I would suggest investing in a service and repair manual for your tractor; here's a helpful link: https://www.tradebit.com/filedetail.php/274757894-ford-new-holland-5640-6640-7740-7840
This way you can get all the electrical voltage and resistance specs for the components you need to test instead of using the process of elimination method that could end up being as costly as bringing it to an authorized repair shop...and eliminate the frustration factor in DIY projects.
The link I posted was not to promote this vendor; and I certainly have no personal or financial interests with this company; I'm just trying to be helpful and pass on good info. You may have your own local tractor dealer that can get you a similar type service & repair manual as well.
In the meantime, I would check my voltage and resistance readings on the switch, fuse, relay, and solenoid components and call the local repair shop to get the range specs on each.
Best regards on your project.