AC on, blows hot while driving but cold while Idle

Is the coolant level ok? Mileage?

 

That pattern very commonly means that one of the servo actuators in the HVAC box is stuck in one position.

There are three of those actuators: the inlet servo (controlling whether you get fresh or recirculated air), the outlet servo (controlling which vents the air comes out of), and the blend servo, a/k/a air mix servo, which controls the air's temperature by changing the blend of how much air goes through the heater core and how much goes around it.

Sometimes you see people say "blend servo" or "blend door" when they mean any of those three, but of course it only means that last one. If it gets stuck in the position where all the air goes through the heater core, what you're describing will happen. Whenever the engine's running, the heater core can completely overpower the A/C, so the air will be hot. When you come to a stop and the engine shuts off, the coolant stops circulating through the heater core and the A/C is able to give you cool air.

You could check with the A/C off, just running the fan. That'll take the refrigerant system itself out of the picture, so you're just testing the airflow and heating. Keep the engine running (say, by shifting to N while it's running), turn up the heater fan, and then change the temperature setting between a warm setting and LO.

On LO it should only blow ambient-temperature air around, while at any setting higher than the temperature in the cabin, it should blow heated air out. If it just keeps blasting heated air even on LO, probably that blend actuator, or the door it moves, is stuck. You can confirm by sticking your head under the dash and watching its motion.

Sometimes the problem is in the actuator itself, or in the control microchip for it, which is actually built into the wire harness connector that plugs into it. If the trouble is the actuator or that wire harness, those are available parts for a reasonable price. Still, it's tedious to take the dash apart to get access to them.

Sometimes what happens is really that the door or shaft inside the HVAC box is broken or jammed. Toyota only wants to sell you a whole HVAC box then, $$$$. I've seen a thread here at least once where somebody did something akin to laparoscopic surgery and got it moving again.

 

This issue, as described, usually isn't about the coolant level.

 

thank you for sharing that Chapman.

can i ask you which side of the center console these blend doors are located on in a 2010 Prius?

my prius ac is running. the sight glass does have some tiny bubbles & could use a squirt of 134a only.

what it’s doing is on full cool it’s heating super bad when the engine runs. Then when it kicks into eco & the engine shuts off it cools.

i’m both modes it cools as the low side ac line under the hood sweats & the high side line is hot.

So it must be the blend door. but if there are 3, is there a specific one I should target?
&
Just a reminder to let us know which side it’s under the dash. Maybe I should go buy a phillips head now for tight spaces that I saw a man using on youtube to get the top screw out from behind the controller there,

thanks again : )

 

In Gen 1 and Gen 2, at least two of the servos were on the convenient side of the unit.

In Gen 3, they are pretty much all on the side where you'd rather they weren't:



(Because of some PriusChat breakage from the recent server migration, that image doesn't show up. You can view it here.)

"air mix" is the other name for "blend".