Truck power to trailer

harley4275 wrote:
Fuse under hood in your fuse box not hooked up or not there. Look in your manual,if they give you one, and see whick it is. Should be a 30 pr 40 a square fuse. Could be the wire under hood not connected yet.

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You dint even bother to read the very 1st post where he plainly said he had 13.8 volts at the 7pin !!

In a small glass pour some vinegar and dip the corroded plug in. Let soak. Also can use toothbrush and baking soda with the vinegar to brush away the corrosion.
In humid areas especially it easily corrodes in a year. This is common.
Scraping with something metal is temporary usually and corrosion will keep growing and be back soon.
Glad you fixed it

Good work. However, note that the full battery should read 12.7ish not connected. The truck is at 13.8. When connected the full trailer battery would be say 13.7ish (allowing for line loss).

If it only goes up by 0.2v that means it is not full.

Apparently this was just the 7 pin not making contact. A spray of contact cleaner, some sandpaper, and several cycles of plug/unplug solved the problem. The truck now turns on the lights without the battery, and with the battery in the voltage at the battery goes up by 0.2 v (with the battery fully charged).

OK, so no big problem anyway! So the issue left is that you do have 13.8 at the truck's 7-pin so pin #4 (charge line) and the ground pin there are ok there.

The trailer battery voltage is reaching the end of its 7-pin cord connector too, so no breaks back from that.

That leaves the actual connection in the two 7-pin connectors not making contact. You can scrape them inside with a screwdriver or knife, and wiggle them until you get good contact.

Outside chance is on a poor wire you can get the proper voltage with your meter, but it cant carry any amps, so you could have a bad wire connection inside the back of the 7-pin (either one) where the wires screw down on their pins. Those connectors come apart so you can get at that.

If you had truck power to the battery, but the battery disconnect open, it still would not run the lights, but the trailer battery voltage would go up.

BFL13 wrote:
What is the trailer battery voltage with the 7-pin disconnected? Do the lights and fans work with the trailer battery connected, but the 7-pin disconnected?

Need to find out if the 7-pin pos is before or after the DC circuit breaker by the battery pos post before blaming that breaker (or it might be a fuse) since the battery voltage did not rise.

(You said the lights don't work with the battery disconnected, which is to be expected.)

Check the voltage at the 7-pin of the trailer cord (that goes in the truck's) That should be the same as the trailer battery's voltage. If none, find the wires from the trailers 7-pin cord that go onto the battery and see if they are connected to the battery. The ground wire of that cord might not be grounded to the trailer's frame. The positive (pin #4 wire) might join in with another pos wire, and then that wire has a fuse before going onto the battery pos post.

With the 7 pin disconnected, and battery connected, everything works fine. Battery voltage is 12.7, and voltage at the 7 pin is the same. If there is a blown fuse, why is there voltage at the 7 pin?

It seems like there should be a fuse between the 7 pin and the battery, but if so it is well hid.

With the trailer battery disconnected at the battery, and the 7 pin plugged in with the truck running, , shouldn’t the truck power the lights?

I’ve been trying to avoid pulling the power panel out to look for more fuses and connections behind it, but I may have to do that.

Fuse under hood in your fuse box not hooked up or not there. Look in your manual,if they give you one, and see whick it is. Should be a 30 pr 40 a square fuse. Could be the wire under hood not connected yet.

What is the trailer battery voltage with the 7-pin disconnected? Do the lights and fans work with the trailer battery connected, but the 7-pin disconnected?

Need to find out if the 7-pin pos is before or after the DC circuit breaker by the battery pos post before blaming that breaker (or it might be a fuse) since the battery voltage did not rise.

(You said the lights don't work with the battery disconnected, which is to be expected.)

Check the voltage at the 7-pin of the trailer cord (that goes in the truck's) That should be the same as the trailer battery's voltage. If none, find the wires from the trailers 7-pin cord that go onto the battery and see if they are connected to the battery. The ground wire of that cord might not be grounded to the trailer's frame. The positive (pin #4 wire) might join in with another pos wire, and then that wire has a fuse before going onto the battery pos post.