there is an internal battery in the pcm. it is kept alive by providing power to the pcm all the time. check your fuses for constant power to the pcm. if you have that power, then the pcm will have to be replaced because the battery is not servicable.
Perform the troubleshooting procedure associated with the P1633 code, refer to the repair manual for this information. If you don't have a manual, please check here: http://bit.ly/mitchell_repair_manuals_online