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  • 12V power from tow vehicle connection?

    "Traditional" SAE wiring shows a 12V output from the tow vehicle on pin 4 of a 7 pin wiring connection. I have a 2021 Momentum 397TH and am wondering if this is wired into my trailer in any way, and if yes, how?

    Thanks, Steve
    My trailer: Grand Design Momentum 397TH with many small modifications and a fairly large (1700W panels, 10KW batteries) DIY solar install.

    My truck: 2020 RAM 3500 Laramie Longhorn 4x4 DRW, 6.7L Cummins HO Turbo Diesel, Aisin 6-speed, 4.10 axle, Patriot Blue Pearlcoat, Longhorn L1, Towing Tech, Max tow, Towing prep, 30K 5th wheel, Cold Weather pkg, Safety group, Body color bumpers, Tow hooks, Auto level, 50 gal fuel, Bed step, Side steps, Sunroof, Aux switches, Air Horn, TruXedo Sentry bed cover

  • #2
    Originally posted by Sckarolek View Post
    "Traditional" SAE wiring shows a 12V output from the tow vehicle on pin 4 of a 7 pin wiring connection. I have a 2021 Momentum 397TH and am wondering if this is wired into my trailer in any way, and if yes, how?

    Thanks, Steve
    Yes, it is wired into your trailer.

    I'm not 100% familiar with your particular trailer, but follow the 7-pin wire back and you'll find a junction box. This is what it looked like on my 315RLTS:

    Click image for larger version

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    Within the junction box you'll find something that looks like this:

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    From there the blue & white wire will tie into the self-resetting circuit breakers (again--not 100% sure where they are on your trailer) that will look something like this:

    Click image for larger version

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    Assuming you are familiar with how to use an ohmmeter, with a bit of investigation it will be easy to find out exactly how this is wired on your trailer. If you do it, please do the forum the favor of posting your results (with lots of pics--we like pics!)

    Howard

    P.S. Inspired by my fellow moderator TucsonJim, I replaced the OEM junction box with a Mictuning setup (with cable pre-installed). A very clean install and very easy to access all the circuits as required.


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    Forum moderators are not GD employees--we are volunteers and owners presumably just like yourself. Unless specifically mentioned otherwise, we have nothing to gain should you choose to purchase a product or engage a service we discuss on this forum.

    2017 Ford F-350 DRW, '19 315RLTSPlus

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    • #3
      Originally posted by howson View Post

      Yes, it is wired into your trailer.

      I'm not 100% familiar with your particular trailer, but follow the 7-pin wire back and you'll find a junction box. This is what it looked like on my 315RLTS:

      {pictures removed from quote}

      Assuming you are familiar with how to use an ohmmeter, with a bit of investigation it will be easy to find out exactly how this is wired on your trailer. If you do it, please do the forum the favor of posting your results (with lots of pics--we like pics!)

      Howard
      Lol... the 7-pin wire goes into my king pin box. I have no idea where it goes from there. I was hoping someone from Grand Design could answer with something specific to the Momentum 397Th.
      My trailer: Grand Design Momentum 397TH with many small modifications and a fairly large (1700W panels, 10KW batteries) DIY solar install.

      My truck: 2020 RAM 3500 Laramie Longhorn 4x4 DRW, 6.7L Cummins HO Turbo Diesel, Aisin 6-speed, 4.10 axle, Patriot Blue Pearlcoat, Longhorn L1, Towing Tech, Max tow, Towing prep, 30K 5th wheel, Cold Weather pkg, Safety group, Body color bumpers, Tow hooks, Auto level, 50 gal fuel, Bed step, Side steps, Sunroof, Aux switches, Air Horn, TruXedo Sentry bed cover

      Comment


      • #4
        Originally posted by Sckarolek View Post

        Lol... the 7-pin wire goes into my king pin box. I have no idea where it goes from there. I was hoping someone from Grand Design could answer with something specific to the Momentum 397Th.
        In the fifth wheels the junction box howson shows is up under the kingpin. You should be able to see it if you stand under it and look up. I replaced mine with the same plastic box to make troubleshooting easier.

        All that said it should be there and connected unless something is broken. I think almost every time I have seen an issue with the vehicle charging it has been on the truck side. Depending on your truck it may be live all the time, or ignition keyed.

        Do not expect much power from it, the most I ever measured on mine was 7 amps. Howard and I have both added 4AWG wire back from the truck to feed a Renogy DC to DC 40 amp charger to get some measurable charging power from the truck into the trailer batteries. Someone else (name escapes me) went with the 20 amp version and was able to utilize the factory wiring.

        Forum moderators are not GD employees--we are volunteers and owners presumably just like yourself. Unless specifically mentioned otherwise, we have nothing to gain should you choose to purchase a product or engage a service we discuss on this forum.

        Neil Citro
        2018 Reflection 28BH Pepwave
        2019 F350 6.7L Long Bed Crew Cab

        Comment


        • #5
          Originally posted by ncitro View Post
          Someone else (name escapes me) went with the 20 amp version and was able to utilize the factory wiring.
          That would be Sir Jim of Guest
          Forum moderators are not GD employees--we are volunteers and owners presumably just like yourself. Unless specifically mentioned otherwise, we have nothing to gain should you choose to purchase a product or engage a service we discuss on this forum.

