2015+ F-150 towing question: How do I know if I'm getting power to the trailer battery?

[Martinjmpr]

Crossposted from F-150 forum because I know there are a lot of F-150 owners here.

Truck is a 2018 CCSB XLT XTR (a Canadian package), 3.5 EB/10 speed/4x4 with the trailer towing package (brake controller and trailer backup control.)

In the "old days" I could just put a multimeter lead into the appropriate pin on the 7 pin connector on the truck and see if it's sending power, but as I understand it, on these newer F-150s, it won't send power to the trailer unless it "senses" that the trailer is connected (i.e. is connected to the lights, brakes, etc.)

So my question is: How do I know if mine is working correctly?

I put a multimeter onto the trailer battery while it was connected and then disconnected the trailer and the meter showed 12.6 v both ways.

I expected to see a drop in voltage when I disconnected the trailer.

FWIW I was running the trailer fridge on battery when I did this so I don't know if having a load on the battery would have made a difference.

This worries me because we have several long trips planned and although I CAN run the trailer fridge on propane, I prefer not to for safety reasons (it's a 3 way absorption fridge that will run 12vDC/120vAC/propane.) Trailer is a 2018 R-Pod 179 HRE.

I've heard some people say there is a fuse that needs to be put in to send power to the charging line but if mine has the tow package from the factory, shouldn't it already have that fuse? If there is a fuse specifically for the charge line to the trailer connector does anyone here know what fuse number it is?

 

[Grassland]

How fine is the resolution on your multimeter?
I'll check my 2014 later but I doubt it's the same set up, especially as mine didn't have the OEM tow package.
The amperage from the truck to the trailer battery would be pretty low as the wire size is tiny I think, and those fridges are power hogs on DC if it's using DC for the ammonia heater.
Try with fridge not on DC and see if there is a difference in voltage from not being hooked up to truck vs hooked up

 

[Hummelator]

Maybe I'm missing something but try disconnecting the leads from the trailer battery and put your meter on the leads.

 

[ebrabaek]

As my f150 shows "trailer connected" in the dash the voltage to the trailer rise to about 13.6.
As the trailer disconnects it drops to battery voltage.
There is not much amperage getting to the trailer as it is a thin wire.
Perhaps 8 to 10 amps at the most I'm guessing so if your drawing heavy it might not ever get over 13 volts. Bottom line thought, it will drop further as you disconnects the trailer.
Remember that in order for the trailer to connect you have to step on the brake in order for the brake lights to show the draw that the onefitty needs to sense that the trailer is connected.
At least thatbis how my 2018 is.

 

[Deleted member 9101]

My solution: run your own power wire and bypass the OEM system. It doesn't provide much power.

 

[WOODY2]

My solution: run your own power wire and bypass the OEM system. It doesn't provide much power.

I agree however put it on a relay so it is only energized when your trucks ignition is on otherwise it could drain the truck battery.

 

[Buddha.]

I looked into this extensively for my Chevy. It puts power to the trailer via the seven way plug but I wanted more.

1.
Run 2awg from truck battery to trailer battery. Not cheap, kinda a lot of work, mightnot give your trailer battery a full charge due to voltage drop.
2.
Same as 1 but you add an appropriate sized DC to DC at the trailer batteries. This is like a battery charger you’re used to but it’s mobile and runs off 12v.
3.
Buy a cheap eBay voltage booster and run 36-48v back to your trailer in a much smaller 10awg wire. You can do this because there’s less voltage drop with higher voltage. You then plug your higher voltage 10awg into a solar controller mounted near the trailer batteries and it functions as a battery charger.

 

[Chunkylover76]

I watch the voltage on my ARB fridge remote monitor or solar charge controller app on my phone. After I step on the brake and put the truck in drive the voltage jumps up to charging voltage.

 

[Martinjmpr]

Maybe I'm missing something but try disconnecting the leads from the trailer battery and put your meter on the leads.

Ah, didn't think of that, I'll have to try it next time I have the trailer connected.

 

[Martinjmpr]

As my f150 shows "trailer connected" in the dash the voltage to the trailer rise to about 13.6.
As the trailer disconnects it drops to battery voltage.
There is not much amperage getting to the trailer as it is a thin wire.
Perhaps 8 to 10 amps at the most I'm guessing so if your drawing heavy it might not ever get over 13 volts. Bottom line thought, it will drop further as you disconnects the trailer.
Remember that in order for the trailer to connect you have to step on the brake in order for the brake lights to show the draw that the onefitty needs to sense that the trailer is connected.
At least thatbis how my 2018 is.

Yeah, mine stayed at 12.6 regardless of whether it was hooked up or not. That's what worries me.

By any chance was there a load on your trailer when you were looking at the multimeter readings?

As for the amperage, honestly I'd be fine with 10A going to the trailer battery. I only use the fridge on battery power when I'm towing anyway. Once we get to the campground I switch it to propane if we don't have shore power and it runs great on propane.

 

[Buddha.]

Yeah, mine stayed at 12.6 regardless of whether it was hooked up or not. That's what worries me.

By any chance was there a load on your trailer when you were looking at the multimeter readings?

As for the amperage, honestly I'd be fine with 10A going to the trailer battery. I only use the fridge on battery power when I'm towing anyway. Once we get to the campground I switch it to propane if we don't have shore power and it runs great on propane.

On the chevys you had to hook up a wire by the fuse box to get power out the seven way. It it was constant hot regardless of key position.

 

[Chunkylover76]

There is a expensive 25A slotted mcase fuse in the pdu under the hood. Mine was already blown when I bought the truck new.

 

[ebrabaek]

Yeah, mine stayed at 12.6 regardless of whether it was hooked up or not. That's what worries me.

By any chance was there a load on your trailer when you were looking at the multimeter readings?

As for the amperage, honestly I'd be fine with 10A going to the trailer battery. I only use the fridge on battery power when I'm towing anyway. Once we get to the campground I switch it to propane if we don't have shore power and it runs great on propane.

Sometimes there were a load from the 12 volt fridge but it only pulls around 3 amps when running.
I'd say measure the voltage before disconnect and as you disconnect it should drop.
This is off course unless your fuse is popped.
There is a fuse that carries the charging wire from the onefitty.
I don't recall in which fuse box but I found it in the manual for the onefitty.
If that is busted then you would not have any.
I'd check that.

 

[Martinjmpr]

So the fuse is no. 30 under the hood. It's one of those blocky "M" type fuses rather than a blade type fuse.

Manual says 25A but the fuse on mine is 30A. However, it does not appear to be blown, though I'm not 100% sure I can tell.

Might try with the multimeter tonight to check it although I would "assume" that if no trailer is connected it won't send power. Otherwise I'll have to wait until our next trip when I have the truck hooked up to the trailer and I can measure at the trailer battery connecting cables.

 

[Grassland]

So the fuse is no. 30 under the hood. It's one of those blocky "M" type fuses rather than a blade type fuse.

Manual says 25A but the fuse on mine is 30A. However, it does not appear to be blown, though I'm not 100% sure I can tell.

Might try with the multimeter tonight to check it although I would "assume" that if no trailer is connected it won't send power. Otherwise I'll have to wait until our next trip when I have the truck hooked up to the trailer and I can measure at the trailer battery connecting cables.

Pull the fuse and use "ohm" or continuity and test it