AC Button Light

Exactly that. I thought something is wrong with the car. I like to turn off the AC when accelerating or going uphill to get a little more mpg, but I don't know when the AC is on or off sometimes, because of the blowing cold even with the lifht out and Auto off. I am wondering if it has something to do eith the car remembering your preferences based on you having the same key fob.

 

I posted a version of this earlier today in a different thread, but I think we're experiencing a minor philosophical change to the HVAC user interface and it's confusing us because we didn't expect it and were so used to the old way.

Old way: When we set the climate controls to AUTO, the lights showed us what it was doing. A/C, vent, recirculate, etc. If AUTO made a change, we saw it reflected on the climate panel.

New way: When we set the climate controls to AUTO(default), the panel basically goes silent and does everything behind the scenes. Notice that under the new AUTO, both the vent and recirculate lights are off; under the old way, one or the other was always on. The lights now indicate when the user has chosen to override AUTO's control of a particular setting.

Neither way is bad or good, they're just different and require a slightly different mindset when approaching them. But fear not, if you like to manually control A/C to maximize efficiency, Toyota made it easy enough. You even have two ways to do it: the temporary and the permanent.


Temporary: While in AUTO, press the A/C button twice to turn it off(the first press takes manual control, the second press shuts it off). The manual control of A/C continues until you press AUTO again(even turning the car off and then restarting keeps it in manual until you eventually press AUTO).

Permanent: On the infotainment screen, press Settings, then scroll to Vehicle Customize, then Climate. Turn off "A/C mode with AUTO". Now AUTO won't control A/C on/off. You have to cycle the A/C button one time after changing the setting for it to take effect. Now the climate control will act just like previous Priuses.



All this being said, you're not likely going to see the same gains in power and efficiency by shutting off the A/C as you did in previous gens. The reason is that the new A/C compressor is a multi-stage unit. The older compressors were on/off. You were either at 0% power or 100% power. The new one has multiple power settings so it can produce cold air at lower power modes. On a moderate day, the A/C system will now only sip power, so there won't be a huge advantage in shutting it off. I suspect the change to a multi-stage compressor is why the designers changed the UI.


*older = at least gen3 and earlier; I'm not sure where the gen4 sits because I've never owned one

 

Pressing that button returns control of the entire climate system to the car, with the exception of the A/C system if you've selected to disable 'A/C mode with AUTO' in settings.

Example scenarios:
1. I'm driving along under AUTO and I see a dead skunk in the road or smoke from a brush fire coming up. I don't want the smell in my car, so I press the recirculate air button. Once I'm clear of the issue, I press the AUTO button to let the car take back full control of the climate system.
2. I generally want to let the car control the A/C('A/C mode with AUTO' is set to ON), but for some reason I want to temporarily shut it off. I press the A/C button twice to turn it off. Once I'm ready to turn it back on, I press the AUTO button to let the car take back control of it again.
3. I'm in an atypical climate situation and I want to override the climate control settings. Maybe it's really cold in the cabin and I want the hot air to come out of the dash vents instead of the floor vents so I can warm up my hands. Once my hands are warm enough, I press the AUTO button to let the car take back control.

 

I could very well be wrong about that part. Still, I think the gen5 has even more fine control over the A/C compressor than previous generations did. I know in the residential A/C and heat pump world, there was a time of only on/off. Then they progressed to two or three power stages. Now it's not uncommon to see seven stages, and I think I even saw one with twenty stages and another one with ability for infinite stages.* Not sure exactly how Toyota chose to do it for the gen4 and gen5.



*But don't get me started on how those systems operate in the real world. If you don't have them installed juuuust right by a tech that knows what they're doing, you'll often end up with a multi-stage unit that only operates in on/off mode.

 

In my Gen 3, watching the "A/C watts" PID on a ScanGauge, I see the car routinely running the compressor at any power level between 250 watts and 2500 or so, That's a pretty big variable range. I guess I generally see numbers that are multiples of 50 watts, but I'd call that pretty fine control, no? So it's not a "multi-stage" unit, it's just a variable unit, and it has been from the get-go, so I'm not sure there'd be much room for the control in Gen 4 or 5 to be even more fine.

 

FYI: in the Prius (starting from Gen3 for sure, for Gen2 I am not sure) the AC compressor is fully electric and not connected in any way to the ICE. So turning it off is not going to affect in any way acceleration or performance going uphill, etc. This is the only way to not die in the heat when stopped at a traffic light without having to turn on the ICE.

It does affect significantly MPG in the Prime in EV mode as it can use up to 15% of battery to heat the car, a bit less (in my experience) to cool the car. That's a 15% reduction in range.
In the regular Prius, I am pretty sure it doesn't do anything in terms of MPG. My experience on Gen3 and Gen4, driving in ECO mode didn't do much, if at all, on MPG that was related to the AC. The driving style was essentially the same, as well as my MPG, so I left the car in "normal" mode (i.e., PWR and ECO mode off), all the time.

In Gen3 using ECO mode would increase MPG in winter was it would not turn on the ICE to heat the car as much when stopped at a traffic light to heat the cabin. It reduced gas consumption by doing this and also reducing the fan speed to make you feel less cold in the cabin (less "wind chill" effect, so to say). But in summer it didn't do anything in terms of MPG that I could notice. In Gen4 it was a nice welcome that you could activate "eco" mode for the HVAC independent of the drive mode.
But in terms of driving performance (acceleration, power, etc.), turning off the AC really doesn't do anything. This is another benefit of fully electric, not belt-driven, HVAC compressors/heat pumps.

 

The Car Care Nut just dropped a video loosely related to this topic. He talks about how many Toyotas (somewhat excluding the most recent) have issues with mold and odors in the HVAC system. He shows how to fix it once it's happened, and how to prevent it from happening again.

Linked to the gen5, he suggests that owners of the most recent Toyotas leave their HVAC controls set to AUTO(especially the A/C) in order to let them cycle according to their program to reduce moisture in the A/C evap ducts. He says the biggest problem is users leaving their system set to MAX A/C all the time in the summer; locking the system in recirculate mode, which traps condensate in the evap and creates a perfect environment for mold growth.

This is obviously a problem that affects people differently depending on where they live, their driving habits, and how they store their vehicle.