Question about 1998 Chevrolet Astro Van

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Only air from defrost vent. Checked vacuum lines. All good. What else to check?

Followed plastic vacuum line from intake manifold to tee in engine compartment.

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You would check for vac at the blend door servo. The servo could be leaking, or it may not have vac from the control valve on the dash.

Posted on May 02, 2015

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2002 jeep laredo interior vents won't blow except


The mode control on your heater panel uses engine vacuum to operate the doors in the heater box. when the system loses vacuum, it defaults to the defrost setting. find the small plastic vacuum line that penetrates the firewall and leads to the engine intake manifold. Check the hose that attaches the line to the engine for cracking or looseness. If that is OK, trace the line back into the cabin. It should lead to the control panel. Check any tees or couplings for leakage. If it's good all the way to the mode control switch, it is likely the connection to the switch or the switch itself is leaking. With the engine running, listen carefully to the switch for hissing. It is also possible that the defrost door actuator is bad. If you can find the door, try applying vacuum directly to the actuator to see if it operates to shift the flow. If it doesn't, replace the actuator.

Feb 10, 2012 | 2002 Jeep Grand Cherokee

1 Answer

My A\C stop blowing out the A/C vents but blows on high and low throw the defrost vent


If air not blowing out any other vents in any other modes. If it continues blowing out of the defrost vent , suspect a broken vacuum hose between the engine and firewall where it goes to the heater controls. Most systems that use vacuum motors to move the mode doors around are spring-loaded to go to defrost mode when there is a failure. That insures that even though your feet might freeze in winter, you will have a clear windshield. There is only one vacuum hose and check valve so there is nothing to mix up under the hood.

Besides that vacuum supply, it is also possible to have a defective vacuum motor under the dash. The clue to finding that is that some functions will change but some things will work improperly. It's when nothing changes at all when you switch modes that the vacuum supply hose should be suspect.-------
--------------- Your vent controller is not working. Is the air coming out the defrost vents? If yes, this is the default when there is no vacuum to the switch. In this case, check the small plastic vacuum line that is connected to the engine intake manifold and going into the firewall. Make sure this line is connected to the manifold and the hose is not cracked. If this is good, go inside the car and see if you can trace that line to the control switch. If the line is good to the switch, see if you can pull the vacuum connector from the switch for troubleshooting. with the engine on, check for vacuum on the line from the engine with your finger. If the vacuum is there, your switch is probably leaking and needs to be replace.------------ This will help.Thanks.Helpmech.

Jul 05, 2011 | 2003 Mercury Sable

1 Answer

My pontiac grand prix is a 1995 the air works fine but will only blow threw floor and defrost vents how can i get it to blow threw dash vents


Vacuum vents not working: Your vent controller is not working. Is theair coming out the defrost vents? If yes, this is the default when there is novacuum to the switch. In this case, check the small plastic vacuum line that isconnected to the engine intake manifold and going into the firewall. Make surethis line is connected to the manifold and the hose is not cracked. If this isgood, go inside the car and see if you can trace that line to the control switch. If theline is good to the switch, see if you can pull the vacuum connector from theswitch for troubleshooting. with the engine on, check for vacuum on the linefrom the engine with your finger. If the vacuum is there, your switch isprobably leaking and needs to be replace. Any other questions, please reply.

Jul 01, 2011 | Pontiac Grand Prix Cars & Trucks

1 Answer

When i place the selector on heat mode it only blows out the defrost vents ....the other vents blow on defrost as intended or at your face ....could the selector be bad or vaccum proublem ? thanks


yes, and yes. It is a vacuum leak and could be in the switch itself. Can you hear it leaking behind the control? If not, check under the hood. There is a small plastic line that hooks to the intake manifold with a stub hose. That hose can crack and leak or fall off. If that's OK, trace the plastic line thru the firewall into the car. If there are any tees in the line, those are prime for leaking too. If you can't find the leak, pull the control panel off and see if the line from the engine is delivering vacuum to the switch. See pics from autozone.com. If it is, the switch is probably leaking inside. Please let me know if you have any questions or need instructions, and thanks for using FixYa.

