judging by the original posters comment (before the question), he already knows the problem...he is trying to trick us!
The problem is that when he gives the plc a 'stroke' (could be kinky, could be medical, could be with a BFH), it overshoots after 3 minutes (could it be that he is 'stroking' it for 3 minutes, if with the BFH I don't see how it does anything afterwards)
...there is the problem! The solution, on the other hand will require knowledge that we do not possess,i.e., the exact set-up that the OP has. Type of plc, the actual program, his definition of 'stroke'...