Temp control knob changes vents but not temperature

[Bmachine]

My whole HVAC system seems to be sick these days.

AC just died, fan only works when speed switch is on 4, etc...

But another issue that suddenly propped up is that when I turn the temp control knob, I hear a distinct change of pitch from the air vents, as if it was opening and closing some air valves, but it makes no difference to the temperature of the incoming air.

Does this automatically indicate either an obstruction in the heater core or a dead temp control mixer unit?

The car runs really well with no heating issues.

Thanks for any help.

Bo

[jalopi]

Sounds like the blend door could be at fault (the flap that directs air flow through either the heater core or evaporator). The question would be as to whether the blend door is cable or electrically controlled on our cars... I'm not too sure.

[vpilarrt]

The reason the blower only works on position 4 is that the blower resistor pack is bad.

[Poorboyz]

The reason the blower only works on position 4 is that the blower resistor pack is bad.

This. Speed 4 doesn't go through resistor which is reason it works on 4 and not on 1-3.

Sent from my Galaxy Nexus using Tapatalk 4

[Bmachine]

Thanks guys.

Yes the blower speed control should be an easy fix. There is a good write up on that elsewhere on this board. I'm not worried about that part.

It's the temp control which is more puzzling at this point. I'm intrigued by Jalopi's idea because as I mentioned originally this is clearly some door opening and closing happening behind the vent as you can clearly hear something change from the inside of the car. But the temp and air speed does not change though.

Bo

[jalopi]

When I used to work at a shop, I did a blend door job for a contractors' truck once. It was less so a blend door job and more so a 'remove 10 or so pencils from the blend door area' job... dude would jam pencils and stuff in the vents after writing up estimates. Eventually they would fall in and jam up the blend door.

It's possible that the blend door control is bad - or - something is preventing the door from moving. From what I understand, removing the heater core is a 20-30 minute job if your car came from the factory with A/C. I'd say your best bet (to check for a non-functional blend door) is to remove the heater core and work the temp knob while you look up into the HVAC box. Confirm with other members that are more experienced with our cars before starting the job, but you should be able to see the blend door moving (if it's working) or see any obstructions. Granted, you'll probably need a mirror and a flashlight.... but it sure is better than removing the dash to diagnose the problem :)

[kungfooren]

I'm running into a possibly similar issue. From what I've gathered, there are two things going on with the knob 1) open/close flat 2) open/close solenoid in heater control valve.

The first is driven by a cable whereas the second is electrical. it's a common issue for the heater control valve to be stuck in an either always open or always closed position.

-jeff

[jeffnhiscars]

Sounds like the blend door could be at fault (the flap that directs air flow through either the heater core or evaporator). The question would be as to whether the blend door is cable or electrically controlled on our cars... I'm not too sure.

The blend door is controlled by a Bowden cable that moves a lever right behind the vent in the passenger foot well

[Bmachine]

Thanks very much for the info guys. I got moved to another part of the project (bringing the oxidized paint back to life) so I will get back to this in a few days.

Bo

[Bmachine]

OK, back to this issue. As I said in the OP, when I turn the knob, I hear a distinct change in air flow from deep inside the vent. It really sounds like an air door is changing position.

So, based on the scenario mentioned by kungfooren, this would leave the solenoid in the heater control valve, right?