Strange Trailer Light Problem - Page 3 - Dodge Cummins Diesel Forum
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post #25 of 32 (permalink) Old 08-21-2019, 11:26 AM Thread Starter
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Oh how I love my classics with point to point wiring.....

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post #26 of 32 (permalink) Old 08-21-2019, 11:38 AM
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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....
Diagram 8W-52-2:

FCM gets the "turn signal command" directly from INSTRUMENT CLUSTER, not from PCM or ECM.

PCM just sends the "brake switch sense " to FCM.

FCM gets the "turn signal command" and "brake switch sense" and make decision on what to do with the two relays.

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post #27 of 32 (permalink) Old 08-21-2019, 11:44 AM
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PCM says "brake pedal is pushed." 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".
This is the case.

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post #28 of 32 (permalink) Old 08-21-2019, 12:06 PM Thread Starter
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Diagram 8W-52-2:

FCM gets the "turn signal command" directly from INSTRUMENT CLUSTER, not from PCM or ECM.

PCM just sends the "brake switch sense " to FCM.

FCM gets the "turn signal command" and "brake switch sense" and make decision on what to do with the two relays.
Yes there is sense inputs from the brake pedal to the ECM and the PCM and the Cluster and the anti-lock module. There is turn signal inputs to the Cluster which communicates with all the above also. My point was that there is communication going on all over the place so I believe it is impossible to rule out a communication error although I'm not leaning toward that. But the brake lights worked a week ago and only one turn signal did not. You can hear the relays clicking with the turn signals and there was no clicking with the right turn signal. I changed both the relays and now there is clicking with both turn signals and both work on the trailer. Now one brake light does not. I'm tearing the fuse box down again and checking everything out. There has to be something stupid causing this...
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post #29 of 32 (permalink) Old 08-22-2019, 06:44 AM
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Oh how I love my classics with point to point wiring.....
Convert it, best thing you can do, keeps a short in truck or trailer from killing IPM, FCM and or EMIC.

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post #30 of 32 (permalink) Old 08-22-2019, 01:51 PM
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post #31 of 32 (permalink) Old 08-23-2019, 08:00 AM Thread Starter
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Ok, I'm in a major internal fight with myself since I'm a "gotta fix it" and "gotta solve the problem" person. I have read a lot of info on the weakness of the TIPM in relation to the trailer harness. I am gonna convert it and be done with it. Since my harness already has an issue I will have to take all the signals from the rear tail lights. Will take a little fancy relay work since the brake lights and turn signals are on separate bulbs on the truck. Did toss around using an arduino and let him do the work but relays will be faster I think. Drawing it up right now....Better yet just found this. This is just the brake/turn signal portion. Another relay for the parking light portion. Fuses will be on the outputs and the 12V input.
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post #32 of 32 (permalink) Old 09-06-2019, 08:23 PM
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OKay, just stopped in because I have a 2004 Ram 2500 that keeps blowing fuse 24. I think the board under the fuse box is to blame.
Anyway, no one has mentioned these relays that flash brake lights and run on the same filament all have to go through the multifunction switch. There are front turn signals too.

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