To debug a tool, try this:
Run a tool interactively by filling in the dialog by hand.
After it runs successfully, open the results and right-click to get "Copy as a Python Snippet"
Paste this into an empty Python script and examine the syntax, particularly the SQL query.
I find that provided you are not using a personal geodatabase (*.mdb) AND you are using valid field names (No spaces or silly chars, starts with a letter, less than max chars, not a reserved word) you can also dispense with the double quotes around the field names. In spite of the help saying otherwise. This makes assembling SQL queries much simpler. Also try printing out your expression to see if you really got the expected valid expression.