just bought this 98 seville sls had been sitting for 3 yrs. I have 2 codes come up P1637 and p1638 I know what they mean and have checked alternator,battery and all fuses relays and grounds!all good. has new battery, the alternator is a delco remy rebuilt by them,had a sheet with alternator that stated that an improved regulator has been installed to make up for a possibaly bad thermosister!! still after about a min the message battery not charging comes on and stays on. could the pcm be bad?