My van wont start whenever the air is damp?

Thanks alot, man! You probably saved me hundreds that a mechanic would have charged. I sprayed water on the crankshaft sensor on a dry day like you advised and it didn't start. Put a new one on and sprayed it and it started right up. The real test will be in a few hours when it rains, but I'm pretty sure that fixed it. You know what you're doing! Thanks again.

I was wrong. It is raining today and the van would not start again. The only thing I havent replaced is the ignition control module. Any other ideas?

I checked out everything I could see without taking it apart. Nothing seemed wet.I give up. Taking it to a mechanic tomorrow. Thanks anyway

I had this problem for years and understand so did GMC trucks.
Somewhere in this period around 2000, GMC went to a low profile distributor.
After a zillion mechanic in several states and a lot of $$$ later I finally found a mechanic who figured it out and a solution.
What he did was put dialectic grease all around distributor. Then he saw that air conditioning lines went directly over the distributor causing condensation so he split a garden hose which he covered the lines with.
I have not had any problems since and that was about 4+ years ago.
Hope this helps