I have a 98 Gmc envoy that just developed some heating issues. Sunday I drove it and the heater doesn't blow any heat, just cold air. I checked the coolant, it was good. I then checked to make sure both sides of the heater core were getting hot, all hoses under the hood are hot. I then checked all the fuses, all good. I checked the actuator that moves the blender flap. The only way the actuator will move is if you turn it to defrost or floor. If you try to move it to the "Vent" position, the actuator doesn't move but you can move it by hand and heat will low out of the vents. So now I am unsure what part to even look at because I cant find any info on how the system operates. I would guess that the vaccuum line is good. This POS uses the fancy climate control box and I would hate to buy one of those if that isn't the problem. Anyone got any ideas.

 

did try maybe changing out your thermostat maybe it is stuck open

 

If you moved the actuator by hand, then you did not check the blend door actuator, but rather, one of the mode doors. The blend door actuator is located on top of the HVAC housing behind the glove box and cannot be actuated by hand without breaking things.

 

Ok. I didn't see any other actuator in there. The one I am moving is connected to a metal rod and it is vaccuum controlled. The only way I can move it by hand is to disconnect the vaccuum line. There is also an accuator of some kind on the right hand side of the glove box opening. It appeares to be electrical and only has and orange wire going to it. Iam thinking maybe this is the selector for either fresh air or cabin air. It doesn't seem to move either.

 

I have a 99 trailblazer with the same problem

took it to a supposed ac specialist in the area and was told the Acuator needs to be replaced. I have automatic climate control so mine was 112$ and still didnt fix the problem.

it seems there is a vaccuum controlled process involved in some way but I am not familiar with heating systems.

hopefully someone with more experience will respond to this thread and give some insight.

 

I ended up replacing my hvac control box with a used one from ebay. Still had a problem. So I replaced the blend door actuator. I now have heat but I cant get it to switch to cold now. Also the fuel pump went out yesterday. Worst vehicle ever!