Understanding how these closers work will help you to understand the adjustment. A spring within the unit does the actual closing. If the closer had just a spring, though, the door would slam shut. The automatic closer uses air (or in some cases oil) to control the spring and thus the closing rate. In a pneumatic closer, for example, there is an air tube inside the cylinder. When the door is opened, this tube fills with air as the piston extends. When the door is released, the internal spring begins to pull the door shut. The movement of the spring is cushioned by the air in the cylinder, with an adjustable valve in the closer controlling how quickly the air is released. The slower the air is released, the slower the closing rate. Either slamming or incomplete closing can indicate either the need for adjustment or the need for replacement. As they age, door closers loose some of their ability to pull your door smoothly as the internal spring weakens and/or the seals in the pneumatic (or hydraulic) system begin to fail. Often times, minor malfunctions can be compensated for by turning the closing rate adjusting screw that is located on the door-mounted end on the closer. Turning clockwise will slow down the closing rate, while turning counterclockwise will increase the closing rate. A second way to adjust the closing rate is by making adjustments at the mounting bracket on the door. Many door closers have multiple holes on the closer and/or the mounting bracket allowing you to increase the tension on the internal spring in the closer. This is known as preloading. This is a good way to increase the closing force, especially in the last few inches of door closure if the door is not latching. The problem, though, is that this adjustment can decrease the amount the door will open. Check the total out swing of the door whenever making this adjustment to be sure that it is opening adequately for your needs. If your mounting bracket does not have multiple adjustment holes, you can also move the bracket further from the hinge. Many brackets have multiple mounting holes allowing for small adjustments. Keep your adjustments small, testing the door function each time... a little goes a long way here! There is another type of closer... the hydraulic door closer, which uses a light oil instead of air as a control fluid. As the door closes, the internal spring forces oil to move at a controlled rate between two compartments inside the closer. The only time you would see the oil is if the unit malfunctioned. Hydraulic closers don't have an adjustment screw, but instead are generally adjusted by physically rotating the body of the closer on the piston. Because the flow of oil is more easy to control with precision, these closers work more smoothly than air-controlled closers. The cost of these units is a little higher, but not staggeringly so. If you need to replace the closer, take the old one with you to the hardware store. If you purchase a close or exact match you may save yourself some work since the hardest part of the installation is installing the door and frame brackets. With an identical replacement part, you can just replace the cylinder and leave the rest of the hardware in place... unless of course the hardware is damaged.