For me, the wouldn't makes it into a question, and expresses a form of negative expectation.
Do you have a wrench?
Simple question, no expectation implied.
Would you pass me a wrench?
Ambiguous. This could be a hypothetical: "Would you pass me a wrench if I were stranded without a wrench?" In this case, you aren't passing me a wrench right now, and the expectation might be negative, that you might not do it, and I want to know. But, the more usual interpretation is that it is a form of polite request. "Hey, give me a wrench, dude." And, as a form of politeness, the expectation is that you can and will do it. I expect you to pass me the wrench, and I am just being polite. A positive expectation.
You wouldn't pass me a wrench.
This sounds like a statement of negative belief, I don't think you would do so. But if we add a question tag (which takes the negative form of the original), it becomes clearly a request/question.
You wouldn't pass me a wrench, would you?
Here, the expectation is more negative than in "Would you pass me a wrench?". I am asking as a favor. Maybe the wrench is far away, or you are busy doing something else. I have less expectation or right to ask. I am acknowledging that I am imposing in some way. I am being more humble.
On the other hand, this could also be interpreted as a hypothetical. This time, the hypothetical has a positive expectation. An expected answer could be "Yes, that's right, I wouldn't pass you a wrench if you were stranded, because I hate your guts and you know it!"