Originally posted by nunzio I've tried with the film and without and there's no difference.
That's interesting. There should be a huge difference. In auto, the camera is metering off the film, so with no film in the camera the auto should do as you describe.
If it isn't the batteries (which it wouldn't be anyway as weak batteries will close the shutter early) and you can't get it working by exercising the ISO resistor, it might be the circuit board itself has gone flakey. I had that happen to one of my LX bodies. It took forever to get it fixed as Pentax had to fab the board for me at the factory. There were so many internal changes made to the LX over it's life that parts are a bit of a nightmare.
I'd send it to Pentax and hope they can resolve it.