I have an old Giant GSR 400 mountain bike (i know it's pretyy dam old). Trying to get it working again as hope to go riding this saturday am. however i have hit a big issue, namely the Shimano SIS derailer and shifters. It will change into the high (small cog) gears, but wont into the low. It's not a matter of the derailer not moving the actual shifter doesn't seem to have anything to "click" into. I simply push it and it doesn't do anything, it's just a lose movement no click.

I've tried the high and low screws but this does nothing. I've also twisted the tension cab;le that goes into the derailer itself, and again no improvement.

Ideas. I totally lost.

Chris