- There is a mechanical linkage between lens and body.
- When you move the aperture ring, it causes a little lever to move, which mechanically interacts with a lever in the body, telling the metering system what aperture is selected on the lens.
- If you remove the lens and actuate the aperture ring, you will see the location of the moving lever on the lens, and then you should see the corresponding lever in the mount, and it should be able to move freely...its position is passively determined by the position of the lever on the lens.
A mechanical link tells the lens aperture to stop down to shooting aperture when the shutter button is pressed.
The FD mount system allowed automatic diaphragm function, and a new signal pin (compared to FL mount lens) supported full-aperture metering.
A second signal pin for the "auto" setting of the aperture dial, plus a linkage to allow the camera to set the degree of diaphragm opening, enabled integral auto-exposure.
Understanding that will help to diagnose where your problem originates, in the inability to change the aperture ring position.