99 Sonoma- Heater control won't change between defroster, upper vents, an lower vents

Discussion in 'Chevy S10 Forum (GMC Sonoma)' started by wrench69, Mar 30, 2012.

  1. wrench69

    wrench69 New Member

    I recently bought a '99 GMC Sonoma for my son. The previous owner told me that the heater control didn't work correctly. Specifically, it wouldn't change between the defroster, upper, or lower vents. It didn't matter what setting on the control was used, the air would basically come out of all vents (including the defroster vents) equally. The temperature was fine- it would blow cold air for a/c settings, and warm/hot air when you changed the temperature to full hot, but you could not change the location of the air discharge.

    I had the lower dash panels off to troubleshoot a headlight problem anyway, so I started messing around with the heater problem. For those of you who don't know (I didn't know this until I started troubleshooting the system), the actuators that direct the airflow to the various ports (defrost vents, upper vents, lower vents, or a combination of these) are controlled by vacuum pressure from the engine, and they are all located behind the dash panels. The vacuum is supplied directly to the control head, which ports the air pressure to the appropriate actuator, based upon the HVAC control position.

    Anyway, I pulled off the HVAC control head to check the vacuum lines. This is very easy to do- just pull the retaining screws on either side of the panel, and then release the clips on either side of the panel- it will pop right out. Then disconnect the electrical connectors and vacuum connector. The vacuum lines seemed ok, and I knew the engine vacuum lines were good, so I continued to troubleshoot. My next step was to make sure I had good vacuum to the control head from the engine. Oh, by the way, the vacuum lines are all color-coded, so you know exactly which line goes to which actuator.

    When I was preparing to check the suction line from the engine, I pulled on the collective group of vacuum lines, and, to my surprise, they simply all came right out of the front of the dash! This was a nice discovery, because it let me know that something had come loose further back in the system. The vacuum lines are routed behind the glove box, so the next step is to lower the glove box out of the way. To do this, simply open the glove box, locate the spring-loaded stop that prevents the glove box from tipping down any further than the normal "open" position, and push up on that spring until the back of the glove box clears it. The glove box will then drop straight down and be out of your way. Make sure you unload it first, or everything that was in it will hit the floor!

    Behind the glove box, there is a junction connection that looks like this:


    You can see the junction block in the background. The group of vacuum hoses was disconnected from this junction block, and therefore no vacuum pressure could get to the control head, let alone get to the control actuators. I reconnected this connection, and the system works as advertised.

    Here is another picture with the connection properly in place. Notice one of the acutators in the background to the right (not sure exactly which actuator this is, but could probably figure it out based upon the color of the vacuum hose):


    Hopefully this will help people who have a similar problem. Remember to work from the source out, and you should be able to isolate your problem pretty easily.
    1 person likes this.
  2. Pikey

    Pikey Moderator Staff Member 5+ Years ROTM Winner 5000 Posts

    Really nice write up with pictures! Glad to see that your issue was a cheap fix. I have never been that lucky. For me it usually ends up being the control knob itself or a melted vacuum line.
  3. Mechnix

    Mechnix New Member

    a lot of these trucks w\2.2 have a vac line that goes from throttle body towards the Master cyl and a t fitting. this line rots and causes the same issue,
  4. Treadnmud

    Treadnmud New Member

    I just recently purchased a 98 Sonoma with the 2.2 and 5spd. I noticed on my 43 mile drive to and from work, that I also have air blowing mostly out the dash vents but also through the floor vents some. There are a few little hills that cause my 4 cylinder to work harder and the more I press on the gas, I lose most of the air coming out of my dash vents. As soon as I let out of the throttle it starts coming out of the dash vents harder. I suspect a vacuum leak too, any ideas?
  5. lowvoltage

    lowvoltage New Member

    I was having same problem. There is a check valve (not sure exactly where, sorry) that was bleeding off. Once that was changed this particular problem was fixed .
  6. bobnj54

    bobnj54 New Member

    I had the exact same problem. Looked at it and the line was rotted. Went to the parts store and for $3 the problem was solved. The heat comes out where it is supposed to.
  7. spac16

    spac16 New Member

    Check if the vacuum resivoir is hooked up the hose leading to it rots very quickly and breaks apart. There is a T over near the master cylinder with a hose off the t leading to resivoir. The resivoir is under master cylinder in side panel wall.
  8. Egrit

    Egrit New Member

    Thank you so much for your post. I starting purchasing parts like the HVAC control module and I hadn't checked behind the glove box for a vacuum leak. Unfortunately I wasn't able to return the control module. The fix that you showed here worked.
    Thank you again
    RayVoy likes this.
  9. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    Too bad you hadn't hopped on here sooner, the S10 uses vacuum motors for a number of functions, rotting vacuum lines are an easy fix.

    We have a classified section, maybe someone with an electric problem might need your control module.
  10. Bill B

    Bill B New Member

    Thank you, thank you.
    This thread help solve my 2000 Sonoma heater vent problems also.
    Rotten vacuum lines at the junction next to the master cylinder. Turned of the Check Engine light too.
    Bill B
    RayVoy likes this.

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