Cottonpicker

Got a few questions but the first one is about the dash vents, The defrost and floor vents are the only ones that work. The dash vents stopped about a month ago. anyone had the same problems?

oliver92

when i bought my truck i had just the opposite problem dash vents but no heater vents..take out the glove box and check that all the vacuum lines are connected to the pods that control the air directions..with the truck running you can switch the controls and watch the arms move and kinda figure out which ones arent working ..then trace vacuum from there hope this helps

SOOT4BRAINS

There are 3 vacuum controlled doors in the hvac. 1st is recirculate or outside air. 2nd puts air to the dash vent or the floor/defrost. 3rd puts air to floor or defrost. Since the defrost and floor vents work, I am guessing that the 2nd one is bad and is stuck in the floor/defrost position. The vacuum pot may have a bad diaphram or the line going to it may be bad. Or possibly even the control unit in the dash is bad. If you remove the glove box you will be able to see 2 of the vacuum pots, its the one on the left. It will have 2 lines to it, a red and a brown. Put the controls on so it should be blowing out the dash. Now pull the brown line off and see if it has vacuum. If it does, you have a bad diaphram in the vacuum pot. If no vacuum is present examine the line for breaks or kinks. If the line is fine, the control unit in the dash is bad. Good luck!

Aaron

oliver92

another way to check on the diaphram working is to remove the lines and move the arm against the spring tension..place your finger over the port and let arm go ..if it holds in place the the diaphram is ok..thats the way i check the vac secondary diaphram on holly carbs..did that with my truck upon 1st inspection then you know if its good or not

SOOT4BRAINS

Oliver, you are correct in this is a good way to test vacuum pots that are one way vacuum and one way spring. Unfortunately the vacuum pot we are talking about is 2 way vacuum. It does not have spring loading. It has vacuum on both sides of the diaphragm to open and close it. You could however put your finger over the nipple and try to pull the arm the other way and you will hear air moving if the diaphragm is bad.

Aaron

Jim5870

It is possible the doors inside the HVAC are siezied. The door hinge pin
can rust at the bottom in it's recess and cause too much friction for the vacuum motor to overcome.

oliver92

thanks for the heads up on that soot..

wannadiesel

Most likely problem is no vacuum to the HVAC system.

IRoss

There is a kind of 4-way vacuum plug behind the glovebox in the firewall...If you have no heat shield on your dp or if, say, your exhaust temps get outta hand it can melt the vacuum hoses going into this connector...justa thought

Cottonpicker

Thanks guys. I found the ones in the fire wall they looked to be fine but I haven't took the glovebox down yet. I'm taking my laptop to work tomorrow so I can read up on this while I've got the truck at the shop. hopefully its something simple.

grouch

What year truck do you have? If you've got a 1994 or later there is a servo that fails. I had a '94 and found that you have to remove the whole dash to get to it. You can sort of see if from the drivers floorboard. Seeing it and getting to it are two different things though.

blackfordd

Interesting. I get heat out of the defrost and the floor but not the dash vents. I just though it was supposed to be like that. Learn something new every forum I read here. Thanks guys.