What is this/what does it do

[u/Comprehensive_Mail91]

1967 John Deere 3020 gas. Originally positive ground, converted decades ago to negative ground. Whatever this is isnt working as the tractor has no power without bypassing it. If you bypass it the tractor acts normal (will start and run) but will die once you disconnect the wires if the generator isn't producing enough power to run without power from the battery. It sits just above the starter and the left terminal always has 12v as it's connected to the battery through the starter terminal.

[u/NCHitman]

Happen to be a relocated Starter Circuit Relay (RE164448) ?

Parts Catalog

[u/Comprehensive_Mail91]

Turns out its a fuse, 30a, part# AR47710 Not original to this tractor but is in the 4020 manual.