Relay for Tow Charge Indicator

Hi All - I am trying to create an indicator in my cab to show when my trailer is being charged (or when any power is being used on the 12volt terminal of the 7-pin adapter for that matter). I’ll share my thoughts on what I tried and ask for help as to why it isn’t working — I’ve tried searching for relay diagrams that may be of help, but I can’t seem to find out why it isn’t working. My thought was to connect a relay in line with the 12v power lead going to the 7-pin trailer plug. I thought once a trailer was hooked up and the battery was properly charging, it would connect the circuit on this lead to ground and activate the relay. Then connect a small led indicator to 87, with power from 30, and I would know when it was on. When experimenting with this set up, I found that the relay does activate when needed, but I am getting reduced power through the 12v circuit to the trailer. I was trying it out with just a test light and the bulb would barely light up — obviously I have something wrong in my plan here but I can’t seem to figure out where it is or if there is a way to actually accomplish this. Any help greatly appreciated!

So as I tried to troubleshoot this is my thinking correct that my problem is I have this relay installed in series essentially with a high current draw wire?

Are you just wanting to know if the trailer side of the 7-pin connector is receiving 12V or do you want to know how much current is being consumed by the trailer?

Good question — I suppose either option would work.   Two reasons why I want the info: 1- I want an indicator that my trailer is being charged because sometimes the fuse blows an I’m left with a dump trailer that has a dead battery 2- I have a small salt spreader that runs off the 12v power of the 7-pin and I’d like to know when it is running Knowing amperage for #1 could be helpful

What is the current rating of the fuse feeding the trailer connector?

20amp

I was hoping to suggest a current measuring solution that was relatively inexpensive - such as this: https://abra-electronics.com/test-equipment-tools/meters/digital-panel-meters/pm-50-voltmeter-and-ammeter-combination-panel.html However, the setup requires that the shunt be installed in series with the ground wire. That is not practical with a trailer as there is usually a ground path through the hitch and safety chains. An alternate measurement method with a shunt in the +12V feed could be devised but that would probably cost about $75 to implement. So my only suggestion (so far) would be to add an additional connector to the trailer that connects to Pin 4 on the trailer side and feeds an LED inside your tow vehicle. At least you will know that 12V is available at the trailer.

Would a clamp-style ammeter work?

jrschatz wrote: Would a clamp-style ammeter work?

Yes a clamp-on meter would work. Just make sure it is rated for DC - many are AC only. A cheap fix for making the item in the link work on the +12V side would be to power the display with a 9V battery.

Thanks for your help. And for the sake of learning — is the reason my relay would not work because I was trying to run it in series with the 12v line?