So I installed a after market radio. Apparently BMW doesn't have a ACC+ wire, only a constant 12V (yes it was weird for me also) I checked all the cables with my multi meter but there wasn't a single ignition 12v So I connected a cable straight from the fuse box where I found a 12V ignition socket. Hooked it up to the radio and it worked, radio turns on and of with ignition. BUT THE PROBLEM IS, my radio won't remember anything, for more or less 15 minutes without the key ignition it does remember everything but if I leave the car without ignition for a hour or so it lost everything, the original BMW radio didn't had this isue at all. So does anybody know what is the problem and how I can solve this ?
Most cars, including BMW, have a key controlled ACC supply - usually position 1 on the key, with position 2 being the ignition (battery warning lights etc) and position 3 being start - (some older diesel vehicles had position 3 as engine heaters, with position 4 as start.
Find the wires connecting to the ignition switch - under the steering column and check there with your multimeter.
You did not supply your year so I just guessed at one. E81 >2005/08 Looking at the power supply schematic you are correct. I did not dig much deeper then the schematic and not being familiar with bmw radios this is mostly just guessing but Your aftermarket is probably designed differently requiring a KAM (keep alive memory) supply at all times. and the power supply is probably being cut off after a hour or so for some reason. BMW and MB like to cut power to modules when they sense certian faults. leave a meter hooked up and shut the car up for a few hours and see if the supply is still there.