1995 Chevy S-10 Blazer - 4.3 L Vortec V6 (vin W with PCM) - 230,000 miles original owner.

I changed my IAC Valve today, and coincidentally, my HVAC started blowing out of the defroster and floor no matter where I put the HVAC selector or fan switches. This happens in all modes, AC, Heat, Vent, etc.

I don't see how this is related to my IAC Valve, but I put the old one back and the HVAC problem continues to be present.

I've had a similar problem in the past where it was intermittent and replacing the HVAC control switch fixed it. This time, the HVAC control switch was just replaced a couple months ago with a genuine AC/Delco part and it was working perfectly until today.

Now, immediately after changing the IAC Valve, I can only get air to blow out the defroster and floor simultaneously.

I did a lot of work on the car over the summer so I remember the recent vacuum readings. I checked the engine vacuum today and it's measuring the same as before.

I also checked vacuum lines and intake manifold for leaks by spraying carb cleaner at these areas. There are no leaks and the engine is running fine... actually running better after changing the IAC Valve.

What could be causing the HVAC to always blow out of the defroster and floor? I've ruled out the IAC Valve replacement as coincidence since swapping with the old IAC made no difference.


Changing the IAC Valve on this vehicle involves two small screws and one electrical connector. NO vacuum lines are involved with this or located nearby.

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I took out the dash panel and could hear hissing at the HVAC Control Switch.

Unplugged the vacuum line assembly from the switch and measured vacuum on the source line. It's measuring the same as engine vacuum, so I can rule this out.

Next, I manually applied a vacuum via pump to each line(s) while following the vacuum line schematic. I was able to get the heater, defroster, and vents to operate independently this way, ruling out all actuators.

While the vacuum lines were hooked back to the switch, I was also able to get the Bi-level A/C, Defroster and Heater working properly by holding the connector on the switch a certain way to stop the leak.

It is seemingly impossible to determine is whether the leak is coming from my "new" switch which was replaced last summer and working fine, or from the rubber connector assembly, which is factory original. Not sure how often these connectors fail or if they can even be replaced. What makes it difficult to diagnose is the fact that the switch assembly itself is clipped and stacked sections that have some play. So not sure if movement within the switch or movement within the connector is changing the amount of the leak... I just know that moving these around affects the operation greatly.


Using a stethoscope I made from some tubing, I determined the vacuum leak is coming from inside the "new" HVAC control switch, which was only installed just two months ago.

2 Answers 2


At the HVAC Control Switch:

  • full vacuum measured at source hose
  • able to manually operate all three actuators using a vacuum pump
  • hissing sound from a vacuum leak

Using a stethoscope, I narrowed the leak down to inside the switch itself, which was only just changed a couple months ago. It's being replaced under warranty.


Changed the HVAC Control Switch and the problem was fixed.

  • 1
    I'm glad you found your problem. I guess that .1% reared it's ugly head against me. This is why I don't go with absolutes! Great diagnostic skills on your part! Be sure to check this as the answer, as I already +1'd it for you. Commented Oct 1, 2015 at 22:38
  • @Paulster2, thanks for your help. You forced me to take apart the dash to prove my point. Otherwise, I would still be putting it off. I'll have to wait until the system allows me to accept. And what are the chances a new switch would have gone bad this quickly... I was not expecting that at all!
    – Sparky
    Commented Oct 1, 2015 at 22:55
  • Nobody expects a new part to go bad, but obviously it happens! Commented Oct 2, 2015 at 1:45

I have heard of an issue with some GM models but I am not sure if it goes back to 95's. On some models if the battery is disconnected the HVAC blend door can fail. Apparently when the battery is reconnected the blend door attempts to home itself. This involves the door moving to the extremes of all open and all closed. In some cases the servo seizes. Can you hear or feel the door move when move the selector?

  • All three door actuators are only vacuum controlled. I started digging under the dash to find them, but they're inaccessible so I'm afraid this will require tipping the dashboard forward. They can't even be reached for simple troubleshooting. Not a happy camper right now.
    – Sparky
    Commented Sep 30, 2015 at 23:34

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