Blend door theory/diags

[event3horizon]

AC gurus, I have some questions about the blend door in a car.

As I understand, a blend door opens or closes (or stays somewhere in between based on temp controls) to direct blower air either through the evaporator (cold) or the heater core in a car.

Questions:

Would a blend door 'problem' in a car affect the evaporator temperature at all? Many new cars have evap temp sensor data that a scan tool can show. For example, can one easily diagnose a blend door issue if the evap temp is quite cool but the vent temps are +10 or +15f higher?

Would a blend door problem affect low and high side pressures at all and in which way would it?

[Tim]

Not really. Blend doors redirect airflow. There will always be some effect with no air flowing over a heater or evaporator.

[GM Tech]

Typically all air flows through the Evaporator first, then gets directed through the heater core if warm/hot air is requested. It is diverted around the heater core if cool/cold air is requested. This is so while in defrost mode, dry air can be used on the windshield. So if you can't decide if it is working or not- an easy test is to pinch off one heater hose to stop any coolant flow through the heater core and see if the resulting air temperature is cool. I have known folks with broken blend doors (Ford Eddie Bauer edition comes to mind) to run with heater hose pinched off every summer and released every winter!

[Tim]

Typically all air flows through the Evaporator first, then gets directed through the heater core if warm/hot air is requested. It is diverted around the heater core if cool/cold air is requested. This is so while in defrost mode, dry air can be used on the windshield. So if you can't decide if it is working or not- an easy test is to pinch off one heater hose to stop any coolant flow through the heater core and see if the resulting air temperature is cool. I have known folks with broken blend doors (Ford Eddie Bauer edition comes to mind) to run with heater hose pinched off every summer and released every winter!

Hello GMTech. Nice to see your post. I hope all is well!

Poster: This is one of the best responders around for AC. Follow any advice if provided.

[event3horizon]

Thank you Tim and Gm Tech for the reply. Very informative and educational!

[tbirdtbird]

Always good to see GM Tech on the board!!!