I need a power wire that is only live when the engine is running
What you're looking for is a "switched power source". This would be much easier if you provided the car's year, make & model. Regardless, here are the generic steps you need to follow:
Older cars have a terminal in the "fuse block" that was labeled "accessory" or "ignition". This terminal provided power only when the the key was turned to "accessory" or the "run" (or sometimes called "on") position. Some cars have more than one fuse block, many times this additional fuse block can be found under the hood or under a seat in the cabin.
You should probe the fuses with a DC volt meter one by one with the engine off and the key removed to see which ones have no power, then check those same "dead" fuses with the engine running. Once located, you should be able to use any of those fuses to power a low power device when the engine is running / key on accessory. Make sure that the power provided is 12 - 14 volts though - some newer circuitry is in the 8 volt range and will not work as expected.
Some newer cars however, make use of electronics that that turn power on and off independent of the key. Some will allow the radio to play, power windows to work, etc. after the engine has been shut off and key is removed for a predetermined length of time (10 minutes or so) or until the door is opened - whichever happens first. You may need to find the power lead for the radio, power window, or other device that works as described here and connect to it for a switched power source.
If you blow the fuse after connecting the new device, you'll need to install a small, inexpensive relay to control the new load. These are available at auto parts stores for less than $20. Instead of powering the new device, you would power the relay coil. Run a wire capable of handling the load (or greater) between the battery and one side of the normally open ("N.O.")switch contact on the relay, and the power lead of the new device to the other side of the normally open switch contact. The result will be that whenever there is power available to the radio (engine running / key on accessory), the relay will be energized. The energized relay will close the N.O. contact and bring power from the battery to the device.
If you oversize the wire from the battery to the relay, you can supply power to other devices such as radar detectors, GPS units, FM transmitters, power inverters for 120 volt devices, etc. Make sure the wire from the battery is fused properly and that the relay contacts are rated equal to / or greater than the amp rating of the this fuse.
I hope this was helpful & good luck!
Sep 08, 2012 |
Cars & Trucks