An automatic timer is a device that has been placed in some combiners to ensure that they separate after a certain amount of time.
Marvel Comics continuity
Piranacon suffers from a serious love of the hunt. It's so intense that he often spends all of his time hunting a target. After he has caught said target, he simply goes looking for another target. It became such a problem that Snaptrap, possibly the most deranged member of the Seacons, was forced to set an automatic timer to ensure that Piranacon would separate into the Seacons again. Despite this one drawback, Megatron, a deranged individual himself, often wishes he has more soldiers like Piranacon.
Generation One cartoon
While the Terrorcons were working with the Quintessons, the Quints installed an automatic timer to cause Abominus to separate after a certain amount of time, ensuring that Abominus didn't get out of control. Computron fired an energy blast at Abominus, cause him to split apart without the timer's help. It is unknown if the Decepticons removed the timer after the Terrorcons returned to them.