Hello, maybe I can help. I am 75 years old with 55 years using the tools fixing these sorts of things.
My first guess or assumption however is not always on the right track, that is the nature of trouble shooting. The model number you gave is not checking, Tell me if the blower is a belt drive unit or not. (virtually all are belt drive)
About the only thing that squeals on these is the belt drive, when you put it on vent the pressure on the belt is reduced so is less prone to squealing.
The belt should have no more than 1" slack in it, ideally about half and inch. If that is OK and the belt still slips, then inspect the sheaves for wear, if the sheaves are worn the belt will slip.
Do not over tighten the V belt drive, too much pressure on the blowers sleeve bearings will ruin them, especially on older coolers.
You can buy a can of 'belt dressing' at any auto parts supply, a quick shot of that onto the belt with the motor running will fix most situations if the belt is not too loose.
Look all that over, tell me if this makes any sense on your end or not, we can go from here without a time limit.