Searched but didn't find anybody having the same trouble. My AC works good but, it has to be turned on before I get on the highway. If I turn it on at highway speeds it just goes to vent w/ no ac. I am pretty sure it is a vacuum leak, but where do I start looking?

The ac works fine, blows cold, I just have to plan ahead when I want to use it.
Any ideas?

 

Check the compressor clutch gap. It's probably excessive.

If greater than .030", remove a shim.

 

99 Could be a vaccum leak in the head unit. Ive seen it quite a bit.

 

I'll go with ProjectSHO's reply as a place to check. With a high head pressure and the higher rpms, it may be possible for the clutch not to apply enough friction on the plates for it to engage. If it is working properly at lower rpm, that is about the only plausable reason left. I also think I've seen him give that advice on a similar thread, and it did turn out to be the guy's problem