cgh1234

This is in regards to my 2009 F-150 supercab, 4.6L. If I am reading the 7-pin diagram here correctly, pins 4 and 1 should have 12V power to charge the trailer battery. However, when I have the truck running and 7-pin cable disconnected, I do not see 12V on a voltmeter. It's basically zero. Does it only show voltage when the cable is connected and it's actively charging the battery? Also, looking in the truck owner's manual, there is a high-power fuse located under the hood called "Trailer Tow Battery Charge." There was no fuse there, so I installed a 30A fuse per the manual. This made no difference to the voltage output. Any ideas what could be wrong?

Ricktwuhk

Your truck likely came with the fuse AND relay in a bag, and the prior owner didn't install them. Now you need to add the relay. Slot 9 for the relay and 21 for the fuse. Check your manual to verify those locations.

cgh1234

You're right - the relay is missing too. Unfortunately, I don't have the fuse/relay that came with the truck. Any idea how I can find the correct part number for the relay I need?

dsims

If the 2009 was the same as my old 2008, it was the same relay as the others installed next to it. I just pulled one and went to NAPA to get a new one.

MikeD134

and at some point in time the trailer connector became "smart" and would not allow power to the battery terminal unless the truck was running, the driver door is closed, and it "sees" the trailer brake lights. Not sure when that "feature" began

cgh1234

I picked up a new relay and now it works perfectly! One other question, with the cable connected and the truck running, if I use my power tongue jack, will that draw power from the trailer battery or the truck? Reason I ask is, a while ago, I used the jack quite a bit with the cable disconnected and it drained the trailer battery pretty fast.

Flamingtaco

The non-detailed answer is... it depends.

The jack, battery and supply are in parallel. When the jack is engaged, if the voltage on the line drops below the trailer battery voltage, the battery will supply current. If the truck keeps the line voltage above battery voltage, the truck will supply all the current to the jack.

cgh1234

That makes sense. Thanks!

StephanEngr

Not sure about older trucks, but this is from my 2020 owners manual. Maybe they left it out on 2009?

1Your vehicle must recognize the trailer before the trailer can receive power.
Recognizing a Trailer

1. Attach the trailer and wiring connector to your vehicle.

1. Switch on your vehicle.

1. Press the brake pedal for a few seconds.

A confirmation message appears in the information display.

Flamingtaco

Most likely it is the same, as the 2009's can determine a fault, shut down the ITBC, and send a message to the display.