my check engine light is on, and when i stop at a red light, my rpm's drop from 1000 to 100 when the AC kicks on...when i turn the AC off, all is well...what causes this and how can it be fixed...as a broke college student, i hope to find an answer so as not to get ripped off by shady mechanics, thanks

You need to find the error code the light represents ... without out it you're just shooting blanks ... many auto supply stores will check it for free ...

I doubt it is AC related, just the load created by the AC running makes the real problem stick out