I am working with a 1965 mustang and have added heated seats.

The seat heater wiring does not have a latching circuit and is controlled by a SPDT Hi/off/Low switch.

In an effort to only allow the seats to be powered when the engine is running, I've added a relay to the control wiring that will have a fused direct battery feed for the heat load and "running only" source for the relay coil.

I thought of using the feed off of the alternator "BAT" output. My problem is that this wire is terminated at the hot side of the starter relay with the positive battery feed, so the coil would be always energized.

To isolate the battery from the Alternator output I was thinking of adding a diode. Not being sure of the alternator currents, I do not know how to accurately size this diode, an I am concerned about the heat it would generate.

Is there maybe a better TAP that I can use for this? Maybe the "fld" or "sta" from the alternator or something off of the voltage regulator that would be a better?

  • Hi, I'm sorry to have placed this in the incorrect stack. I am not sure how to move it or select the appropriate stack when asking it. Any help would be appreciated on this too. Thanks and have a great day, John
    – TopDownCar
    Mar 10, 2015 at 14:21
  • @topdowncar the close vote is part of the migration process. It's now on the auto stack exchange where it should get better attention
    – cde
    Mar 10, 2015 at 14:55
  • 1
    Why do you feel you need something in addition to a normally open relay that closes only when the key is turned on?
    – Paul
    Mar 10, 2015 at 15:03
  • HI Paul, The switches are located inside the glove box, their lights are out of natural view. If one or both seats happen to be left on, during maintenance or any other reason the ignition is turned on. There would be a potential of high load battery discharge. Most new cars employ a latching circuit to prevent this. If I could eliminate them from turning on unless the engine is running, Id feel better about the install. There are multiple drivers 3 being my children. Thanks, John
    – TopDownCar
    Mar 10, 2015 at 15:39
  • Wouldn't this same concern apply to all other electrical loads (vent fan, radio, lights, etc.)?
    – Paul
    Mar 10, 2015 at 15:43

1 Answer 1


I have two solutions for you, depending on your skill level.

The first is using a zener diode to turn on the relay once the voltage is above 12.6 volts. This will energize the relay anytime the battery voltage is above 12.6 volts. You could go as high as 13 volts, but that might turn the relay off under high electrical loads with the engine running. The 12 volt battery in the car tops off at 12.6 volts, so other than a surface charge the only time the voltage will be higher than that is when the alternator is running. I might would start off with a breakdown voltage of 12.8 and see how that works out for you.

Option two is to go with a commercial solution. Lind electronics make several shut down timers that you may like.

The T2 Shut down timer turns off after a preset time or voltage.

enter image description here

They have several other models you may like as well.

  • these are both great ways to solve my issue! the shut down timer is of great interest and with a short time with the meter, I think that the voltage for the zener diode option could be easily selected and much less expensive. But if the shut down timer is a better solution, I will go that route. Thank you for the info, I have some learning to do! :) Thanks again and have a great day, you've been a wonderful help! John
    – TopDownCar
    Mar 10, 2015 at 22:42
  • @TopDownCar If this answer helped you and completely answered your question please click on the check mark next to my answer. This lets future visitors know that this was the correct answer. Mar 10, 2015 at 23:56

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