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1998 K3500, no air coming from upper vents #5457932 06/19/20 05:55 PM
Joined: Jun 2002
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wtd Offline OP
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I tried using my AC today in this truck and can only get air coming out of the lower vents. When I select just upper vents on the control panel, it sounds like the air is at the upper vents but nothing is coming out. When you select both lower and upper vents, you get air out of the lower but nothing out of the uppers.

How can I tell if there is an issue with the control panel which I know that as they get older, they can have problems, or a blend control door problem? I used to be able to hear a door close or open behind the dash when I moved the knob but I don't hear anything like that now just a louder rush of air when I select upper vents only. Truck has 119,000 miles on it.


2014 Mustang GT
1998 Chevy K3500
1998 Chevy K1500
(2) 1992 Cavaliers
1970 Monte Carlo
Re: 1998 K3500, no air coming from upper vents [Re: wtd] #5457981 06/19/20 07:07 PM
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nascarnation Offline
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If it's like the S10/Sonoma of that vintage that I have, the system is controlled by vacuum actuators. (Newer stuff is electric force motors)
It's a pretty complex setup so I recommend finding a good manual for troubleshooting.

Those vacuum lines tend to age and leak after 20 years, and also the actuators can fail a diaphragm and not pull.
It's a cheap fix most likely, but some amount of labor to find the failure point(s).

Last edited by nascarnation; 06/19/20 07:08 PM.
Re: 1998 K3500, no air coming from upper vents [Re: nascarnation] #5458092 06/19/20 09:25 PM
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14Accent Offline
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Originally Posted by nascarnation
If it's like the S10/Sonoma of that vintage that I have, the system is controlled by vacuum actuators. (Newer stuff is electric force motors)
It's a pretty complex setup so I recommend finding a good manual for troubleshooting.

Those vacuum lines tend to age and leak after 20 years, and also the actuators can fail a diaphragm and not pull.
It's a cheap fix most likely, but some amount of labor to find the failure point(s).


A K1500 has electric actuators, no vacuum. I would try a new mode door actuator first, it's cheap and easy. The control panels rarely go bad other than the blower switch, so I doubt that's the problem.


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Re: 1998 K3500, no air coming from upper vents [Re: wtd] #5458111 06/19/20 09:48 PM
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clinebarger Offline
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1. Make sure no Mode Door links fell off or broke....The Mode Door Actuator & links are above & to the right of the Gas Pedal on the end of the HVAC Module

2. At Mode Door Actuator.....Verify key on power power on the Brown wire and a good ground on the Black wire.

3. Check for a varying voltage on the White wire. Check for the voltages below.
10.5 volts in vent
8.2 volts in vent/floor
5.5 volts in floor
3 volts in floor/defrost
0.6 volts in defrost

These are approximate voltages, Defrost could be a full Volt & Floor could be 6 volts & still be within range, If they are way off.....Replace the Control Head.

If the voltages are okay, Remove the Mode Actuator & manually operate the Mode Doors.....If they feel good & free....Replace the Mode Door Actuator.


2001 Chevy Camaro L92/4L80E
2006 Chevy 2500HD LBZ/Allison 1000
2010 Toyota Corolla 2ZR-FE/U341E
2000 Toyota Avalon 1MZ-FE/A541E
Re: 1998 K3500, no air coming from upper vents [Re: wtd] #5458608 06/20/20 03:51 PM
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wtd Offline OP
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Thanks for the replies. I will go check the door actuator, links, and voltages.


2014 Mustang GT
1998 Chevy K3500
1998 Chevy K1500
(2) 1992 Cavaliers
1970 Monte Carlo
Re: 1998 K3500, no air coming from upper vents [Re: wtd] #5458663 06/20/20 04:59 PM
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wtd Offline OP
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I checked all of the voltages and they were all close to what clinebarger posted other than the vent was over 11 volts.

I discovered that two of my blend doors were stuck. The one for the vent and the one for the defrost. I freed them by hand and then they all worked with the turning of the knob. I'm not sure why they were stuck or if they will stay unstuck but for now everything is working like it should.

I guess if they stick again I will have to open the case up which I'm not sure how easy or hard of a job that is. Thanks for all of the help.


2014 Mustang GT
1998 Chevy K3500
1998 Chevy K1500
(2) 1992 Cavaliers
1970 Monte Carlo
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