I have a 1999 Blazer 4wd automatic and like a number of folks here, I have a heater vent control problem. I have proper temperature at the vents but I do not have the ability to select the vents despite the position of the selector switch. I realize that checking the vacuum, vacuum lines & the solenoid voltage is what is needed.... I do have vacuum at the T connectors located near the vacuum canister. I have the 1999 shop manual. So correct me if I'm wrong, but it looks like I need to totally remove the instrument panel carrier to get access to the vacuum, vacuum lines & the solenoid? Or is there a better way to access these? While the shop manual is informative, those pictorial drawings have room for improvement and BTW where is the "vacuum electrical solenoid"?

Any assistance would be most helpful.

 

i know on my blazer i was able to here a light hiss comming out of the switch i found a replacement in the junkyard and the problem went away

 

i'll second bonecrush. i too had a quiet hiss coming from the director switch, but all it took for me was pushing the rubber plug with six(?) vacuum lines in and making sure that air wasn't escaping.

Also, when it is set to "off", there is always that same small hiss. Im not sure if this is supposed to be there, but it lets me know that vacuum is indeed happening

 

I've done the troubleshooting on my '98 Jimmy twice in the past six months. You don't need to remove the entire dash (instrument panel carrier in the book) to test all of the vacuum actuators. If you can maneuver in tight spaces you can do it without removing anything except the lower kick panels.

Remove the glove box by releasing the retainer catch and swinging it down out of the way. Remove the lower kick panels underneath the dash. That will give you access to all of the actuators. Test each of the actuators with a hand vacuum pump to make sure they're holding vacuum.

Most likely your problem is with the rotary switch in the HVAC control panel. Check for ATF in the vacuum lines. If the switch gets contaminated with ATF it won't seal up properly and deliver vacuum to the actuators. I've had success with cleaning the rubber diaphragm and the backing plate and then putting it all back together. If it won't seal up after cleaning try getting a stronger spring to put back in the switch.

 

Near the vac reservoir, there is a hard, plastic vac line that comes from the harness that runs thru the firewall. It goes to a "T" near the vac reservoir. My hard line broke, causing me to lose the ability to control where to direct airflow.

I couldn't find the line at the time, because it broke right at the harness so I couldn't find where the line originally came from, I ran an entire new length of vac line from the vac connection below/behind the dash thru the dash and into the engine bay, where I connected it to the line that the hard line use to go to. I fixed the problem, which was that I couldn't control which vents the air came out of.

 

I just wanted to respond to the hiss issue. If you pop the hood, in the center of the firewall there is a vacuum switch for the 4×4. It does hiss there and can be heard from the inside. Even a new one will so if you can't find the leak check there. I'm having problems with my temperature control won't work, stuck in hot position.