1script - 9:56 pm on Jun 8, 2013 (gmt 0)
The only possible way of negatively effecting a site's ranking in Google by adding site search is if the search results have become indexable and Google picked up a lot (in theory it could be millions) of URLs, each of them a search result for a different keyword. I've had that happened to me before. There's a simple [but by no means quick] fix: no-index all search pages. Be sure to still have them allowed in robots.txt else they won't be able to read the no-index meta tag.