Vaccum???????

Need some help here, 99 with about 118k. A/C switches to defrost when under load. Been doing some research and have checked the vaccum line from the manifold all the way to the vaccum ball in the front bumper. Nothing seems to be loose or cracked, and the only check valve I've found I took off and seemed to be fine. Any suggestions would be awesome, just tired of it switching back and forth.

What do you mean by seemed to be fine?

I'd check the vacuum ball again. Maybe even try bypassing it temporarily if you can't actually test it for vacuum. The balls can crack, and if there's been a fender bender, sometimes a mounting foot will tear off, and leave a tiny hole.

Check carefully too where the vacuum line passes under the battery tray. Corrosion can eat little holes there.

Many people put the ball inside the engine compartment, where it's a little less likely to break.

There's also all sorts of vacuum behind the heater controls that can leak. Vents typically default to defrost with loss of vacuum and there is less vacuum at higher load or wider throttle opening. You have a leak somewhere. Get the propane bottle or smoke machine out.

A hand vacuum pump such as a Mity Vac can be a real asset here. Perhaps not worth actually buying one, but if you can borrow one it will save some time.

Try to find someone in your area who deals with old Mercedes cars, which have vacuum everything. A real Merz nut probably wears one on a holster.

The ventilation system is designed to default to defrost when the vacuum drops out.

As previously stated, high loads (climbing hills, towing & lots of gear) will cause the vacuum to drop out.
Cracked and decayed vacuum tubing is also a common issue. It only takes a pinhole to cause a loss of vacuum.

Cars with high lift cams will frequently run low on vacuum.
This IS A BIG DEAL if they have vacuum assisted brakes or distributor vacuum advance.

An old school trick to locate a vacuum leak.
Start engine and let it idle.
Remove vacuum lines one at a time and spray starter fluid into the line.
If the RPM's increase then you've found the leak.
The replace that section of line or fitting.
Be methodical. Do one line at a time.

Once everything is fine and operating properly AND if you still have poor vacuum, then consider having the engine checked out.

Another alternative for modified cams is to add an electric vacuum kit available from almost any hot rod shop.
They turn on only when the vacuum drops out, keeping all of your vacuum operated devices working properly.
Stainless Steel Brake PN:28146 sells for around $250.

Check out the diagram(s):

These are all the lines affecting the HVAC system. The engine lines are not associated EXCEPT for the line coming from the intake manifold. If all these lines are solid, AND the lines inside the dash are solid...you're good to go.

The suggestion above seems a bit ambiguous, as removing a line and putting starter fluid into it should always send the fluid into the engine and speed it up. Either I'm misreading, or there's a step missing, I think.

I have heard of spraying some combustible fluid, or propane, around suspected leaks with the line connected. If the engine speeds up, you've found your leak.

Needless to say, if you're dealing with combustible substances in the engine bay, you need to be careful and keep the quantities small, and make sure you do not have any stray sparks. WD-40 can work, but it's messy unless you're already pretty sure where the leak is.

If I were doing this, I'd actually get a vacuum gauge, since such things are extremely handy and useful for other diagnoses as well. Hook the gauge up to the manifold, and check the reading. You can now disconnect and plug vacuum takeoffs one at a time, and see if the reading changes. If plugging a line at the manifold sends the reading up, that line is leaking. If the reading is low but none go up (don't forget to include the power brake), then it's time to suspect the intake manifold itself.

Although modern engines often have somewhat lower idling vacuum than they did 50 years or so ago, the gauge is still a very handy diagnostic tool, which can identify all sorts of ills pretty quickly.