I do not use impacts but i do have a wide range of knowledge about them.
The head is broken, your spring which releases the arm, you have to install a new
head or better yet convert to gear driven without diging.
Buy Hunter ATR impact replacements which slip into the impact canister
Ray
Rays Above and Beyond Automatic LAwn Sprinklers
Raymond Padula Jr.
Thanks
Ray
Rays Above and Beyond Automatic Lawn Sprinklers