I'm a vintage road bike guy -- don't know from nuttin' about MTBs, but I picked up an older, dark blue, almost black Diamondback Sorrento today for $20. It's a 21-speed with Shimano Acera X derailleurs and crankset, Shimano Altura shifters and brakes, Cheng Shin SemiSlicks. According to labels on the frame, it's designed in the US and built in China.
It's been used but not apparently abused. The wheels are straight, and it goes down the road like it ought to. The main puzzler is the shifters (this is where the indexing is, right?); neither of them do anything. pressing either lever has no effect on the derailleurs, although they're both in apparent good working order.
I'd like to try to find out about what year this thing is, and what gives with the shifters, like whether these shifters belong with these derailleurs. If they do, I need to find some info on servicing and troubleshooting them. Like I said, I'm a friction-shifter person and this is the first bike I've had with indexing, so I'm a complete noob about maintaining them. Any info or suggestions will be much appreciated. Here's pics of the bike and components: