Resistance should be very high at the M4 and depending on the MB around 10k ohm as the cap across the input charges with the multimeter. Thats measured with the MB and wih the M4 unpowered.
I have the same problems as you guys. I have 2 M4-ATX power supplies and i will try to narrow down the problem so if anyone can help us. Both of the units, if you connect the multimeter to the on/off pins of the power supply and you give power on the IGN then you can hear a beep on the multimeter for about half a second which means everything works normal. I have 2 motherboards. A new one and an older one. Everything applies to both power supplies. The new motherboard fires up just fine but the older one seems not to get the on/off pulse at all although i know that it should. If i short the motherboard pins myself the pc fires up fine. So i connected a relay and let 12 volt current to pass through the power supply's on/off pins to arm the relay and guess what. The relay armed but it stayed armed forever. While the psu was on i disconnected the power line from the relay and connected it back right away and the relay didn't arm again! So as you see i managed to reproduce the other problem, where on some motherboards the psu holds the on/off switch until the pc shuts down after 4 seconds.
Now after long time and emails, i l found out from the man than built the M4-ATX that the unit uses a 1 bit bus switch which has an internal resistance of 5ohm, to turn on the motherboard and the max current is 50mA. I don't know what all that means, but i hope that someone in this forum is and he will help us. Maybe with a specific relay we all will manage to power up our car pc's.
Oh btw, is it possible for you to give the contact info of the guy who made the m4-atx?
Now i am trying to solve the problem with the amp remote. One psu is turning on the amps right after the psu goes on and turns them off about one minute after i cut the ignition line but thats ok for now because that pc doesnt produce a pop from the speakers. The other psu turns on the amps only in p0 mode and thats a problem because the pop is so loud that i think it will destroy the speakers and my ears so i have a little switch for now to turn on the amps myself.