Will an alternator off a 1996 Oldsmobile Ciera 3100 engine fit 1999 Chevrolet Malibu with engine?

It's unlikely a 1996 Oldsmobile Ciera 3100 alternator will directly fit a 1999 Chevrolet Malibu. While both are GM vehicles, there are subtle differences in alternator mounting, pulley configurations, and electrical connections between models and even years within the same model. The engine itself (3100 vs whatever engine is in the Malibu) is a major factor.

You'll need to cross-reference the alternator part numbers (found on the alternator itself) with the correct part number for a 1999 Chevrolet Malibu to be sure. Trying to use the wrong alternator could lead to damage, incorrect charging, and other problems. Using a parts website and entering the year, make, and model of your Malibu is the best way to find the correct alternator.