It's hard to advise also, without knowing what year your car is.But, it sounds like you have a MK2 car with AC. It's the only one I can think of that has a long ALT bracket with no adjustment. If that's the case, then the adjustment is in the small idler pulley that sits just beneath the alternator. You loosen (slightly) the 17mm center nut, and the you use the 13mm adjustment bolt to pull the pulley tighter. When you get the correct tension, then lock the 17mm nut back down.