Weird voltage on 7 pin plug

[EastSide]

So I have a flat bed trailer that gets a lot of abuse from banging around. I was getting a lot of "trailer connected / trailer disconnected" warnings and lamp light warnings. It's an old trailer so I figured it was the trailer's fault. I pulled the boards/deck off so I could get good access to everything, and noticed that I was getting about ~5v at the tail lamps. Truck in park, off, hazards on. It would flash between 0v and ~5v. I assumed it was the wiring, but I checked the 7 pin connector and seem to be getting the same ~5v. I checked the 4 pin connector and I get 12v down there.

Am I missing something? Maybe it's supposed to be ~5v there?

above: the 7 pin, negative on ground pin, checking each tail lamp

above: checking the 4 pin

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[EastSide]

Also, just did it again with the engine on and still ~5v.

 

[airline tech]

I would have to dig some, but this is what I know:


Trailer Lamps - Vehicles Equipped with a TRM

The TRM supplies voltage to the trailer tow stop/turn lamps when the vehicle stop or turn lamps are commanded on.

So, without the parking or headlight switch being on, you may only see 5-volts, as it is being controlled by the TRM and then when you apply the brake you should see 12 volts.

Update: Edit: I checked and with brake applied 12 volts and brake released less than 7 volts, per the manual pinpoint test of the lighting circuit.

The BJB trailer tow park lamp relay supplies voltage to the trailer tow parking lamps when the vehicle parking are commanded on.

And with the parking or headlights on - you should see 12 volts

The BJB trailer tow reversing lamps relay supplies voltage to the trailer tow reversing lamps when the vehicle reversing are commanded on.

The TRM provides voltage to the trailer tow connector for trailer battery charging when all the following are true:

• the TRM detects that a trailer is connected
• the ignition is on and engine is running
• a brake pedal application has been detected in the present ignition cycle
• the BCM load shed strategy is not active (a message will be displayed in the instrument cluster, such as "Low Battery Features Temporarily Turned Off" or "Turn Power Off To Save Battery", to indicate that BCM load shed strategy is active)

The TRM directly (no relay) supplies fuse protected voltage for trailer battery charging.


Per the manual less than 7-volts is normal voltage (for turn signals) as the TRM would regulate it.
However, if you were to do the same test, but have a helper apply the brakes, you should see 12 volts on Pins 1 and 4 - LH/RH Turn - 7 Pin Connector


For your initial complaint - Trailer Connected/Disconnected

There is a TSB for that, check the connector (7-Pin) for corrosion on the back side and the inline connector (C405) for same
C405 is located around the LH Taillight, just follow the harness from the 7-pin over to it.
Now the above TSB mainly covered, without a trailer connected the Trailer message would randomly pop up.

So, I have a feeling being that it appears the Flat Bed gets a lot of use, which means that you are connecting and disconnecting the trailer connector quite a bit.
From what I have learned on this forum is that the Trailer Connect / Disconnect sense comes from the TRM sensing a power draw on the exterior lighting circuit. (Mainly the resistance change-(Greater)
So, if your connector plug is worn, dirty or weak contact pins, on either the trailer or truck then I can possibly see this message popping up. This would be my first check and for good measure check the inline connector C405.
and then dig deeper into the trailer wiring itself, but an issue here would be more shorts and causing other issues than the Connect / Disconnect.

 

[airline tech]

I edited and updated post #3 - 5-Volts is normal, until you hit the brake pedal then 12-volts.

 

[EastSide]

Dude that is so helpful, thank you! Gives me a lot to digest. At a basic level how can the car hazard lights being on *not* send 12v to what I assume are 12v lights back there?

 

[Grandaccess]

on my last truck the wire came away from the bumper about 8 inches so my trailer hook up is about a foot short of even plugging in, so I just run without its not a big deal, its legal to run without them in the day time in N.Y as long as its a flatbed and they can see your truck lights.... I will make a pigtail someday

 

[Utope]

Dude that is so helpful, thank you! Gives me a lot to digest. At a basic level how can the car hazard lights being on *not* send 12v to what I assume are 12v lights back there?

It is probably going between 5V and 12V. Being 12V only when the lights are lit. Your meter probably isn't reading it fast enough to display. You would need a scope to capture the waveform. Try again with the brake pedal depressed.

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