New relay circuit for ham radio

[MythX]

Hi, I wired a ham radio to my truck a couple years ago. It's wired directly to the battery, so it's always on unless I turn it off. I would like to have it come on when my truck comes on though. What's the best way to add a new relay to do this? I'm figuring to use the existing wiring from the battery and just letting the relay complete that circuit for me. The existing wire comes from the battery, over the wheel, through the firewall (under the left foot rest). From there it goes under the carpet to the center console where it meets my radio.

Thanks in advance for the help.

 

[J_C]

As you probably know, you'd wire the relay coil to a ground point, ground wire or chassis ground, and the other terminal of its coil to a circuit that becomes live with the vehicle "on".

It would help if you can track down some wiring diagrams but may not be necessary. Instead you can observe what circuits don't work when the vehicle is off but do work when it is on. For example if you have more than one cigarette lighter outlet, it might be that one switches off with the vehicle and could be used to trigger the relay. Another example is the main power (not clock/memory power) to the radio, or the HVAC fan (or the rear console fan control if present), there should be a wire going to the console control module that you measure 0V on when the vehicle is off but 12V on when it is on. The same is true for any wire you find under the dash, that you can have a multimeter probe on a chassis ground and the other one probing for 0V on the connector when vehicle is off, and 12V when on.

The point is that which wire you choose could be a matter of convenience, what is accessible whether you want to crawl under the dash, or take the radio out, or take the glovebox out, etc, to get to it. Any relay box under or behind the dash is going to have some wires that go live with the vehicle in run or accessory mode.

In some cases you have to be mindful of how much additional load you put on a circuit but in this case not so much as a relay tends to only draw a few mA current.

 

[MythX]

Thanks JC. I think I was hoping to find out that these trucks had been built with this sort of thing in mind that I could just plug into. Unfortunately, all my cig. lighter ports are unswitched. I think I can tap into the line going to my stereo. I appreciate the response.

Thanks

Jason