My 2003 S10 has good heat but it only blows out of the dash vents. Anybody know how I can troubleshoot this rather than just throw parts at it?

Thanks

 

Need a little more info. Is your system auto climate or manual? Do you have heat only or can you get cool or cold air? Typically, when you lose vacuum, the system defaults to defrost mode with fan forced air coming out of the top dash grill. I'd start under the hood and inspect the vacuum lines from the intake to the check valve and on to the fender. These lines degrade and split over time causing you to lose vacuum. You have a 2003 so it is time to replace them anyway. Inside your vehicle, the HVAC box has 3-4 vacuum operated actuators that open and close various doors based on the selector **** setting. It also has an electric motor activator that opens and closes the blend door, though that doesn't sound like your problem. It may be someting as simple as an object falling into the box causing one or more of the doors to stick, or your switch module might be bad. A vacuum line could have come off an actuator, or an actuator may have failed. If you can check your vacuum, you should have ~14-17 at idle at the check valve under the hood. You can open your glove box fully and see behind it to find 2 of the activators. See if they move when you select recirculate. If it something inside the box causing the problem, then that will most likely require disassembly.

 

It sounds like your A/C door actuator has failed in the fully open position (closing off flow to the defrost & heater ducts) or has vacuum at all times to the brown vacuum line that is keeping it locked in position.

Here is the schematic from a '99 with Automatic Climate Control:

It shows the door I am talking about schematically.

If you have manual temp controls, then the schematic below is of your setup:

And the problem would be a combination of the A/C mode door and the slave door (and/or their actuators).

 

I am having a problem with my hvac system. Hot air only blows out dash vents. Cannot select different zones. Cannot regulate temp. Can control fan speed. 98 gmc jimmy with auto hvac system. Have swapped hvac control head. no luck there. I can put vacuum on vacuum lines to actuators and they move doors. Any ideas?

 

I am also having heater issues, hot air at floor and defrost, only cooler air at dash vents, 2000 Blazer LT, with automatic climate control. Can anyone help, Ive been working with this for to long.