1999 gmc sierra no heat in defrost but heater and A/C work

SOURCE: 97 Sierra no heat on passanger defrost or floor

I assume you have the dual climate control feature on your truck. it sounds like you have door problems. The heating system has several doors inside the box and duct work that divert the air from defrost to face and feet...i'm not 100% sure if the ones in your truck are electronic or vaccum, but i think they are electronic. From time to time the actuators that control the door go out, as every electric motor does. You need to find the ones that have quit and replace them. It is possible that the arm(s) on the actuator(s) have broken loose from the doors and are working but not pushing or pulling the doors properly...if might help to remove the front seats...just to help save your back....working under the dash is hard on your body....and it take a good half day to remove the dash, so working under it is actually less work.

SOURCE: 2000 gmc 2500 pickup heater fan works good but not a lot of air

http://www.fixya.com/cars/t1386554-91_ford Please read this post, I hope it will help you.

SOURCE: 2001 gmc jimmy defroster-heater blower will not direct heat

you have a vacuum leak

Colin Stickland

• 22516 Answers
• Posted on May 30, 2009

SOURCE: a/c and heater only blow air out of defrost

either a vacumn pipe is broken or the ends split so its not passing a vacumn to open the flaps,where they are and how to get to them i cant help because i have never worked on one as they are not here in spain.check the engine end first though .Look for some small rubber pipes running from the inlet manifold

SOURCE: I replaced heater core in 1999 gmc sierra 1500 5.3

no,, the system works from a vacuum and has to be worked on in a service garage. sorry about the bad news...