When I have my car PC connected in the car, I can't get it to post. Fan comes on, but I get no beep, no VGA..

When connected in the house to a standard ATX power supply it works fine.

The CPU and fan come on for 10 seconds, then it shuts off and just keeps restarting until I unplug the power. 10 seconds on, no post, restarts. Always the same.

Voltage is all OK and I'm using a good ground. Connected directly to the battery so amperage should also be fine.

I have tried reversing polarity of J8 without any success.

I have also tried unplugging everything and trying to boot with just the RAM and CPU, in case I was overloading one of the M2's rails.

Can anyone help? Thanks