The reason could be one of two things: Either the plug is wired incorrectly or the computer is sensing the higher voltage and causing the battery light to come on even though the alternator is charging properly. Please refer to the instruction manual tab on the website for specific instructions that will help you with wiring. For some older vehicles, the factory style regulator is not available with the hairpin style alternator. This is why your high output alternator will come already “upgraded” with the 15 volt regulator. It will have an oval plug which requires splicing into the factory plug. This is a simple ,fast step and these instructions are detailed for you in the instruction manual.