Air exchanger problem

[Gus]

I have a Lennox air exchanger original to the house '94. When the defrost cycle runs the door that opens to let in warm air and close off outside air motors all the way to the closed position then slams back down. It does this over and over. It won't stay blocking the cold air.

I took it all apart and cleaned everything I could find. Looks like a really simple fix to just replace the motor. But at $150 I don't want to guess if the motor is the problem. Is anyone familiar with these enough to tell what the usual problem is? I am just concerned that maybe the brains of the unit are telling the motor to keep running rather than stop in the closed position?? The motor seems to lift the flap fine it just won't keep it in the closed position and it slams back down.

What say the experts out there.

Thanks!

[pikechaser]

Not framiliar with lennox units..dont get into much residential work anymore...does sound like an issue with the motor..have you looked at it closely? something broken off of it..or stripped out..in either case probly need a motor..