it's a long time since i last used my sabertooth 2x5.
it's in dip switch up down up down down up. (serial 38400)
but it now starts rolling the motors right when I feed it power.
S1 and GND are connected to an unpowered axon.
What is S1 connected to? So it turns on when the Axon is turned off? If the Axon isn't connected, does it do the same thing? If the Axon is on, does it do this?
(tell us about your entire setup, what your multimeter says, and your attempt to debug it - otherwise I'll ask a million questions first