Brain module

A Transformer's mind and personality is housed in its brain module. The brain module is a tiny cluster of microchips located in the Transformer's head. Transplanting the brain module into another piece of machinery effectively transfers the Transformer into a new body.

They give off a distinctive odor, which can be detected by Transformers with sensitive enough olfactory sensors.

The Transformers cartoon
While shrunken inside Megatron's body to remove the Heart of Cybertron, Brawn had to be restrained from damaging Megatron's brain in case it caused the unstable energy source to explode. Instead they proceeded to ride evil brain impulses (!) to the Heart.