Strange Trailer Light Problem - Page 2 - Dodge Cummins Diesel Forum
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post #13 of 32 (permalink) Old 08-20-2019, 03:26 PM Thread Starter
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Originally Posted by PWong View Post
Are you talking about the right brake light on the truck?
There are the LEFT STOP LAMP & RIGHT STOP LAMP DRIVER outputs from FCM.
Not the truck, the trailer. Appreciate the reply but I did state this....

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post #14 of 32 (permalink) Old 08-20-2019, 04:50 PM
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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.
Itís just my thought.

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post #15 of 32 (permalink) Old 08-20-2019, 04:55 PM
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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.....

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post #16 of 32 (permalink) Old 08-21-2019, 07:52 AM
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Quote:
Originally Posted by smeek View Post
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.

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post #17 of 32 (permalink) Old 08-21-2019, 07:59 AM
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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.

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post #18 of 32 (permalink) Old 08-21-2019, 09:51 AM Thread Starter
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Quote:
Originally Posted by PWong View Post
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.
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.
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post #19 of 32 (permalink) Old 08-21-2019, 11:01 AM
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Quote:
Originally Posted by smeek View Post
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.

Blinking brake light = Turn signal light

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

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post #20 of 32 (permalink) Old 08-21-2019, 11:26 AM
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Quote:
Originally Posted by smeek View Post
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.

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post #21 of 32 (permalink) Old 08-21-2019, 11:42 AM
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Quote:
Originally Posted by smeek View Post
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.
If your left trailer brake light is working while the right trailer brake light is not then FCM is the issue, not communication.

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post #22 of 32 (permalink) Old 08-21-2019, 12:06 PM Thread Starter
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Quote:
Originally Posted by PWong View Post
Diagram 8W-33-6 and 8W-54-5 and 8W-51-7:
8W-33-6 shows the sense to the PCM is pulled low when the brake pedal is not pushed. So I will give you that pulling fuse 24 may not cause all the brake lights to go out. But the high mount will go out for sure.

8W-54-5 shows how the electric trailer brakes work.

8W-51-7 shows the normally open contact between 1 and 2 pulled out of the 8W-33-6 diagram. Doesn't show anything different for that circuit.
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post #23 of 32 (permalink) Old 08-21-2019, 12:23 PM
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I think we just got the operation of the trailer tow circuits straight out.

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post #24 of 32 (permalink) Old 08-21-2019, 12:23 PM Thread Starter
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Quote:
Originally Posted by PWong View Post
If your left trailer brake light is working while the right trailer brake light is not then FCM is the issue, not communication.
PCM probably tells the FCM to close the left and right turn signal relays to activate the trailer brake lights. Can't really say its not communication unless someone knows how this is all coded. For example PCM could say "hey FCM, the brake pedal is pushed so close both your turn signal relays". Or it could say "close the right relay but the left turn signal switch is active so make that flash". Or it could be the PCM says "brake pedal is pushed, close relays" but the FCM knows that a call for one or the other turn signal is active so I'll do my thing with the relays, "thanks PCM for letting me know that the brake pedal was pushed". Isn't this fun....
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