Hmm, maybe what we seen was already a "iterated" version?
First time, no Terminator sent back and no wreckage was found, the robots were rather stupid. Still they sent back a robot which boosted the research, getting smarter robots beeing sent back next time.
If you have no problems accepting timetravel, then the first Terminator movie should be one of the most consistent movies featuring it. Well, maybe not answering why the resistance-figther cant take a literal assload of weapons with them
The second movie is quite bad tough, for the reasons noted in that link and some more. Highly advanced robots is one thing - a energy-source and an AI beeing contained in a homogenous blob of metal beeing able to morph into skin, hair and clothes at will is something different.