I have a 2020 F350 6.7. Towinga 21’ travel trailer. Truck is not adequately charging the two 100 amp trailer batteries. Voltage from charge pin in the 7 pin trailer plug is only 2.6 volts. It was recommended I use 40 amp upfitter switch to run 8 gauge line to trailer batteries so i could charge them at will while towing. Easy enough to run line from batteries to hitch, connect to line on truck with Anderson connectors, and run line to vicinity of passenger side battery. I can’t find any You Tube that has specific step by step instructions for how to make the connection to the uppfitter switch for a non mechanic. I’ve located the relay and fuse box by the battery. Anyone kknow how i go from 8 gauge positive and negative wires to the tiny switch wires? Thanks.

 

The right way to do this would be to protect the circuit from the point of origin (alternator) to the plug going to the trailer. It would need to be a fairly high fuse current rating.

Picture this, you have an unprotected power supply using 8 gauge cable to the trailer and that wire manages to to get caught /jammed where it has a nice path to ground. All possible power in that cable now is ready to do some welding or just burst into flames, not a good picture.

Please have someone that is electrically experienced set this up for you, OK ?

 

If you have an owners manual, it should show you where to make the connection to the up fitter switches. It's in the engine compartment on the fire wall if I remember correctly, so you don't have to worry about how to get the cable into the cab. Also, those are fused for protection. Rather than make a connection out where the elements could get at them, I hooked up a NAPA backup camera using one of the up fitter switches on my 2012 F-350. Works great. I can look at the hitch from time to time just to keep an eye on things while going down the road. I just liked the idea of not getting into the wiring harness thereby eliminating the potential for corrosion in the future. Besides, I've been looking for a reason to use those switches.