Trailer Lights Gurus--Got a Question.

[CausticUrbanCoast]

MY 2005 Chevy has no flasher or control other than turnsignal switch, and all truck body lighting is controlled by PCM (power control module) and there is a complete set of different fuses and wiring harness (from body lights) to rear of truck for trailer lighting... Wondering if PO truck may have same setup?

Sounds like your truck had an option for towing if the harness is separate.

My F350 has the factory towing kit; the towing fuse box lives in the engine compartment on the drivers side and has harness jacks under the dash for the controller and at the end of the frame that you are supposed to utilize to prevent splicing.

For this truck, I have to have my lights on for trailer tail lights and trailer markers since that is the way Ford wired it. Brakes and signals work as they should but the police prefer the taillights and license plate light on when moving.

 

[IndyJay]

MY 2005 Chevy has no flasher or control other than turnsignal switch, and all truck body lighting is controlled by PCM (power control module) and there is a complete set of different fuses and wiring harness (from body lights) to rear of truck for trailer lighting... Wondering if PO truck may have same setup?

So you are saying that instead of a simple 12v DC circuit going to the lighting with dedicated wiring for Running , RH Sig, LH Sig and Neg/Grnd that a single wire is used with some sort of signal from the computer telling which lamps to light? Does the "complete different fuses and wiring harness" also use this computer controlled lighting instead of dedicated DC wiring? Is there a transducer available for those that have standard trailer wiring?

 

[Bearsixty7]

So you are saying that instead of a simple 12v DC circuit going to the lighting with dedicated wiring for Running , RH Sig, LH Sig and Neg/Grnd that a single wire is used with some sort of signal from the computer telling which lamps to light? Does the "complete different fuses and wiring harness" also use this computer controlled lighting instead of dedicated DC wiring? Is there a transducer available for those that have standard trailer wiring?

Most pickup's today have similar, The on board computer may control the signals but they still go through 7 individual wires.
There is a set of fuses that the signals go through before going to the trailer connector receptacle at the back of the truck. This way if there is an issue with the trailer lights or wiring, it only blows those fuses and the pickup's lights are still fully operational.

 

[IndyJay]

Most pickup's today have similar, there is a set of fuses that the signals go through before going to the trailer connector receptacle at the back of the truck. This way if there is an issue with the trailer lights or wiring, it only blows those fuses and the pickup's lights are still fully operational.

Understood, but DLM said that if his "truck lighting is controlled by 'computer' (ECM/PCM/BCM) getting trailer lamps to work is whole different ball game than a non computer controlled (old school) system"....

I was wondering how this changed how you troubleshoot trailer lighting.

 

[aczlan]

Understood, but DLM said that if his "truck lighting is controlled by 'computer' (ECM/PCM/BCM) getting trailer lamps to work is whole different ball game than a non computer controlled (old school) system"....

I was wondering how this changed how you troubleshoot trailer lighting.

On the trailer its the same, at the back of the truck its the same the only difference is that the computer turns power on and off vs a switch, a relay and a flasher module. More reliable, but when the computer fails, then it gets expensive in a hurry.

Aaron Z

 

[WranglerX]

So you are saying that instead of a simple 12v DC circuit going to the lighting with dedicated wiring for Running , RH Sig, LH Sig and Neg/Grnd that a single wire is used with some sort of signal from the computer telling which lamps to light? Does the "complete different fuses and wiring harness" also use this computer controlled lighting instead of dedicated DC wiring? Is there a transducer available for those that have standard trailer wiring?

Correct... BUT no transducer needed... All truck does is supply a different path for all lamp & brake functions to trailer than the paths to lamps on truck body.... Trailer wiring always stay the same...

 

[WranglerX]

Most pickup's today have similar, The on board computer may control the signals but they still go through 7 individual wires.
There is a set of fuses that the signals go through before going to the trailer connector receptacle at the back of the truck. This way if there is an issue with the trailer lights or wiring, it only blows those fuses and the pickup's lights are still fully operational.

Correct...

 

[WranglerX]

Yeah, i have not seen this trailer frame grounding thing. Must be something new. But i can see the benefit of ground to trailer frame as long as the white wire is "also" run back to the tow vehicle frame through the connector.

Way back in dim times (around 1954) many trailers used to ground frame through coupler and ball.... Having intermitted success keeping lights working.... My father being the sharp person he was actually connected a wire to trailer frame and uses huge alligator clip on other end to clamp on tow vehicles frame.... As a very young lad I learned something there... This was before it was really popular to include ground wire in trailer wiring harness/connectors...

 

[WranglerX]

On the trailer its the same, at the back of the truck its the same the only difference is that the computer turns power on and off vs a switch, a relay and a flasher module. More reliable, but when the computer fails, then it gets expensive in a hurry.

Aaron Z

Yeah.... PCM was $365 plus labor... And it did not fail towing, truck was running fine and parked in drive way, shut it off and when inside, next morning went to start it up and NOTHING worked properly.... As a side note PCM only has 2 clips and two large plugs, it physically can be changed in about 10 minutes.... Killer situation is the PCM has to be programed with vehicles VIN number or it wont communicate with other modules, and programing can only be done at dealership unless you want to spend $2500-3000 for all the programing hardware/software...

 

[CoyPatton]

With a 4pin connector the taillights and signal lines are only energized when the vehicle's are.

Whatever the vehicle behavior is will be what your lights are: so if your vehicle has no markers unless the headlights are on, the same goes for the trailer

To further clarify this (at least for me)!
Your turn signals (green and yellow wires) should be energized when the turn signals in the tow vehicle are activated. Brake lights should be activated when breaks are pressed (energizes the green and yellow wires, turn signal is primary function. Meaning blinker will work on that side over brake light). Tail and marker lights (brown wire) are only energized when tow vehicle lights are on (regardless of auto lights or not).
If wiring is correct, then ground is your issue. Many 12 volt electrical issues are ground related. A lot of the time the trailer frame does not provide a consistent ground due to rust. If ground is dependent on the ball/hitch connection, a faulty ground is a given!