Wire a switch to the ACC line and it will funtion as a valet switch, so you can shut off the PC properly when needed, as well as prevent the M2ATX from turningthe PC on when you don't want.
I'm wondering is it OK to add in an external switch at the ignition switch connection so that I could turn off auto run sometimes especially when the weather is too hot, in Malaysia it is average 32 deg C all year round and if the car is park under direct sunlight you could fry an egg on the dash therefore during such condition I would preffered not to run the Car PC until the air cond cool down the interor. Thank You.
If you want to run the PC when the ignition is off too then a 3-way switch is best. One for the acc line, one for off and one for always on
also, make sure the switch can handle a high enough current (15A or more), otherwise use a relay.
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sorry, I misread somewhere that a switch was to be put on the main line. of course the ACC line doesn't need a high current switch.
Saying that, In my case, I have a power conditioner inline with the power supply and that leaks a tiny current, so total off is important and for that I used a relay on the +12v line, which I can override.