How do pass/fail classes look to PhD admissions committees? I'm a junior hoping to do a PhD. I'm in a graduate level class right now in a different field. I believe almost all the other students are PhD students. The first couple weeks of the course were just barely accessible, though very challenging. Now, however, I really find myself unable to complete the problem sets. There are no office hours either. 
How bad would it look to pass/fail this class? It's not particularly related to my intended research focus in the PhD.
I believe I am a strong candidate aside from this class. 
Also, I'm really not sure what grade I would get in this class. Probably a B at worst. I'd rather not mess up my GPA though. All of my other technical classes are A's or A minuses.
In short, this looks bad.  

When I see a "Pass" or "S" grade on a transcript that I'm evaluating, my default assumption is that the student did work equivalent to the lowest passing letter grade (e.g. a "D" in the US system.)  

I tell the undergraduate students that I advise that if they want to go on to graduate school they should avoid taking courses on the S/U grading scale.