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.
Makes sense being the shifter. I can't get at the bike right now to take a photo. It was bought in the 90s, has the brake built in and has two levers for the gears, one up one down. Would have been pretty cheap then, let alone now. very basic.
Do a basic cable tension check by shifting all the way to the small sprocket in back, and then see how taut the cable is. It should have some tension, but not be tight. It most certainly shouldn't be slack. If there is perceptible slack, loosen the pinch bolt and pull the excess cable through. Tighten the pinch bolt and then see how it shifts onto the bigger sprockets.
You'll probably need to check the high and low limit screws, too, to make sure the derailer doesn't go too far either way.