I would make sure that everyone understands that you cannot, no matter how good you are as a leader, manage "up". The current Manager runs his/her department ultimately. He/She has deliberately made themselves central to every process in that department. No need to focus on their the maanger's not cooperating. You shouldn't insinuate yourself further if you meet resistance. Ultimately you are trying to help and improve things as requested. I suggest you review the success you have had so far with your management and suggest that because the Manager of the group is making further change difficult or unnecessarily complicated due to that manager's need for control, you suggest that that Manager must now take the reigns moving forward. If this is not a highlighted and well meaning strategy, you will not be successful in future endeavors with the department. You will butt heads and clearly not be as ineffective. I would step aside and allow management to deal with the manager and his department (as clearly he has issued or why would your help have been enlisted in the first place). The management will will either address the issues directly and their need for your continued leadership or will allow the Manager to try to keep improving his/her department alone. You need to be strategic about your efforts and not unwittingly set up an adversarial position.