Vacuum problem

Hi,

I have a Ranger 98 and I have a problem with the vents. At idle, I can play with the vents (defrost, floor, ...), but on the road it goes directly on defrost. I think it is a vacuum problem, how can I check that? Where can I find the vacuum diagram with the explanation about how it wooks.

Thanks

I have a '99 Ranger and I have the complete factory manual however try buying either the Haynes or Chilton Ranger Repair Manual for your year truck. These manuals are fairly informative. I don't think you are loosing engine vacuum cause otherwise your truck won't run right. Sounds like a control issue. The two deflectors may not be working right.