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Ask Dave: Why doesn’t the tow vehicle charge RV’s batteries?

Dear Dave,
The car does not charge the battery to the RV when traveling, but the battery charges when hooked up to shore power. I’m wondering what could be the problem? All the fuses and the 7-point plug on the vehicle are good. —Matt, 2019 Forest River R-Pod 191

Dear Matt,
You need to verify there is 12-volt power coming to your 7-pin plug first with a multimeter. Start the car and find the pin on your tow vehicle, which should look like this.

1 – Left turn signal
2 – Ground
3 – Tail/Running Lights
4 – Aux 12V+ Charging
5 – L Turn/Stop Lights
6 – R Turn/Stop Lights
7 – Backup Lights

Put the dial on 12-volt DC then the black probe to the ground (#2) and red probe to the 12-volt + (4) with the tow vehicle running and verify you are getting voltage to the plug. If not, it is most likely a fuse under the hood in the engine fuse block.

You can also purchase a handy 7-pin tester that will test all the connections. It’s available on Amazon here.

The black label on the bottom is for 12-volt hot and white is for ground. However, this will not tell you the voltage.

What to do if you have voltage going to 7-pin plug

If you do have voltage going to your 7-pin plug on the tow vehicle, plug the pigtail in and with the tow vehicle running, use your multimeter to check voltage at the battery. You should see more than the 12.6 volts of a fully charged battery, and it should match what you found at the tow vehicle plug. This would indicate you are getting a charge while driving. If you are finding your battery or batteries are not fully charged when you get to the campground and set up camp, but verify a charging voltage during your test, your batteries are sulfated and not holding a charge.

If you do not see a charging voltage at the battery during this test it would indicate a short in the wire, or you might have a battery solenoid in-line. You should be able to do a continuity test from the RV’s 7-pin connection back to the battery to verify it is connected.

Keep in mind, if there is a considerable distance between the alternator and the RV batteries, you might not be able to get much voltage as the typical wire gauge is too light and you will experience voltage drop. This can also be verified by a multimeter test at the tow vehicle plug and then at the RV batteries.

Since it is a 2019, you should be able to get a wiring diagram from Forest River to determine if there is a solenoid, relay, or inline fuse between the RV 7-pin plug and the battery/batteries.


 You might also enjoy this from Dave 

When RV is plugged in to shore power, why isn’t house battery charging?

Dear Dave,
When my RV is plugged in to shore power it doesn’t charge the battery. Is there a fuse inside the charger? When I turn on power I hear the cooling fan turn on briefly. There’s power going in but nothing out. —Claire, 2017 Winnebago Minnie Winnie 25B

Read Dave’s answer.


Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

Read more from Dave here

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Cary
1 month ago

There may be another explanation for not charging. Some GM vehicles have variable alternator voltage to improve MPG. To charge a tow vehicle the manual recommends either turning on tow/haul or the headlights, both of which will increase the voltage of the system and charge the remote battery.

bill
2 months ago

More likely an “open” on the trailer wire than a “short” which should blow a fuse or melt a wire.

Impavid
2 months ago

Dave, in routine checking of the plug in my 2020 F350 I found no 12 v power going to the trailer. I used a plug-in tester plus my volt meter. Neither time did it show voltage when the truck was running. I checked with the dealership and was told unless there’s load needed then power isn’t provided.

Vernon R Lacy
2 months ago

I see two left turn signals #1 and #5, but nothing for electric trailer brakes.

David Solberg
2 months ago

Opps, you can tell I made the pin diagram myself! I’ll fix it as the right #5 should be #6. Thanks for the catch!

Bob M
2 months ago

On the connector diagram there are two #5’s and no #6. On the new vehicles the the brake controller is controlled by the vehicles computer. How about the vehicles 7 pin connectors wiring for lights? It might seem like a stupid question, but I had trouble opening my drivers door from the inside and the outside handle. I thought something froze with the mechanism, but they had to reprogram something in the computer. I also had two computer updates on my brake controller that took 3 hours. In the past I had a technician install a break controller years ago on a truck in one hour. New vehicles have between 1000 & 3000 computer chips controlling vehicles. My truck is a F150 hybrid.

kat
2 months ago

Thank you for this one. I knew the pins were for different things but until I saw the diagram I didn’t really understand for what!

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