Ac controls stuck in vent mode. no vacuum to control head. where does vacuum line tap into under the hood?
Comment by schochiometr, posted on Aug 24, 2008
No changes to fuses. I rechecked all fuses and they're good.
Comment by schochiometr, posted on Aug 25, 2008
To clarify, I have a 1999 GMC Jimmy, 2 wheel drive. I checked for vacuum at the ac/heater contol and there is no vacuum. I found where the vacuum line comes out under the center of the dashboard to the left of the console. It runs back to the firewall and into a large wire bundle. I can't see where it goes from that point. I've looked under the hood at the firewall and can't see the vacuum line. I'm trying to find the vacuum source to determine what the problem is. Is there a switch or relay I'm not aware of? I have a Haynes manual but it's no help for this problem.
Just fixed same problem with my '98 Jimmy for under $3. The vacuum line comes thru firewall (semi-rigid 3/32" hose) with the main wire bundle just below the brake powerbooster. After about 7" this line goes to the first 3-way 'T' connector with an inline adapter going up to 5-32". Hose 2 from the tee goes to connection on the inside of left front fender. Hose 3 from the tee goes to another 3-way fitting where the system gets vacuum ultimatley from a larger hose going to the plenum where the PCV hose attaches. Check ALL these hoses for rot or splits.
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Pull the heater/AC control head and check for vacuum at the mode control switch, if none is present go under the hood and trace the vacuum line to the engine and correct the loss of vacuum which is causing this problem.
Dustin, Sound like vacuum loss to position switch. Defrost is the default position when vacuum is lost in vent control system. Check any and all vacuum lines and connections under hood first, especially any that run through firewall. Check the attached links,instruction and guides, Good luck
"I hope this helped you out, if so let me know by pressing the helpful button. Check out some of my other posts if you need more tips and info." 2001 F150 V6 4 2 AC Heat Stuck In Defrost Easy Repair
You have a loss of vacuum from the engine check the vacuum lines to the vacuum storage canister under the hood. If the vacuum form the engine is ok check the lines to the control head in the dash one may be pinched or restricted or the vacuum switch may be faulty and leaking.
Vacuum supply starts on the drivers side of the engine. There is a vacuum line going into the brake vacuum assist from the manifold. branching off of that line is a double line that snakes under the intake manifold, behind the valve cover, along side the valve cover on the passenger side headed to the front of the engine. Then it curves to the right side of the engine. One set of hoses from this line goes to a valve control (red and green i think). The other line goes to the vacuum storage ball, it should be black. The vacuum circuit continues out of the ball in another black line that snakes into the firewall. It goes under the heater core black box and disappears under it. Now go into the passenger compartment and drop the glovebox. look to the back right you will see a black and a white vacuum line coming out of the firewall. The black line is your vacuum supply from the engine. It goes up to a little rubber manifold. disconnect the lines coming from the dash switch and check for vacuum on the manifold where the black line is going in. The white line controls the cabin air intake under the glovebox and the thermostat shutoff under the hood. It should only have vacuum when the switch is in a certian position, AC probly. The other three lines coming off the rubber manifold snake behind the dash vent door assemble and come out near the gas peddle. The red one controls the dash vent. apply vacuum to the red hose and see if the vents go to the center dash position/AC. The yellow and blue? ones control the floor vent control near the gas peddle. yellow gives you half floor and half defrost. blue gives you all floor. you can use a small vacuum line to connect the black vacuum supply to each individual line on the manifold and check the function of each vacuum control assembly individually. I hope this helps. On my truck each control unit works fine when i apply vacuum to them individually but when I hook the dash switch up to the rubber manifold it goes to defrost. I tried changing the dash switch and the lines running from the switch to the rubber manifold and the vacuum ball under the hood, nothing works. I suspect I may have leak i just haven't found yet. next step is to get a vacuum gage and start checking the amount of vacuum i am getting at each vacuum line. I'm sure it is supposed to be getting a specific amount of vacuum and will not work if the vacuum drops below a certian level due to a leak in a line somewhere? God bless.
Look for a vacuum line passenger side rear under hood near hood hinge area. Follow this line back to the intake manifold. checking for crack and leaks if any breaks in this line repair them this should fix your problem if not check all vacuum lines under the dash as well as the vacuum diaphrams that control heater door operations. if you have a hand vacuum pump you can check all these at (previous found) intake connection for the heat controls
Defrost is the default position for your blower. When everything is shut down, or if your loose control it goes to default and since you need to be able to see out the window more than be warm.
Open the hood and with the engine at idle, see if you hear a hissing noise. Most of the less sophisticated systems use vacuum to control the vents. Sometimes when someone does a tune-up or any kind of under hood repairs one of the vacuum lines can come off causing there to be no vacuum to the control head of the hvac.
If you can't find a vacuum leak, you may have an electronics system. Then you need to determine if the blend door actuator is functional. A dealer can usually determine that with a body control scanner. If it has that upgrade. Few of the Lumina's had that in the 90's.
Go to an autoparts store and see if they carry a temperature control valve. When you see it you can find out if it has a cable or vacuum control to operate it. Then you can have someone move the dash controls and see if anything moves under the hood. If it is a cable and does not move the cable is broke or unhooked under the dash. If it is vacuum controlled a vacuum line is wrong and keeping the valve open.