1993 G2500 van heat issue

I have a 1993 conversion van that only the defrost setting on hvac pannel works, put it on vent n it hissis changed the round wheel on controls that the vacuum lines plug into and same issue only the defrost works change it to heat or vent n it hissis.. please help very frustrated with it! Thanks

I have a 1993 conversion van that only the defrost setting on hvac pannel works, put it on vent n it hissis changed the round wheel on controls that the vacuum lines plug into and same issue only the defrost works change it to heat or vent n it hissis.. please help very frustrated with it! Thanks

Make sure you do no have a vacumn leak or line unpluged on heat or vent.

Thank you mt i cant find a leak except were its coming out of round vacuum dial on back of heat and a/c control panel. I've changed out that n it still forces air out of it not sure if its pinched sum ware tried following them but can't see a pinched line.. i took out radio n tried to follow it.. it hissis on all settings except defrost. N defrost is the only form of vent activity that blows..

Not sure if related but driving it to Florida now and noticed cruise losses speed up hills n its not kicking down going 70 hit a hill losses speed down to 55 or so..

Not sure if related but driving it to Florida now and noticed cruise losses speed up hills n its not kicking down going 70 hit a hill losses speed down to 55 or so..

I would ckeck all vacumn and vacumn balls and lines from engine intake to heter controls and cruise control..