light switch.
It’s just my thought.
 

·
Registered
Joined
·
5,355 Posts
Try this:
Without hitting the brake pedal, use a jumper jump the brake switch and the check the high center brake light and trailer brake light.....
 

·
Registered
Joined
·
5,355 Posts
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.
Turn signal lights and brake lights are two totally independent circuit.

Diagram 8W-54-5 and 8W-51-7
Brake Lamp Switch doesn't go to FSM.
And I think this is the circuit to the trailer tow connector for braking and brake lights.
Do the jumper test and tell me I'm wrong.:wink2:
 

·
Registered
Joined
·
5,355 Posts
An alternative test to the jumper test:
Hit the brake pedal and make sure you have a trailer brake light ON and center high mounted stop lamp/cargo lamp ON.
Then pull Fuse 24 to see if those lights go OFF.
 

·
Registered
Joined
·
21 Posts
Discussion Starter #18
Turn signal lights and brake lights are two totally independent circuit.

Diagram 8W-54-5 and 8W-51-7
Brake Lamp Switch doesn't go to FSM.
And I think this is the circuit to the trailer tow connector for braking and brake lights.
Do the jumper test and tell me I'm wrong.:wink2:
I will do your test after I get home from work. I'm pretty positive that pulling fuse 24 is gonna shut down all brake lights since you will kill the power to the brake lamp switch. The pinout for the switch is on 8W-80-12. I disagree with turn signals and brake lights being totally independent. In the tail light itself the turn signal and the brake light are the same filament in the bulb. 8W-80-101 shows the pinout for the trailer plugs. There is only left signal and right signal on either type of plug. No brake light wire. Every trailer I've ever worked on the lighting circuits are always yellow=left, green=right, brown=tail/marker, white=ground. Never a brake light wire. That is why foreign cars and some domestic with separate brake and turn signals at the back require that trailer converter box to bring the turn signal and brake lights back together for the trailer. Remember I changed those turn signal relays that are soldered into the PCB in the fuse box. That is what got my trailer turn signals working again. Now I have one brake light out that wasn't out before I changed the relays. The brake lights on the trailer have to be both the left turn and the right turn relays being driven high constantly since they are the same filament/same wire at the bulb. Something drives both those relays closed when you push the brake pedal. If you are turning right with the brake on then the left relay stays closed while the right relay clicks on and off to flash the signal. Going by the diagrams I don't see any other way for this to work. Now the brake lamp switch does send a sense to the ECM so I'm thinking that there is communication between the ECM and FCM which controls the two turn signal relays. I appreciate your thoughts. Made me keep digging into the diagrams that only reinforced my conclusion. If nothing else I hope this helps others since from what I've read elsewhere those relays have been a common problem with these trucks.
 

·
Registered
Joined
·
5,355 Posts
The brake lights on the trailer have to be both the left turn and the right turn relays being driven high constantly since they are the same filament/same wire at the bulb. Something drives both those relays closed when you push the brake pedal. If you are turning right with the brake on then the left relay stays closed while the right relay clicks on and off to flash the signal. Going by the diagrams I don't see any other way for this to work. Now the brake lamp switch does send a sense to the ECM so I'm thinking that there is communication between the ECM and FCM which controls the two turn signal relays.
Ok I didn't know that trailer brake light bulb uses the same filament for turn signal light and brake light. :spank:

Blinking brake light = Turn signal light >:)

It's the FCM drives those two trailer tow relays. :wink2:
 

·
Registered
Joined
·
5,355 Posts
I'm pretty positive that pulling fuse 24 is gonna shut down all brake lights since you will kill the power to the brake lamp switch. The pinout for the switch is on 8W-80-12. .
Diagram 8W-33-6 and 8W-54-5 and 8W-51-7:

There are three switches in the BRAKE LAMP SWITCH.
Pulling Fuse 24 will not shutdown all brake lights.

Brake light switch status is sensed by PCM.
PCM tells FCM to do the braking stuff.:wink2:
 
1 - 20 of 32 Posts
Top