in my 2000 cherokee laredo, 4.0 2wd , when climbing a hill or accelerating heavy the ac blows thru the defroster, then moves back when condition goes back to normal driving . also, without the ac or anything on the inside of the windshield fogs up doing the same thing , up hill or acceleration. What do i look for and appprox. where ?

 

check your vacuum lines

 

Test the check valve thats inline with the vacuum hose going from the intake manifold to the fire wall. That was my problem. The vacuum was escaping when the engine vacuum dropped.

 

near the battery there is a hard 1/4" vacuum line that runs to the vacuum reservoir under the bumper. Ive had 2 that rubbed on the battery putting a hole in that line.
Also ck the reservoir cannister under the bumper and make sure it hasnt been knocked loose from it.

 

i dont see a reservoir canister under the bumper.

 

Any idea if this would apply to my '98?

 

My '98 also does this whenever I am on the Freeway, but not at surface street speeds. It actually switches back and forth between the Defrost and the Vent continuously while on the freeway.

 

when the engine is under load or at low vacuum it ed energizes all vacuum actuated accessories mainly the A/C to return the vacuum to the motor for optimum power. this also means you probobly have a small leak somewhere!

 

Easiest way to track a leak....get a can of carb cleaner and spray around vaccum lines when the rpm's raise ur in/on/around/near the leak good luck!!