Is there a lemon law on used cars even if you have signed an "as is" agreement?

There is no lemon law in South Carolina for used cars (only new cars are covered).

Generally when a person buys something "as is", that cuts off any claims against the seller.  An "as is" sale subjects the buyer to the chance that what is being purchased has flaw or defect. Additionally, car sellers are generally not responsible for repairs or the like in the absence of a warranty.

However, if you can show that the seller defrauded you in some way; that is that they knowingly made specific claims upon which you relied on in buying the car, then if such claims turned out to be untrue, you might have a legal cause of action against them.  But the statement would have had to have been detailed and specific, not just something like, "This is a really great car".  It must also be relevant to the problem at hand and can be proven to be false.  For example, if the seller said, "Don't worry, we just put new brakes on it" and then the brakes needed to be repaired.  If upon such repair, it is discovered that they were not new, then that would have been an intentional misstatement/misrepresentation of the facts that would in all likelihood give rise to a claim against them.