There are a few things that can cause a closer to slam.
The closer could be adjusted incorrectly. Turn all regulating screws clockwise until they stop. Then open each valve 1 and 1/2 turns. If the closer now seems to work, it can be adjusted to your closing preferences.
The closer may have lost some oil. If there is evidence that the closer has lost oil, nothing can be done to get it to work correctly. It will need to be replaced, or sent back to the factory for rebuild. Contact Sales Support for warranty/repair information.
There is internal damage or component failure. The closer will need to be replaced / repaired. Contact Sales Support for warranty/repair information.
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