Remove the blower housing and disconnect the white wire from one module and crank the engine while checking for spark on both cylinders. If spark on both then reconnect that one and disconnect the other wire. If still have spark on both that would indicate a bad steering diode in one of the two ignition modules, but both will need to be replaced.
If the disconnected module has no spark then that module is bad but will still require the replacement of both. If you don't have spark in the module that is connected to the white wire and doesn't matter which module then the issue is in the tractor wiring harness and will need to find what is grounding the kill system.
