In our Volkswagen Touran, I have already modified the fuse box in a way that the auxiliary 12V power outlets always deliver power, without ignition enabled, by connecting two piggyback fuse holders with inserted circuit breaker fuses from the always-on rail (lower rail in fuse box) to the fuse place of the power outlets, which is on the upper rail (only enabled on ignition).

This has the advantage that mobile phones and powerbanks are able to be charged without the battery-draining ignition enabled (key in ignition position) or the engine running. In addition, it allows charging devices while absent, which can be very practical on trips.

But the 40A fuse in the fusebox under the steering wheel (, which is for the air conditioner's fan and temperature regulation) is already on the always-on rail.


  • While the key is in ignition position,
    • The air conditioner's fan works, but not the temperature regulation (as appropriate, because temperature regulation would drain the car battery rapidly.), but I only need the fan to work, but without key in ignition, because with ignition enabled:
    • The battery power consumption is crazily, estimately beyond 10A, which is not worth wasting when needing only a fraction of it for a fan to turn or to charge a mobile phone using a Powstro car plug phone charger.
  • While the 40A air conditioner fuse is not inserted,
    • The user interface of Climatronic (setting fan speed, temperature, etc.) does still work, there is just no air from the fans.

What I want to accomplish is somehow allowing that Climatronic user interface to be able to enable the fans, without the key in ignition position, because as already stated, the enabled ignition wastes battery power that I would like to only be used for the fans, not for everything else the ignition activates.

There is even no way to turn off the car's lamps with ignition enabled. The dimmest one can get is the parking light, which still draws approximately 3 to 4 ampère. (Not yet tested using a clamp multimeter.)

I could use an external fan connected to the vehicular power outlet, and I will do so if there is no other viable solution, but making the car's internal fans work without key inserted is just a more elegant solution.

1 Answer 1


I would consider a relay and a handful of diodes (so preventing backfeeds) to supply the fan when ignition is off. But diodes for 40A may not be practical, you could set up the relay to choose between two supplies ie normal and permanent though.

  • 45V 40A diodes only cost a dollar or two each in "one-off" quantities - nothing much impractical about that. (That sort of rating is common in solar power installations)
    – alephzero
    Jul 26, 2019 at 14:55
  • @alephzero keeping them cool?
    – Solar Mike
    Jul 26, 2019 at 14:56
  • They don't even need a heat sink, unless the temperature inside your car is going to be over 100C. Another 20W thermal dissipation from a diode isn't going to be a big deal for an A/C system to handle, considering sunshine on the car roof may be generating 500W or more.
    – alephzero
    Jul 26, 2019 at 15:01
  • @alephzero OP is not running AC system just the fans if I understood the question correctly...
    – Solar Mike
    Jul 26, 2019 at 15:12
  • Wouldn't piggy backing of the fuse rails invalidate any manufacture warranty or even insurance for that matter if things did go wrong.
    – Dan K
    Jul 26, 2019 at 15:47

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