'98 gmc sonoma-only air from vents-none from defroster or floor-no hissing sounds?

sprocketcity said:
replaced mode actuator-still the same
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First check for vacuum lines that goes to the vent control, you may have a loose or cracked line which would cause the vent doors not to work properly. Make sure the lines are not kinked and that there is vacuum to the switch that it's connected to. You could have a vacuum supply problem to the HVAC control head. Look for the "t" in the vac line behind the abs under the hood. Remove it and blow through it, might be clogged. Also check vac lines for holes/pores. My "t" was clogged with dirt, but it all came from a problem with tranny fluid in my vac lines. Faulty 4wd switch one day. Check that vac cannister on the fender well for holes, if you find one put some silicone on it. 98+ have the vacuum reservoir inside the driver side (Left side) fender. A hand vacuum pump with a gauge will let you know if it is still functioning properly. Just put the pump onto the connection and pull a vacuum. If it is sealed, it should maintain the vacuum. Good luck and hope it's just a simple vacuum line that has a crack in it.
 
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