need help regarding my truck. Last night I turned the key off, and instantly we

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need help regarding my truck. Last night I turned the key off, and instantly went to start it back up and the starter wouldn't turn. I've changed starters in this truck before, so last night I put in the new starter and same problem. I left the truck over night. 1992 GMC 5.7 350 automatic transmission. so today I went back and really went through things. My first thought was maybe i had seized my engine. So i decided to check the flywheel and crank pulley to verify they'd move by hand, they did. (Did a full turn on the pulley) I ended up buying a third starter and verifying my wiring was correct. What is happening is my starter is engaging the flywheel but not spinning. After releasing the key the starter stays engaged, until I turn the crank pulley, I can hear it click back into the solenoid. I'm feeling like perhaps this is an electrical issue(Not enough juice at starter) today I did hook up starter number one and two to the battery but not connected to the flywheel and the starter engaged and spun just fine. Makes me think it never was my starter. I know I need to take a voltage meter and do some readings but I'm also curious is there a starter relay? I can't seem to find information about this online. thanks in advance