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?

 

Oh, it is a 99 4.6 4x4.

 

Look between the battery and the fender.

 

Thanks, I'll go check it out!

 

that would be my suggestion too.. good luck fixing it

 

Also check behind the Batery for a vac line going in the firewall

 

Are you certain the compressor is engaging? How many miles? I had a 97 and I had to reshim the compressor clutch. Worked for a while, but the clutch was worn too much, and eventually failed again. New clutch kit solved the problem.

 

I you have the climate control that is set by the temp, instead of using the three dials the o-rings go bad inside, there is a how-to somewhere if you search im sure it will come up.

 

I have the three dial hvac control. What vacuum lines am I looking for? I took out the battery and battery tray. What are the two plastic boxes that fit behind the battery tray? The upper one looks like it might have a vac line but the bottom one looks like it has wires?

 

There is a whole gaggle of vacuum lines that come torether at one junction there between the battery and the firewall. One on the lines goes out into the fender where there is a vacuum resiviour canister hiding. But the line that I've ussually seen go bad, plugs into a grommet looking piece of rubber that soes through the firewall, just below the big electrical junction. It is just one small little vacuum tube. The last one i worked on, i had trouble getting my hand down to the place it plugs in, because there is so much crap around it.
It just kinda hides there, but it is above the evaporater drain, so if you're looking down there look a little higher, up below the electrical junction. Also since you haven't lost all control function, I'd say the line is just cracked and not seperated.

 

I just read all of the posts again. I thought you were saying it was blowing cold, but just out of the vents only.
Is it blowing cold air? That would be a different set of circumstances all together.

 

Yes my ac is working, but for example, but I can only switch to ac while not under a load. If I try to turn the ac on on the highway it doesn't turn the ac on it just goes to the fresh air vent setting. If the ac is on, it stays on.

 

That problem is a vacuum leak 99.9% of the time. You just have to keep looking till you find it.
Did you find the tube going into the firewall?
If not I'' go take and post a picture if I can get a good one for you.

edit: But it is usually defrost, where the air comes out, so the leak you have must be tiny.
Have you got a high speed idle since this started?

 

Sorry for the delay, things get busy in the summer. I found the line that goes into the firewall. I didn't see any slits or tears in it. There is not much behind my battery tray, the two plastic boxes that mount on the battery tray and the vac line that goes to the firewall and thats about it.

I occasionally have had a higher than usual idle speed. It doesn't happen every time.