Hi all I need to find ignition wire as I need to connect t to my ower supply. This should be behind the key lock right? How do I approach to it and how to know which one is right wire? Truck s Mercedes Atego 2005 year


I wouldn't necessarily jump to the stereo wire; this would depend on where your unit will be mounted and what it is. As stated; you shouldn't draw excessive amounts of power (>2-3 Amps or so) and it should be fused. If you will be needing lots of power then use the ignition wire to switch a relay with battery power, not to provide power itself.

If you're mounting it behind the stereo; then by all means use the stereo. Yellow is usually constant 12V from the battery (unless Mercedes is different in this regard) and Red is usually switched (only provides 12V when the ignition is in Accessory or On).

There may be other reasons you would not want to use either of these; first if the unit will not be anywhere near the stereo and second if you need ignition-on only (off for accessory).

My first stop would be the fusebox. That will contain most sources of power to tap into; accessory and ignition-on, ignition-on, and start. You'll need to find an appropriate fuse (avoid important ones like 'fuel inj.', etc.) and tap into the fused side if you want to do this; they even make terminals for such a job that straddle the fuse pin and present a terminal above the fuse. If you get lucky, there are many times empty slots with a live pin on one side that is the type you are looking for, so you can add your own fuse and not piggyback on another circuit's.

  • Hi thank you for your information. So basically why I need ignition off and on is to power a PC in the car. I have this power supply which gives me stable power. So what do you suggest me for this item? LINK: mini-box.com/DCDC-USB – user123_456 Jun 6 '12 at 21:01
  • I would want that on for both accessory AND ignition-on, if you want it switched at all. If you're mounting it near the stereo, pick up the wire that carries this to the stereo (most likely the red wire). Note that you should not add more than 2-3 amps to this wire; so depending of what you mean by 'PC', you should probably use a relay. However, if it were me, I would want the computer to stay on until shut off, even with the car off; maybe with some hidden on/off switch. – Ehryk Jun 7 '12 at 0:18
  • I will put it near the fuse box. I will take free one from the board for the input power. But there is not ignition on signal there right? I need to draw that wire from stereo for example. This unit that I have showed you is really great it starts after 20sec and delays shoutdown for 20sec and then cut the power source. – user123_456 Jun 7 '12 at 7:14
  • That will be in the fuse box as well. Probably 'Radio' or 'Accessory'. Take a test light and probe the tops of the fuses while you change the key position (start in accessory or ignition, whichever one you want) - you'll find one that's the right one. – Ehryk Jun 7 '12 at 9:10
  • Can I ask you what is the best way to connect to the fuse box? Are there any special wires? and when I will use ignition, it is a wire that is coming from fuse box? do you have some link of what elements I can use to connect to the fuse box? As I don't want to be screwing something and I'll do it by myself.. Please help. You can send me an email if you don't want to be talking in here – user123_456 Jun 11 '12 at 9:24

What are you trying to power? The ignition wires are generally only intended to power the ignition - if you want to run anything else you are better off taking an appropriate feed from the fuse box, or a relayed feed from the battery, depending on the power draw of the intended application.

Either way, the best place to look is a wiring diagram for the truck in question - usually found in the back of any decent workshop manual. This will give you the colour codes of the wires you are looking for - ignition-fed live feeds are usually red, but this does vary by manufacturer.

  • I need to send the signal to my unit when key is turned on or off. That's why I need this. Problem is there is now manual for it. Not that I can find of any – user123_456 Jun 6 '12 at 11:01
  • Ah, in that case you want to look for the signal wire going to the stereo - Yellow according to the proper ISO colour code, but again this may very according to manufacturer. The best place to look for that would be behind the stereo. – Nick C Jun 6 '12 at 15:51
  • Thank you so much. So I just pull out the stereo and see where ign is going right? Yellow one you say? Yea that's usually the right color. I think it should be according to ISO as this is Mercedes :) They have standards – user123_456 Jun 6 '12 at 18:09
  • As the other reply says - check with a multimeter first to work out which is which! Looking at the link you posted below, I would expect the PSU to have two inputs, one expecting a permanent live feed and one taking a switched feed, so that it can send the 'power off' signal to the motherboard when you switch off the ignition. It'll draw more current from the permanent feed, so make sure you use a good feed, separately fused, for this. – Nick C Jun 7 '12 at 9:48

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