AC blows hot air only when engine starts

Is the change from cold to hot instantaneous, or does it take a while?

A stuck blend door in the HVAC system would allow the car to give cold air when you first get in, but then after the engine runs and warms up the output of the heater core can overwhelm the output of the air conditioner.

 

I have a 2012 Toyota Prius C, and when the ICE turns on the AC instantly changes from cold to hot, what could be causing this issue? i would like if you describe the possible problem as clear and specific as you can, thanks.

 

The box under the dashboard is used both for A/C and for heating. For A/C, there's a cold coil that refrigerant circulates in from the A/C condenser. For heating, there's a hot coil that hot engine coolant circulates in from the engine.

If the air is blowing through both, the heat from the engine coolant is going to win.

There's a flappy door inside there that can direct how much of the airflow goes through the hot coil and how much goes around it. The computer makes the delivered air temperature be as warm or cool as you want it by moving that door around. If the door gets stuck in the position where the air is blowing through the hot coil, the symptoms will be like what you've described.

 

Is such mechanism present in the Prius C too? mine had the exact same problem that fsab888 described.

I disconnected the 12V battery for a few minutes to let the computer reset to its factory settings, connected it back and the problem was solved. Perhaps it could be a bug in the computer that avoided the flappy door to close when the ICE turns on to avoid the hot air to come in? i think it was most likely that, makes sense as far as i know.

 

Yes, the system @ChapmanF described is a very common method to do automatic HVAC in cars, and that's what our Prius c uses.

It's wonderful that you were able to restore yours with a power-out reset. We will have to see if it helps others.

 

Yeah, i was getting stressed since i don't want to take it to a mechanic, i know they will charge a lot for something simple, plus a lot of mechanics don't know how to do their job and may even damage the interior of the car.

Also do you know if it is possible to reach that area by taking all the dash covers out? maybe reaching that flappy door and see if it's getting stuck by dirt or something else can save people decent amounts of money. But mine was stuck with no doubt, most likely by a bug in the computer.

 

Try disconnecting the 12V battery for a few minutes, mine was having the problem yours have, but disconnecting the 12V battery for 3+ minutes fixed it.

 

As things to try goes, it won't hurt much (besides the radio presets, clock, etc.)

But it might not do the trick for everybody; two people having the same symptoms are not always having the same problem.

 

I understand that some problems with the blend door are only fixed by accessing the device directly, and that for the Prius c this means complete dashboard removal.

Getting the entire forward interior out of the car takes a lot of time and effort, and ideally you have some soft, clean and dry storage space for the pieces while they wait out of the car.

You can learn to do this yourself to save lots of money, but it's not for everyone.

 

I see, but by removing the dashboard you mean only the plastic panels and disconnecting cables, or the steering wheel and everything basically?

I found that if i turn the AC to and then back to LO, after that it will only blow cold air if the ICE engine is off, it's like if the flappy door gets stuck if i make it move all the way to allow only hot air to pass, but then it can't move to only let cold air to pass, still i don't understand why disconnecting the 12V battery for a few minutes fix this, it's weird.

 

I mean the whole works coming out. It's not really for the faint of heart. It's not terribly hard either. And it isn't even unusual- several other makes and models have the same problem with their automatic HVAC setups. It's just very labor intensive, which means it costs a lot if you pay a mechanic.

The root problem is a blend control door driven by an electric servo. Sometimes the electric bits die, usually the gearbox strips or the bushings on the blender itself bind up and can't be overcome by the servo.

Disconnecting the 12v is apparently causing something to reset, you're at least gaining temporary control afterwards.

 

The servos changed between Gen 2 and Gen 3. In Gen 1 and Gen 2 they were plain DC motors with potentiometers attached to read back the position. The pots would routinely get noisy and send back jittery position reports, which would make the computer actually jitter the doors back and forth trying to make the position hold still.

