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Discussion Starter · #1 ·
On my last trip with my 5th wheel and also when I picked it up after service (the trailer) I found that the 12v coming from the truck is very weak. The dealer had left the 12v reefer on, which drained the battery, it was DED Dead. With the truck hooked up, I could not run the front jacks, they would start pushing up and then voltage dropped too much and threw the controller into error mode. When we went on our trip the trailer battery was fully charged, and by the time we reached camp the reefer had drained the battery down to 10.5 volts and the truck wasn't even registering.

Is there something I can look at, maybe a corroded relay, or is this all in a module that can't be looked at? I know the truck is supplying 12v, but any current drawn causes it to drop voltage. I have the twin outlet harness, the one at the receiver and one in the bed post.
 

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More likely your battery in trailer is now toast if the dealer left fridge on it killed it.. May need a new battery.. Truck will maintain but will not charge it up travelling

RoadrunnerII

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Discussion Starter · #3 ·
You would think it is, and I thought the same thing, but I put my load tester on it and ran it about 6-8 times and it stayed in the green every time. I had turned the disconnect off when I plugged the truck in so I culd raise it up, and the truck is not supplying enough current. My F150 supplies more current than that and it is a 2018.
 

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Corroded trailer connector pins will cause low voltage at the trailer. That's the first thing I would check.

I carry a can of electrical contact cleaner with me and use it frequently.
 

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Discussion Starter · #5 ·
Thought of that too, but its a new harness installed last year. I am going to pull the connector at the base to see if there is something hinky there for starters, but I installed a new splitter harness that adds the 7 pin to the bed. I have two points to test from too.
 

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Is there something I can look at, maybe a corroded relay, or is this all in a module that can't be looked at? I know the truck is supplying 12v, but any current drawn causes it to drop voltage. I have the twin outlet harness, the one at the receiver and one in the bed post.
I am assuming this is the 2012 F350 in your signature. Your 12V battery circuit at the trailer connector is relay powered and should be powered at all times with the ignition in the run position. If I suspected a circuit issue like resistance I would load test the 12V circuit using the trailer tow ground with a headlamp bulb for example. If the lamp burns bright there's likely no problem with the truck wiring. This is important because you want to quickly establish whether the problem is the truck or the trailer - almost every time a customer presents a truck with trailer wiring problems the truck is not the issue. However if the bulb lights but is dim you likely have resistance somewhere in the circuit - either the power side or the ground side. You can test further by using a basic automotive test meter (DVOM) to perform a voltage drop test to locate the problem. Here are two resources to help you understand and learn how to do a voltage drop - I selected the video because of the way it was presented using a demo board. The link to Fluke will give you full and complete technical details on the subject. Grab your favorite beverage and check out both if your testing leads you to this step or if you just want to learn something!

Voltage Drop Testing ~ The BEST Way To Test Wiring - YouTube

Diagnosing Voltage Drops Electrical Automotive Troubleshooting | Fluke
 

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Discussion Starter · #7 ·
I will RTFM and look up the relay and start from there. After 9 years, it is always possible the relay is hinky from not being used. The receiver had NO wear in it and the 7 pin still had factory grease in it when I got the truck. I wasn't sure if the 12v+ is Module or relay based. Thanks, gives me a place to start. I'll check to make sure there is no corrosion at the relay then hook up the trailer and test. It just takes time to hook up since I have to raise the rear jacks so I can lift the front to get the truck under enough to plug in. The trailer is leveled and set up in my drive.
 
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