How to cut 12v power?

[Midwilshire]

Our mc5 has a vanner 60 amp equalizer.  When I cut the master disconnect switch, my chassis batteries nevertheless drain.  At first i thought it was the equalizer, but it now appears there's a 12v phantom load somewhere.  I disconnected the 12v tap, leaving only the vanner hooked to the batteries, and the drain is much much slower.  Slow enough that I can live with it.

I want to be able to pull in to a temporary parking spot, shut it down, and know that when I wake up, I can leave without exiting the bus.  If  I could just cut the 12v power from inside, and thus cut the phantom load, that would do the trick.  Sounds easy, right?

Here's the rub... I need 12v power to turn the engine off.  So, I cannot just hook a normal relay into the master switch and have 12v turn on and off with the key.  That would be the elegant and foolproof solution.  I tried it today: disconnect the 12v tap and then turn off the key... Bus keeps running.  And running.  Reconnect 12v power and the bus shuts right down. 

Right now, the only solution that comes to mind is to divert the 12v power up to a switch in the dash, then back.  That seems clunky... to use a technical term.  My 12v tap has a 30amp fuse.  Seems like I'd have to run 10 awg wire to a bigger-than-normal switch and back... About a 20 foot round trip.  I can do that, but it seems hokey.

Thanks for helping me find a better solution.

Mike

P.s., the bus also won't turn off if the headlights are on.  Turn off the key, bus keeps running.  Turn off headlights, and both the headlights and the engine die.  I can also live with this, but I mention it here just in case it's somehow relevant to my problem.

[gumpy]

So, your bus has been modified.

As for the 12 volt, install a relay at the battery. Connect the hot side of the coil to positive battery. Run the ground to the panel, through a toggle switch, through an indicator light and attach to ground at the panel. Either frame, or ground stud in the driver electrical compartment. Now, when you turn on the toggle switch, it will activate the relay, and you have a nifty indicator light to remind you that the 12v is engaged. Down side is that if the bulb in the indicator light burns out, you can't activate the relay. So, you can dispense with the light if you don't like that "feature".

BTW, the coil only requires a few hundred miliamps so 18 ga wire should be sufficient. You should probably fuse it between the battery and the relay.  A 3 or 5 amp fuse should suffice.
Keep your 30 amp fuse on the switched 12 volt line.

[lvmci]

Craig, how about a 12V LED indicator light, long lasting. How about tapping the 12V toggle to the led light to ground separately,  if just using as an indicator, lvmci...

[gumpy]

I don't think you can use the LED in series to complete the circuit. The coil would draw too much current through it and burn it out.

I'm sure there's a way to attach it to the switch so it's not in series, but I'd have to play with it to figure it out. The circuit I posted
was one I used on a friends battery crossover relay which he uses to charge his house batteries off his alternator when driving. He
had a convenient switch and pilot light available on his dash. It worked great until the bulb burned out and he couldn't figure out
why the relay wouldn't switch.

[Midwilshire]

Thanks, that might have to do it. 

Do you think a 30 amp relay is sufficient, or do I need to build in a cushion?  The fuse is 30 amp.

[gumpy]

I would think 30 amp should be sufficient unless you've added a considerable amount of 12v stuff to your bus, in which case you'd probably
be overloading the wiring and would have blown that 30 amp fuse by now.

It should be rated for continuous duty, though. If you removed the bus A/C, there was a large relay in there that works quite well for this
kind of stuff.  Otherwise, get yourself a good quality continuous duty SPST relay.

[Midwilshire]

Thanks very much!

[thomasinnv]

hey gumpy how about your original idea and use an iluminated rocker switch? when the switch iss on it lights up but the illumination in the switch is isolated so if the led burns out the switch still works.

Sent from my SGH-T989 using Tapatalk 4 Beta

[gumpy]

Illuminated toggles switch the hot lead, and have a ground for grounding the light.  If the switch has an incandescent bulb, it MIGHT be possible to wire it backwards so
it switches the ground to the relay, and wire hot into the light ground terminal. Somewhat unconventional. Would not work this way if the light is LED.

If course there's no reason he can't run a hot wire to the relay, vs running the ground wire. It's still only one wire being run. Honestly, I don't know what I was thinking.  <Dope Slap>