Gen 3 went from the pots to a more digital position encoding using two contacts that just get pulsed to ground alternately as the actuator rotates. (With one pulsing contact, you could count pulses to keep track of the position, if you knew which direction it was moving. With two contacts cleverly offset, you can look at the relative timing of pulses to know the direction of motion, whether to count up or down.) I think that solved the jittery-pot-feedback issue: I don't see nearly as many "hey what's that random flapping noise in my dash?" PriusChat posts beyond Gen 2.



That's not the only thing they changed. Used to be. each servo had its motor and pot wires separately carried all the way back to the A/C controller. New arrangement just uses a three-wire bus shared by the different servos. Less wiring. Each one knows its own address, so the controller can just shout over the bus "hey, servo #2, position 42 please" and it happens.



(Lower drawing is the old way, upper one is the Gen 3 way.)

The drawing suggests (as do some other passages in the manual, I think they really mean it) that the communication IC at the servo end, the part that knows what address it has, watches the two servo pulses and counts up/down to remember position, is in fact not part of the servo, but built right into the connector of that little wiring harness. So the servo itself stays a cheap dumb box with a motor and two switch contacts.

The problem description here sure sounds like something is losing count, forgetting its current position. With Techstream you can see what position (0 to 255) each servo is currently believed to be in, which would be interesting to see.

If it's actually the bus connector in that wiring harness that is forgetting the count, I guess that harness would be the thing to replace. Fortunately, it ain't the whole instrument panel harness, just a little one dedicated to those servos and maybe the evaporator thermistor.

Big warning on all this, I'm looking at a Gen 3 liftback manual, don't know what might be different in the c.

 

That drawing kind of breaks it all down to "old 10-wire version" or "new 3-wire version" pretty quickly, no? I would hope that this makes it easy to work out once you've gone spelunking under all the plastic.

 

They simplified the drawing. There are really three servos (fresh/recirculate, outlet select, temperature blend). So it's really "old 15-wire version" vs. "new 3-wire version". So they did save themselves some copper.

Interestingly, the drawing of the wire harness in the 2010 manual shows not 3 servo connector plugs but 4, able to drive separate temperature blenders for the driver and passenger. I think it's a drawing reused from another model; the Prius controller certainly doesn't give you his-n-hers temperature settings, and I've never seen anything else suggesting there's a fourth servo hiding in there.

But in whatever model that's really a drawing for, they saved themselves seventeen wires.

The c doesn't use the old system. The pictures at parts.toyota.com make it look pretty much like the Gen 3 liftback HVAC design. The magic smart wire harness is 82212-52080. (The image on that page is some artist's conception of a generic wire harness, and I want to know what the artist was on.)

 

I see, well for now i will leave it as it is since is not giving any problems for now, but i guess when the time to fix it comes i will have to do it myself, mechanics here don't know how to do their job and will most likely break or scratch the interior of my Prius C.

 

Just wanted to say thank you, disconnected power, rebooted power and worked, no more heat with AC on 2117 Prius. Work on commercial HVAC all day. Didn’t want to dig into this. Thank You.

 

Good to know it worked. Looks like this is a common problem in the Prius line, quite annoying how Toyota managed to fail when engineering something so simple.

 

I am now having the problem on my Prius C 2015. I took it into the Toyota shop near me and they reset the computer. It worked fine.
They charged me $150 labor and sent me on my way, however I was told if it didn’t work they wanted to charge me $6,000 in repairs because parts are broken and need to be replaced.
(I can post a photo of parts / money requested for repair)
But I cannot afford that.

I knew it had something to do with when I changed from hot to cold. (from looking at these forms I think it has something to do with the blend door). As long as I didn’t change the temperature I wouldn’t have a problem.

however we had a really bad storm and my windshield was foggy and I had to use the heat.

After I turned the heater on of course I am now having the same problem. Is there any tutorial on how to do the 12 V battery reset?

I definitely don’t want to pay $150 to have them reset it again if it’s something I can do.
Would also like to take any advice on repairs that can be done myself. Or any advice at all, thanks!