A/C Switch - - - why ?

Don't all cars with A/C have an A/C switch? If the ambient outdoor temp is "cool" you can turn on the fan without A/C to get cool air in the cabin with minimal affect to your mileage. If its hot outside and you want cool air in the cabin, A/C needs to be turned on and you will take a hit on mileage

 

It's a feature, not a bug. And I'm not using that in a sarcastic way.

The standard mode prior to the gen5 was that the car controlled the bulk of the climate settings, but it left overall control of the A/C compressor to the driver. Honestly, that's not really needed or wanted anymore by the vast majority of drivers, so Toyota has changed things in the current generation of cars.

Now, the car controls the A/C compressor like everything else by default. This is what you're experiencing. But Toyota also understands that some people still want to control the A/C like before, so there's a setting to do that. If you go into the climate settings on the infotainment unit, you'll find a setting called 'A/C mode with AUTO'. It's set to ON by default. If you want your climate control to behave the old way, set it to OFF. Basically, the setting indicates whether AUTO has control of the A/C or not. If you do this, the A/C button will work just like cars you've had in the past. When the A/C light is on, the compressor is enabled(if needed). When the light is off, the A/C system is disabled.

Now if you usually want AUTO to have full control, but you encounter a situation where you want to shut off the A/C quickly without going into a bunch of menus, you can do that by pressing the A/C button twice. The first time overrides AUTO and turns it on, the second time shuts it off. It will stay shut off until you press AUTO again, which let's it take back control.

 

Interesting. With 3rd gen, as far as I know, the only way the A/C compressor would run without the AC switch being on was in heat/defog mode:



Without lighting the AC button, stealthy.

In this mode ScanGauge showed values for ACWATTS. Other'n that, and the hell's-a-popping, full on defog mode:



it was user controlled. And in that mode, the AC button would light up, if it was off.

Count me in for minimal AC use, actually use it more in winter, for defogging, as often as note just by setting to the aforementioned heat/defog mode. In summer we just shut everything down and roll the windows down some or all.

One thing: constant AC use makes the system more vulnerable to mold getting started, and if spray cleaners don't kill it, it's not a barrel of laughs accessing the innards for cleaning.

 

Good to know. Thanks.

 

On the flip side, it can actually be bad for your hybrid battery in the long run for an owner to not run A/C, depending on conditions.

Say you're in a hot(and somewhat dusty) climate and you want max mpg. You don't want to take the A/C hit, so you leave it off and open the windows for the breeze. The temp in the cabin gets into the 80s-90s+(high-20s into 30s C).

But what about that AIR COOLED HV battery under the rear seat? How's it doing trying to get rid of the heat it's generating on a hot day using hot air? And if that air is also dusty, it might be clogging the filter and reducing the amount of air the battery needs. And we all know the worst thing for the long term health for our HV batteries is to let them overheat.

In those cases, it's better to leave the windows up and the A/C at 72F/22C. Take the small hit to mpg to keep the HV battery healthy.

Obviously doesn't apply to cooler climates/winters, but it can be important in some areas/seasons.

 

Good info, but does this particular advice apply to the phev, with a liquid cooled traction battery?

 

No. Just the air-cooled models.

The flip side to the refrigerant-cooled PHEVs is that you don't have as much control over the A/C system as the HEV owners do. Even if you use any of the methods to try and keep the A/C compressor off, I'm 99.9% certain the car will override that and kick on the compressor if the HV battery needs it. It's possible you won't get any of the cooled refrigerant in the cabin air box, but the HV batt refrigerant loop will be cold.

The other thing the PHEV owners need to remember is that a problem with the A/C system is much more critical for them than for HEV owners. Losing A/C is an annoyance for HEV drivers, but it's potentially damaging for the HV batt in the PHEVs. But I think the PHEVs are programmed to give major dash warnings if the A/C system is compromised, so I'm pretty sure PHEV owners would be well aware to get their car to a dealership quickly for repairs.

Last side note for PHEV owners is that the book recommends an additional A/C service at a fairly high milage. Something like 10y/120k miles or something like that(check your manual/maintenance guide). I assume it's to test the system for leaks and make sure it has the proper amount of refrigerant.

 

Plus, doesn't the PHEV use a heat pump for heat? I'd think that'd be part of the A/C.

 

Back when I had a scangauge connected, and monitoring the middle temp sensor (of 3 along length of hybrid battery), we’d been parked our ‘10 for at least 30 minutes, on a hot summer day in direct sun.

came back, started car and drove, purposely avoiding AC, windows mercifully cracked, but oppressively hot. Drove a solid 10 minutes thus, monitoring battery temp. It dropped some, then stabilized.

then started AC, set to comfortable cabin temp, maybe 24C. Chill air ensued, quite a relief. About another 10 minutes, hybrid battery temp “had” dropped more:. two degrees Fahrenheit.

 

A heat pump uses the compressor in heat or in cool mode. It simply uses a reversing valve (in simple setups) to reverse the heat transfer from outside to inside. AC mode heat transfer is from inside to outside. The more elaborate heat pump systems are capable of cooling one area while heating another, all through the use of normal vapor compression refrigeration. In a vehicle that could mean cooling the battery while heating the cabin.

High temperature in Vancouver is not the same as high temperatures in Texas where it may remain 85f at night and the uv radiation is off the scales. So ac use in high heat index areas while the car is running can be beneficial to air cooled hybrid batteries.

However efforts to cool them when not operational (as proposed in another thread) is a waste of money.

 

Sure Texas is warmer, but we're not exactly chilly in summer. Google AI summary:

AI Overview


In 2025, Vancouver, BC experienced an exceptionally warm year with record-breaking December temperatures, while Dallas, TX saw a relatively mild summer compared to previous years, though it still experienced significant heat.

Vancouver, BC (2025 Temperatures)

• Hottest Temperature (Summer 2025): The highest temperature recorded at Vancouver International Airport (YVR) in 2025 was 34.4°C (93.9°F) in July, with August closely following at 33.3°C (91.9°F).
• Mid-August Heat: West Vancouver hit 30.1°C (86.2°F) on August 23, breaking a 38-year-old record.
• December Record: On December 15, 2025, a record-breaking 15.7°C (60.3°F) was recorded at YVR, breaking a 63-year-old record.
• Summer Trend: Overall, the summer of 2025 was characterized by several heat waves with coastal temperatures often reaching into the mid-to-high 20s Celsius (70s/80s Fahrenheit).

Dallas, Texas (2025 Temperatures)

• Hottest Temperature (Summer 2025): The hottest recorded temperature at DFW Airport in 2025 was 104°F (40°C) on July 31.
• 100°F+ Days: 2025 had a surprisingly mild summer with only 7 days reaching 100°F (37.8°C) or higher, significantly fewer than the 23 days in 2024 or 55 days in 2023.
• Christmas Warmth: Christmas 2025 in North Texas was exceptionally warm, reaching 80°F (26.7°C), which tied for the second warmest on record.
• August Wind Event: While the summer was milder, an 89 mph wind gust was recorded on August 4, 2025, at DFW, breaking a 40-year record.

Note: Temperatures in the search results are often listed in Celsius. Conversions to Fahrenheit have been provided for clarity based on the data available.