The belt comes loose when the ac is on. Need help

Hi, My car has been vibrating strangely lately and the belt will come off when I turn on the AC.

My car is 73 cougar, 351c. Does anyone know the cause and how to fix it? Thank!

Does the car vibrate without the a/c on ? My first choice would be change the belt or at least try and tighten it up. :smoke:

I assume by “loose” you mean the belt is slipping.

Not all belts are the same. They come in multiple widths and drive angles. The wrong belt (too wide or too narrow) will not properly engage the pulley and will slip. Guaranteed

There should be two idler pulleys, one fixed and one adjustable. Do you have both?