Tow vehicle not charging RV batteries

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AStravelers

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Nov 14, 2016
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1,591
Location
San Antonio, TX
The typical 7 pin round connector that connects the tow vehicle to the trailer has a +12V lead.  You will need to check the tow vehicle or trailer owners manual to find which pin.  It also should be labeled on the connector.

This +12V lead does supply some charging to the trailer vehicle.  However because of the length of the wire from the alternator to the trailer and the relatively small wire size it doesn't do a very good job of charging the trailer batteries.  If this is a truck the wire size to the connector is probably #10.  In the trailer it may be smaller.

If you only have 1 or 2 batteries, group 31 or smaller a pair of + & - wires of #6 or #4 gauge will do a better job.  These wires need to be run from the alternator all the way to the batteries with a heavy duty connector at the trailer/truck interface. 
 

kdbgoat

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Joined
Apr 16, 2014
Posts
6,313
There's a good chance that there isn't a fuse installed in the fuse block that feeds the connector. It seems like the factory doesn't always install it. I understand that your truck is 10 years old, but if the connector was never used on an RV, the fuse may have never been put in. There's also the possibility that the fuse is there, but may be blown.
 

GaryA

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Joined
Mar 20, 2016
Posts
129
Location
Waxhaw, NC
I had a Ford Expedition and the batteries on my travel trailer weren't charging.  There was a fuse and a relay that were associated with that function and the relay was not in the slot from the factory.  Bought the correct $21 relay at NAPA === problem solved.
 

Memtb

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Aug 25, 2017
Posts
951
Location
North-Central Wyoming
AStravelers said:
The typical 7 pin round connector that connects the tow vehicle to the trailer has a +12V lead.  You will need to check the tow vehicle or trailer owners manual to find which pin.  It also should be labeled on the connector.

This +12V lead does supply some charging to the trailer vehicle.  However because of the length of the wire from the alternator to the trailer and the relatively small wire size it doesn't do a very good job of charging the trailer batteries.  If this is a truck the wire size to the connector is probably #10.  In the trailer it may be smaller.

If you only have 1 or 2 batteries, group 31 or smaller a pair of + & - wires of #6 or #4 gauge will do a better job.  These wires need to be run from the alternator all the way to the batteries with a heavy duty connector at the trailer/truck interface.

  AStravelers, pretty much ?nailed it?! If you want the tow vehicle to charge the RV system, you will have to ?seriously? upgrade the charge wire to the truck 7 pin connector, and then likely from the RV 7 pin connector to the RV charge system.
 

regval

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Joined
Jun 4, 2015
Posts
426
Location
Lawrenceville, Georgia, USA
Gmc and Chevy do not connect the aux power wire to the stud at at rear of the main fuse box when it leaves the factory. Under the hood, drivers side is the main power fuse box. At the rear of the box (as you are facing the windshield), are two large square fuses. Outside of the box near the fuses you'll find two studs with a nuts. One is for the trailer brake control (30A) and the other stud is the auxiliary power, protected by a 40A square fuse. If there is no red wire attached to the 40A stud, feel around below the fuse box for the red wire they tuck out of the way. It may be taped to the wire harness.  attach it to the stud. Once attached, there should be 12V at the 7 pin trailer connector auxiliary pin. I suggest you upgrade the 40A fuse to a 40A auto reset circuit breaker if you plan on sending the aux power along to your RV battery.
 
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