[ericbneverscared]

I'm adding an aftermarket 7-pin trailer wiring connector (for lights+brakes) to a 2006 Dodge Dakota. The instructions for the kit I bought result in there being an "always hot" 12V pin in the connector (with a 40 amp breaker). This seems like a safety hazard (e.g. little kid opens and touches the connector and shorts the battery across their finger) and could also cause the trailer to drain the truck battery if it's plugged in for a long time (e.g. at a rest stop).

Ideally, I'd like to add a relay so that the power supply is only "hot" when the vehicle is on.

I'm still researching this, but does anyone have any ideas on how to accomplish this easily?

I'm hoping there is some sort of standard relay for this type of application that can plug into some available slot in e.g. the fuse box or something. I could splice into some other "hot only when running" wire, but I'm considering that a last resort.

Thanks for any suggestions!

 

[radioactive]

The 12 volts hot pin is not really a shock hazard, wouldn't worry about that. It's hard to get more than a light tingle off 12v.

As far as the drain, it also should not be a factor if your trailer is wired right. Nothing should be energized when you are parked, not lights, which are under control of your light switch, nor brakes, which are controlled by your controller.

The always hot wire is kind of handy, in my opinion, you can have a courtesy light that you can turn on inside your trailer without your key on. But then keeping that from draining your battery is on you.

If you insist that it's a problem, wiring a relay is is trivial, and you can just wire tie it to your connector harness under truck.

 

[ericbneverscared]

The 12 volts hot pin is not really a shock hazard, wouldn't worry about that. It's hard to get more than a light tingle off 12v.

Thanks! I'll admit I've never tried touching both the terminals on a car battery. I guess dry skin (which the Internet claims has a resistance of 1k - 100k Ohms) wouldn't yield a serious shock, but I think wet skin probably would. But you're probably right that I'm being overly cautious. And I agree that an "always hot" wire could be useful. The only trailer drain I could imagine would be if the trailer battery was low and its recharging drained the car battery (probably not worth worrying about, you're right).

If I DID want to add a relay (which I'm NOT sure I want to do, at this point), is there some convenient way to do that (e.g. in the fuse box) or would I just have to splice into some existing circuit to control the relay (and use some generic electronics parts with the correct amperage rating)?

I've tried searching on auto parts sites, but all I can find are replacements relays which are specialized and assume that the circuit being switched is already wired up to the fuse box by the manufacturer (whereas in my case, I'm adding a new circuit and running the wires myself).

Let me know if that doesn't make sense. This is my first automotive electronics project (although I have a basic understanding of DC electronics generally).

Edit: was your last comment about a trivial relay basically just adding a manual switch near the connector at the back of the truck? That's basically my backup plan if I can't find a good way to make it work "automatically" (i.e. hot when the car is running and cold otherwise).

 

[radioactive]

Last summer I was working on my pontoon battery, standing in the water, and I came across both terminals with wet hands. I felt it, but it was basically just enough of a buzz to make me notice it. Didn't even drop my wrench.

I'm not familiar with your type of trailer -- with a battery (is it a camper?), but I imagine there is a reason that terminal should be live all the time. If it's to charge the battery, then there is little harm in leaving it live, as your batteries are in parallel and both available for starting the vehicle -- although of course there is some IR loss over the wiring.

You would buy a generic 12V sealed relay with a sealed socket, and terminals, and wire the coil across whatever you want to control the relay, such as your parking lamps. Or a 12V signal from a dashboard switch, or your keyswitch line, etc. Then use the contacts of that relay to interrupt the "live" 12V feed to the plug. Google "relay pinout" and you will learn about the standard pin numbering scheme for automotive relays.

I would not advise trying to use your vehicle fusebox unless the wiring is already there....

I've never looked for one (I have them at work) but I gotta believe that every auto parts store will have the relay and socket you need. If not, I'm sure Amazon will have a thousand to choose from.

Here's one: Amazon.com: DC 12V 40A Universal Car Vehicle Waterproof Sealed Integrated Relay Kit & Base (4-Pin Relay): Industrial & Scientific
You supply and crimp the wires. There are others that include the wires as a pigtail.

A few crimp splices, some heat shrink, and couple wire ties, you're in business. No Scotch locks, and avoid soldering unless you really know how....

 

[ericbneverscared]

Last summer I was working on my pontoon battery, standing in the water, and I came across both terminals with wet hands. I felt it, but it was basically just enough of a buzz to make me notice it. Didn't even drop my wrench.

Ha, ok, I guess I'm just worrying too much. Still not going to try it, thought

I'm not familiar with your type of trailer -- with a battery (is it a camper?), but I imagine there is a reason that terminal should be live all the time.

Yes, it's a 2,000+lb pop-up A-frame camper with a battery (and electronic brakes, which is why I'm upgrading from a 4-flat connector to the 7-pin one).

As for the live wire, the only reason the one pin is live all the time is because the "4-flat to 7-pin trailer wiring" kit I'm using basically just has a lead from the positive battery terminal straight to the pin in the connector at the back of the truck, with only a 40 amp circuit breaker in between. I don't think there is a good reason for the wire being hot all the time other than it being the simplest way to wire it up.

You would buy a generic 12V sealed relay with a sealed socket, and terminals, and wire the coil across whatever you want to control the relay, such as your parking lamps. Or a 12V signal from a dashboard switch, or your keyswitch line, etc. Then use the contacts of that relay to interrupt the "live" 12V feed to the plug. Google "relay pinout" and you will learn about the standard pin numbering scheme for automotive relays.

I would not advise trying to use your vehicle fusebox unless the wiring is already there....

This is exactly the kind of info I was looking for, thanks! I'm now leaning towards NOT adding a relay (since, as you noted, it's mostly unnecessary), but now I at least feel like I could do it correctly

 

[radioactive]

Hope it helped. I've been in the industry over 30 years.... So I know a little about vehicle electrical systems.