I recently noticed that my Nikon F3's light meter stopped functioning normally. The battery in it is brand new, and I've taken it out and put it back in multiple times, making sure to clean it for any possible contact issues. Everything electronic works great, except that the light meter as of late seems to think everything needs to be underexposed. Even at 8 seconds shutter speed with the apperture fully opened and pointing directly at the sun the light meter shows a "-" telling me to open up more (this is at 400 ISO).
When I set the camera to Auto mode the shutter speed seems to adjust just fine, although I'm not entirely sure how accurate the adjustments are (they seem pretty accurate from experience, though). In manual mode, the shutter is basically forever stuck in the "the shot is underexposed" mode.

The thing is that my light meter worked fine before, and this problem started happening just recently out of the blue and it's really puzzling me. Any ideas on what's wrong or what I could try?

Photography - Nikon - F3/T 35mm SLR Camera