Machine spirits are limited Artificial Intelligence's. Think around the level of a dog, although this does change depending on the machine. Having read Priests of Mars there's an incident where a Titan princepes is overwhelmed by the machine spirit of his Titan which then proceeds to have a hallucinatory flashback to their previous war zone, takes control of the weapons systems and opens fire on a friendly Titan. Then there's the incident with the crimson fists, where a Land Raider goes on a killing spree with no crew. Machine Spirits are Artificial Intelligence's but they are not truly sentient. The only possible exception to this could be the Titans who are sort-of borderline. What creates this intelligence is quite mystifying, it could be just computers but I doubt that. Vehicles have taken an enormous amount of damage and retained their "personalities" suggesting that it is not just in databanks. Perhaps it is some kind of by-product from the STC machines.
titan also absorb part of the princeps personalities, like a titan is the melding of all the princeps who drove it.
