# 'Easy' reversable conversion of cigarette lighter to switched on with ignition



## risottto (Jun 13, 2017)

I've gotten a lot of good info and DIY guides from this forum since I got my A3 several months ago, so when I came up with what I think is a pretty genius 'hack', I thought I would post a quick guide for future European car owners to reference. I haven't found this solution online before.

So my A3 and most german cars have constantly on cigarette lighters, great if you want to leave your phone in the car charging or need to light up and don't have your keys on you. However I use a GPS tracker for my business that does not auto-power off when I'm not moving, so it sits there tracking my location when the car is parked, just trickling the battery and screwing up my trip data.

I wanted to make just the 12v outlet in the TRUNK switched, as that's where I plug in my tracker, but this hack would work for the front outlets as well.

I found guides that involved splicing into the back of the fusebox, sharing a fuse with another circuit, cutting wires, etc. My solution is plug and play, totally reversible, and keeps all the same OEM fuse ratings. 

The first step was to find what fuses in the driver's fusebox were switched with the ignition. on the A3 the first two rows of mini fuses were switched, and had power on the top terminal, 5, 8, 9, and 11 had switched power. #40 on the next to last row also is switched and unused. Some of the other empty sockets didn't have any power terminals installed at all. 

So, the rear 12v outlet I want to have switched is fuse #26 (always on), so I planned to use the empty #5 to switch a relay that would turn on #26 when the ignition is on.










I bought a mini Fuse Tap and standard size Fuse Tap at the local parts store, some 14ga primary wire, a basic 14V Bosch relay, various fuses, and insulated terminals. That's pretty much it, around $20 total.

The idea was to pull a 12v ignition switched signal from Fuse 5 (Beam control) into the trigger terminal the relay (86), then the switched terminals on the relay (87 and 30) which would sit in-between the two terminals where the fuse #26 would be plugged in, activating the circuit only when the ignition is on. It's easier to see in a diagram:










The sneaky bit was using the Large fuse tap inappropriately, putting two 20 amp fuses facing OUT in the bottom two terminals, then connecting those to two primary wires that run to pin 30 and 87 on the relay. The top two terminals are not used and neither is the tap wire.










On the fuse 5 trigger tap, I just put a 10A fuse on the top two terminals and connected the tap wire to Terminal 86. You HAVE to plug this in with the tap wire facing DOWN, as there is only power on the top terminal of the car.










The last connection was connecting terminal 85 on the relay to ground. The whole relay look like this:










And installed in the A3, I found a handy place to screw in the relay facing backwards on a ground screw and tucked the wires in front of the fusebox.










That was it! Now the trunk outlet turns on and off with the ignition, and if I ever want to go back I just pull everything out and replace the original 20A fuse. The trunk outlet circuit is still protected with two 20 amp fuses in series, a bit of extra protection than the original wiring.

Hope this can help some other people out, it would work in just about any car with constantly-on 12v outlets. The wiring can be done differently to not block a bunch of fuses around it, (like soldering into a blown fuse) but the ease of hacking the large fuse tap worked well for me.


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## risottto (Jun 13, 2017)

*Oooops .. that killed my battery!*

OK, so I rigged this up, then came back to my car 3 days later and the battery was pretty much totally dead. Not sure exactly what the culprit was. My best guess is that there was ...some.... current leaking from the ignition switched fuse 5 through the relay to ground (the relay resistance between 85 and 86 was 84 ohms, and that circuit from fuse #5 was pushing a tiny bit of current with the ignition off (0.24 volts if I remember correctly), so this path to ground could have drained the battery over a few days. If anyone else has a better idea, let me know. I double checked the wiring and made sure nothing was hitting ground or loose.

After being rescued my my Anker jump starter (that thing is a life saver!) .. I had a silly idea to do things the EASY way! ..... I just put in a wire with an in-line 15amp fuse between the top (hot terminal) of fuse 40 (the unused switched Heater Blower fuse) into the bottom (wired directly to the rear cigarette lighter) of fuse 26. Fuse 40 and fuse 26 are both powered by Fuse FA5 in the engine compartment, so I'm not stealing power from a different source, and I de-rated the fuse by 5 amps to be safe.

1 day and battery is still good, will see how it works in the long run, but does the same job without the relay.


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## PowerslavePA (Dec 9, 2014)

You do not need all that wiring, relay, nothing, to keep the AUX 12V plugs hot all the time.

Here is how you do it on an MK6:









Basically, you remove the existing fuse for the 12V ports, then install a
jumper from another 12V source, and viola, they're on all the time.

There is no need for a relay, and all that extravagant wiring. If the coil
is energized when the car is off to keep the AUX ports hot, you placed
a load on the battery, which exceeds the normal parasitic drain by the
other modules in the car when it is off. 

Automotive relays are typically 150-200mA ("72 ohm coil") current draw, on TOP
of what is already being drawn.

Smaller 12V reed relays are in the 12mA range ("1K ohm coil).


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## risottto (Jun 13, 2017)

*xactly*

You are totally right, My fusebox looks almost exactly like that now, and no power drain. Only difference is that I'm turning the always-hot by default trunk outlet to switched with ignition.


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## PowerslavePA (Dec 9, 2014)

risottto said:


> You are totally right, My fusebox looks almost exactly like that now, and no power drain. Only difference is that I'm turning the always-hot by default trunk outlet to switched with ignition.


Then you can re-route (jump from) the fuse for the interior ones, so it has power with
ignition only. Or connect it to any other ignition switched fuse location with your plug
in fuse holder. You don't need a relay.


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