Relay to Detect/Switch Engine Running?

Hi,

there are devices out there to detect engine running giving out ignition feeds on vehicle with few ignition feeds available, such as most modern CAN Bus cars. Useful for fitting towbars and head units.But i think they`re expensive for what they do.

Has anyone made one?

I`ve attempted this using a small low draw Relay and a variable resistor. But guess what.. Didn`t work.

any ideas???

I can tell you how to do this on newer GM and Chrysler products.  I think that there is a pink wire on the GM units, the problem is that if you connect anything with even the least bit of current draw, it freaks the computer out.  Simply connect the transistor as it is in the picture. 

http://www.radioshack.com/product/index.jsp?productid=2062611&cp=&sr=1&origkw=npn&kw=npn&parentPage=search

I know that this is a 15 amp transistor.  Without a heat sink it will easily supply 2 amps of output.  If you need more than 2 amps, you MUST use this device to trigger a relay.  You MUST insulate the metal tab, it has 12 volts on it.  I build these things for every shop in town.  The original need for the device was as stated on the pic, was to provide a switched out for an XM or Sirius tuner. 

Thanks very much for that, but that`s not what i meant either.. I`m gonna have to think of a way to `re-phrase` my question.. lol