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Dec 25, 2010 | 1998 GMC Jimmy

1 Answer

My heat gose from vent to defrost when i accelerate i think its a vacumm leak but no clue where to look


Yes, there is a plastic vacuum supply line the runs from the intake manifold thru the firewall to the heater control panel. Check where that line attaches to the intake manifold. There is usually a stub of regular vacuum hose glued to the plastic line that can crack and leak at the manifold. If it's ok there, trace it into the truck. There may be a tee somewhere that is leaking inside, or else the connection at the mode switch or possibly the switch itself. Take the control panel out (you may have to pull the bezel off to expose the screws) and listen to the switch with the engine running. Try tugging on the lines to see if the leak gets worse. Worse case, you will need a new switch, but try to rule out any tees or hoses first. OK?

Dec 22, 2010 | 1995 Dodge Ram

1 Answer

My heat only blows out the defrost even if the switch is on the floor or the middle vents? 1997 Ford Ranger 4 cyclinder.


The system defaults to defrost when it has no vacuum supply from the engine. check the plastic vacuum line from the engine intake manifold to the heater control panel. Especially look at the rubber hose that connects the line at the engine. If that;s good, follow it thru the fire wall and see if there are any tees, etc where it could leak. Listen for a vacuum leak at the mode control switch. Could leak at the connection or inside the switch. Please let me know if you get stuck or have any questions.

Nov 03, 2010 | 1997 Ford Ranger SuperCab

1 Answer

The air coming out of vents is very little even though the switch is on high and the blower motor sounds like it is running full speed


Your vent controller is not working. Is the air coming out the defrost vents? If yes, this is the default when there is no vacuum to the switch. In this case, check the small plastic vacuum line that is connected to the engine intake manifold and going into the firewall. Make sure this line is connected to the manifold and the hose is not cracked. If this is good, go inside the car and see if you can trace that line to the control switch. If the line is good to the switch, see if you can pull the vacuum connector from the switch for troubleshooting. with the engine on, check for vacuum on the line from the engine with your finger. If the vacuum is there, your switch is probably leaking and needs to be replace. Any other questions, please reply.

Sep 10, 2010 | 2004 Dodge Ram 1500

3 Answers

My 96 dodge ram 4x4 has a vacuum leak somewhere the air blows from the vents until i accelorate then it shifts to the winshield defroster vents once i let off the gas it switches back to the vents at...


there are two small black vacuum lines near the A/C accumulator. These Tee into a single line that goes to the intake plenum. If you install a vacuum check valve in the one that comes from the intake it will solve your problems. Just did it and fixed mine that was doing the exact same thing......Good Luck

Aug 07, 2010 | Dodge Ram 1500 Cars & Trucks

1 Answer

Air conditioner doesn't blow from right vents. keeps alternating from top to bottom to face vents on its own, most changes occur during acceleration.


The air/duct controls on this are vacuum operated and you most likely have either a leaky vacuum line or a bad vacuum check valve. When the engine is running at a steady cruising speed there will be quite a bit of vacuum in the intake manifold and thus in the vacuum lines and control system. This vacuum is held there by a check valve so when you are accellerating and the intake vacuum drops you won't lose this vacuum. If there is a leak or the check valve is bad then as the vacuum drops the air doors move to the default position which is where the air is directed to the defrost vents. The vacuum line under the hood is usually black plastic and about 1/8" diameter. The line and check valve are on the right side of the intake manifold area towards the rear. I'd start my search there as that's where you'll probably find your problem. Hope this helps!

Jul 14, 2010 | 1999 Dodge Ram

2 Answers

I have a 2004 Jeep Grand Cherokee. The operation of the manual Climate control no longer allows me to select which vent I want air, center, floor, and defrost. I only get air out of the defrost vents.


This system uses vacuum from the engine intake manifold to steer the vacuum actuator of the mode door. Fail safe position is defrost. Most of the time problem is caused by a loose vacuum line at the manifold or a shaved trough line somewhere under the hood. The line runs on the LH side off the engine (as seen from drivers position) starting on the intake manifold.

May 20, 2010 | 2004 Jeep Grand Cherokee

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