Oldpeddaller

Can anyone help?

Briefly I acquired an old Y frame full suspension MTB with Shimano 18 speed and SRAM 'gripshift' twist grip shifters on Sunday. The bike was stuck on the small chain ring. I thought this would be simple as I discovered the front gear cable was seized.

Went to change the inner cable. I finally persuaded the fixed grip off the bars and removed the twistgrip, twisted it and separated the two parts and fed in a new cable. Put it all together and - NO INDEXING. The front changer shifts well enough now, but the moment you release the grip it flies back onto the small ring. The twist grip won't click or stay in position to hold the cable tension on the middle and big rings. I can't see what's wrong and now it's dark I've given up. I suppose it could have been broken before the cable seized I suppose.

I thought I'd got a brand new SRAM unit I bought cheap years ago but it looks as though I may have used it although I can't remember when. I don't really want to buy a new one or fit a new thumb shifter - I've got plenty of those.

Sorry no photos, my new camera phone hasn't arrived yet. Can anyone offer advice on the correct way to reassemble this gear shifter - in case I've done it all wrong. I rotated both halves until they could be pushed together, then turned. The right hand grip clicks strongly between gears but the left one now just turns smoothly.

Would really appreciate your expertise. Thanks.