what funtion does it serve?

it has only the one line to the tank, what is it for and where does the line go from the tank to what and what is it supposed to do?????????????????????????????????????

I am having trouble with the vents, its 98 degrees and 72 % humidity, when I turn on the AC it blows from the defroster but the display shows MID but no air out of the vents, just the defroster and i can switch it to the floor and it come out of the floor and defroster

thanks for the reply about a vacuum tank, but just exactly what function is it for? does it have anything to do with the vacuum lines that controls the doors that direct the air flow on the AC? or perhaps something else?

Basically, what a vac tank is, is a reserve, for when a vacuum is needed, when there is no vacuum being produced....

How is vacuum produced? Basically, at or near idle.......this vacuum is used for vac actuators in the HVAC system, and in some cases, the cruise system.....

If there were no vac tank, the only vac supply would be the line, itself, and that wouldn't amount to much.....there is usually a check valve in the line, so that when the throttle plate opens during acceleration or cruising, the vacuum in the line is not lost....but suppose the car is cruising, throttle plate open, and no vac is produced.....and you change the mode of the HVAC system from vent to another position....whatever vacuum is in that line, will quickly be used up....and if it gets to the point that all the vacuum is used up, then the system will "default" to the defroster position...this is what happens when there is no vac in the system......however, with a vacuum tank, you have a reserve of vacuum available for use......this is the purpose of the vacuum tank, to provide vacuum when no vac is being produced during acceleration or cruising.....

So obviously, you have lost vacuum because you only have air blowing out of the defrost vents.......there could be a leak, like a slit in a vac line.....there could be a vac line disconnected...there could be a crack in the vacuum tank.....so you will have to make a visual inspection of the vac line to the tank and to the HVAC system, looking for cracks or a disconnected line.....

Here is what I would do.....start the car and let it idle, this is when max vacuum is produced....now pull the vac line off the tank, and plug it.....do you hear a hissing sound from the tank? If yes, vacuum is being produced and held in the tank......if there was no noise, and if you have your finger on the end of the vacuum line and don't feel a sucking feeling, then there is no vacuum in the line and it has to be open somewhere.......

If you have vacuum at the tank, you will have to drop the glove box and find the vacuum valve solenoid box.......there will be a purple vacuum line....disconnect it....if you no vacuum there, there is an opening between there and the tank....

If there is a vacuum, then there is a problem with either the vacuum solenoid box or the HVAC control head....

thank you for your reply, I just happened to notice this thin plastic line by the tank and the end that plugs into the tank. It had broken off the line and i dont know how long it had been like this, the 90 degree plug to the tank was very deteriorated, i had taken the tank off from under the car to inspect it it looked ok, but when i went to reinstall some water came out of the tank,i drained it out. i was able to get a 90 degree plug from the junk yard and spliced it onto the plastic vacuum line and attached it to the tank, I then tried to see how the AC worked and it still came out of the defroster, but after 15 minutes of driving, cold air came out of the vents but it was weak, the longer i drove the car the air from the vents got stronger thank you for your suggestion on finding any leaks in the vacuum line

i still have more work to do to get it right but thanks again

I am including this picture of where the vacuum lines come together, if you notice i have 5 lines hooked up but there is one that is missing, is it supposed to be like that or is there another line that is to be attached to it?

I believe there are only 5 lines........

I never liked this set up......reason why? where the vacuum manifold lines, attaches.....I believe there is a 7 or 8 mm nut holding the manifold on....for test purposes, you remove the nut, and pull off the manifold....and then with a vacuum pump(manual), you can apply vacuum to each one of the lines to see if the individual actuator moves.....here'* the problem....there are small tubes that fit into each vacuum line.....sometimes they break off, causing a vac leak.....sometimes when you try to pull the manifold off, the manifold may stick to one of the tubes and breaks off when when you try to pull off the manifold after removing the nut.....

Question is, is there enough vac at the purple line, to operate the actuators? A vac gage or pump would verify this.....

You can also grab the manifold lines and give a slight tug on each line....if you hear a hissing noise, chances are one of those stubs is broken....

I was able to trace down where the vacuum line goes from the vacuum tank, it travels a long way and finally found it attached to a port that is on the back side of the upper intake manifold, i removed the vacuum line at the port and put my finger over the hole and got a good draw of vacuum from there, what is strange is that there is just one line to the vacuum tank