from the web... 'system high voltage'...
Potential causes for this code to set are: High resistance in the cable between the alternator and battery - possible Low resistance/open circuit between the alternator and the control module - possible Alternator faulty – most common
curious... what's the voltage at high RPM, just after starting ?
There is a 'technical' issue. Most DMM measure about 3 times a second,gives us humans a chance to read the meter... The tractor's computer may read faster, and see a 'high voltage', maybe once, trigger the error code. It'd be nice to 'clear the code' ,see if it happens again.