Are you using a setting for AC (not DC) volts? Not sure how you could sometimes be getting a negative number for AC volts.
I had it set for DC. My only defense to the charge of idiocy is that the meter is new to me as of yesterday, and I accidentally threw away the instructions with the packaging, and the manufacturer's website has been down all day. Even so, I find myself guilty as charged. I am an idiot.
The meter is a Gardner Bender GDT 3190. I don't think it's sensitive enough to perform this test. So I need to go out and buy another meter. For the love of...
bc