El GranLuchador

Hello this is my first post although I have been "lurking" on these forums for about a month so I kind of know a little about some of the people here...

Anyways, I have a question about probable cause and unauthorized searches. What if I am stopped by an officer for any reason (speeding, broken tail light, etc)...he asks if he can have a look around the car, and I say "I dont consent to a search officer, I really need to get going."

The car does not smell like anything, and the only weed i have is a little baggy in my glove compartment and the car does not smell at all...So the cop wants to search, and I wont consent, all he has to say is "the car smells like marijuana" and he would have probable cause to search?

This seems like probable cause is a sham and the officer can say whatever he wants and use that as probable cause, because in a courtroom is the cop says it smelt like weed and i say it didnt it will be his word they believe...so the question is, is it really this easy for a cop to say he has probable cause? is there anything i can do to protect against this kind of "probable cause abuse"?