The repair had done a poor job, the check engine light is on again  

Hello king, Are you saying that your check engine light was on, and it was because you needed an alternator or your car was dead and the diagnostic was you needed an alternator? As an owner of an auto shop, and many years in the automotive field, I can't remember a code for an alternator, but for arguments sake we'll say that there is one. When you do a repair on a car and the check engine light goes off, the vehicle will run through monitors (this is done by the computer) when the computer detects a problem 3 times in a row it throws a check engine light, it could be that after replacing the alternator, monitors that hadn't run due to an alternator issue ran after replacing the alternator and the computer has now detected a new problem. This is quite common with certain things on a vehicle such as emmission problems, some monitors won't run when certain problems exist and when those problems are corrected the computer starts it's monitor system again and now has detected a new problem. This can be very difficult to explain to people that have no idea what a vehicle monitoring system is. I would give the auto shop the benefit of the doubt, that the original problem was fixed and now you have a new problem as that is the case more often than not.

Not sure where you live but I will give a simple example. In the state of CA we have to have our vehicles smogged, and on OBII vehicles one of the tests that are performed is that you have to hook up to the computer, a simple battery replacement that was performed just prior to the smog test will cause the vehicle to FAIL, the reason for this is that the vehice hasn't run through it's monitoring system, so the computer sees no data as a problem. So the customer is advised that they have to take the vehicle out and drive the vehicle in stop and go traffic and also on the freeway most shops here will tell a customer they have to drive between 75 to 100 miles to complete the monitoring system. After doing so most customers will return and the monitors will have all ran.

So probably more info than you wanted, so sorry for the run on. Hope that some of this made sense and best of luck to you at the shop on Monday. I do know how frustrating it can be. Cars are much more complicated now than they use to be and it takes a lot of training for mechanics to be able to work on cars. It would be so nice to have a machine to hook up to and state this is the problem fix me, unfortunetly it doesn't work this way.