Sounds counter intuitive, but if you want to dry out your crawl the best time to ventilate is during the coldest days of winter.  Why?  Warm air holds more water than cold air.In the winter, the air in the crawl will be warmer than the air outside.  Substances move from areas of high concentration to areas of low concentration.  The water in the wet, warm crawl air moves to the cold air as it enters the crawl, thus reducing the overall humidity in the crawl and drying it out.  In the summer, the opposite happens. Warm wet air enters the cool crawl and transfers the water to the crawl.

There are many factors at play in drying out a crawl, warm outside air being one of them. Most crawls that I have seen that have water problems are from bulk water intrusion, i.e. a disconnected sewer pipe or high water table, especially when that water impacts the wood members of the house.

So what to do?  First, follow the international residential code and ventilate your crawl (1 sq ft ventilation per 150 sq ft of crawl, or 1 sq ft per 1500 sq ft of crawl if you have a vapor barrier).  Next, physically inspect your crawl.  If there is standing water present, deal with the source.  If there is no standing water, then leave the ventilation alone.