Have a 1989 Ford F700 with a gas engine. Having starter issues, it will turn over and let go of the switch and starter continues to crank until you unhook battery. The starter and the starter relay have both been replaced. Thinking the ignition switch may be next place to check. What would be the way to test the switch? Also anything else I should be looking at? Thanks in Advance!!

 

Disconnect the battery and check the resistance across the switch while attempting to start and check the s terminal for voltage to make sure it's not getting power from somewhere else

The relays can weld together I've only had it happen once but I'm pretty sure it's only the old kind that did that and you have to hit it with a wrench to make it quit starting