          2017 Ford F-350 DRW, '19 315RLTSPlus

          Comment


          • #6
            Sir? lol.

            ncitro,

            I used an Renogy 20 amp DCDC charger and ran the black lead directly to the input side of the charger. The D+ (or ignition circuit) that is used to activate the charger was connected to the marker lights of the Imagine. This circuit is protected on the truck side. The output was connected to the positive busbar off the trailer battery where a 25amp inline fuse was used.

            The circuit on my truck is protected with a OEM 30 amp fuse at the trucks power distribution panel, where this is the only protection on the input side. Tinned marine 8 awg wire was used to extend the TV black charge wire from the trailer frame mounted junction box to the Renogy. This was done to keep resistance as low as possible.

            It's really neat to fire up the Victron BMV712 app while driving and watch the charger function when the marker lights are activated.

            There is another fellow on the forum that has successfully used a Victron unit to accomplish this same task where he may explain his setup.

            Jim

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            • #7
              Guest

              Yeah I’m really happy with mine, even with the solar, we were in the rain most of the way home and I was still able to run the crock pot making dinner while driving and arrived at the Cabelas we stopped at for the night with 80% battery left for the night.
              Forum moderators are not GD employees--we are volunteers and owners presumably just like yourself. Unless specifically mentioned otherwise, we have nothing to gain should you choose to purchase a product or engage a service we discuss on this forum.

              Neil Citro
              2018 Reflection 28BH Pepwave
              2019 F350 6.7L Long Bed Crew Cab

              Comment


              • #8
                Originally posted by ncitro View Post

                In the fifth wheels the junction box howson shows is up under the kingpin. You should be able to see it if you stand under it and look up. I replaced mine with the same plastic box to make troubleshooting easier.

                All that said it should be there and connected unless something is broken. I think almost every time I have seen an issue with the vehicle charging it has been on the truck side. Depending on your truck it may be live all the time, or ignition keyed.

                Do not expect much power from it, the most I ever measured on mine was 7 amps. Howard and I have both added 4AWG wire back from the truck to feed a Renogy DC to DC 40 amp charger to get some measurable charging power from the truck into the trailer batteries. Someone else (name escapes me) went with the 20 amp version and was able to utilize the factory wiring.
                Thanks, will check it out. Frankly, I'm not looking for my truck to supply power to the trailer. I'm more concerned whether I need to do something to isolate the two to prevent any issues with my solar system and LiFePO4 batteries.

                Steve
                My trailer: Grand Design Momentum 397TH with many small modifications and a fairly large (1700W panels, 10KW batteries) DIY solar install.

                My truck: 2020 RAM 3500 Laramie Longhorn 4x4 DRW, 6.7L Cummins HO Turbo Diesel, Aisin 6-speed, 4.10 axle, Patriot Blue Pearlcoat, Longhorn L1, Towing Tech, Max tow, Towing prep, 30K 5th wheel, Cold Weather pkg, Safety group, Body color bumpers, Tow hooks, Auto level, 50 gal fuel, Bed step, Side steps, Sunroof, Aux switches, Air Horn, TruXedo Sentry bed cover

                Comment


                • #9
                  Originally posted by Sckarolek View Post

                  Thanks, will check it out. Frankly, I'm not looking for my truck to supply power to the trailer. I'm more concerned whether I need to do something to isolate the two to prevent any issues with my solar system and LiFePO4 batteries.

                  Steve
                  I ran my solar and Lion brand lithium batteries with the standard charging connection to my Ford truck for a year and 10,000 miles with no issue before I added the Renogy. It’s generally recommended in order to prevent damage to be trucks alternator, but in my experience the computer in the truck controls the alternator output based on the state of the trucks battery and even if the RV batteries would be capable of taking 300 amps of current from the truck, it’s never going to see it. Now if you ran jumper cables from the truck batteries to the RV or something you could maybe fry something, but just don’t do that lol.

                  Forum moderators are not GD employees--we are volunteers and owners presumably just like yourself. Unless specifically mentioned otherwise, we have nothing to gain should you choose to purchase a product or engage a service we discuss on this forum.

                  Neil Citro
                  2018 Reflection 28BH Pepwave
                  2019 F350 6.7L Long Bed Crew Cab

                  Comment


                  • #10
                    Alternators today are not specifically calibrated to be connected to a high draw lithium battery where alternator (diodes) can overheat if sufficient air is not moving through the alternator. This would be at sustained idle under high load. If the load is high enough and the TV battery SOC becomes low enough, the alternator will work to make up this deficit where load shedding can be used to help to bring up the TV battery. This action may shed systems like heated seats and so on, but will not turn off the TV charge circuit to the RV. Its not recommended to connect different battery technologies in parallel since their charge requirements vary. This makes the DC DC charger desirable with a dedicated lithium charge profile with a consistent draw on the TV. I still do not charge using my Renogy until the TV is moving since the additional 25amps are applied to the alternator at all times. A higher output DC DC would generate more heat to the alternator at idle.

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