Keeping the A/C on all the time

Hi all,

I tried searching for this info but didn't find any discussion about it, and I hope I'm asking this question in the right place.

Growing up, we were told that it's best not to keep the A/C on all the time, rather switch it off couple of seconds before switching off the car, and turn it on after turning on the car etc.

But at the introduction of newer/ digital A/C clusters, we were told now it is fine to keep the A/C on all the time, just like you treat the automatic lights.

I'm not sure what has changed exactly, I'm aware of some changes manufacturers did about 20 years ago such as changing from mechanical fans to electric ones, but I'm not entirely sure about the statement itself: That we had to switch the A/C off to prevent harm and nowadays we don't have to because manufacturers account for it and there is no harm or the harm is negligible.

Now to my question: I have a 2020 Tundra SR5, it has dials as A/C controls and I read here somewhere that it's not the same system as the full option trims that has digital controls. If I had the digital one i'd treat it like my other cars and never switch off the A/C when Turning off the truck, but them dials made me sceptical that I have a somewhat inferior A/C system and I should not keep it on all the time.

Kindly share your mechanical knowledge on this matter.

Thanks all !

 

I'm curious about this as well. I've heard the same thing growing up but I have always still just left the AC on when shutting off the vehicle and have never had an issue. Interested in what people more familiar with system has to say though.

 

run AC as much as you want, whenever you need it. No concerns.

 

For one thing, I believe that modern AC systems are controlled by the engine ECU, and not allowed to come on (clutch not engaged) for a short period after engine start. That time varies with different manufacturers and models. Not sure what the delay period is for our trucks.

Secondly, I think the older advice had more to do with the starter motor having to turn both the engine and the AC compressor when starting, putting more strain on the starter motor and the battery.

 

I always turn my AC off before I get home to run warm air through the ducts to try to dry them out. The ducts are good areas for bacteria to grow. My old 1987 Toyota 4X4 had bad smells coming out the ducts and I had to run a lot of germ killing products into the ducts to stop the smell. The one my Toyota Service Manager gave me worked the best. I also turn it off so it isn't an extra load on the engine when it cranks. Probably my old timey ways. LOL.

 

Modern vehicles are engineered to automatically disconnect most of the accessories (A/C compressor, blower fan, radio, etc.) during the engine cranking period. The A/C can be left turned on and the computer will actually run it once it's booted up and done it's checks (mere moments).

The digital HVAC controls that are an option on upper trim levels are computer controlled and allow the system to dictate where to direct airflow and to set a temperature for the cabin. Either style of HVAC panel will control the on/off cycling of the A/C compressor depending on how you've set your desired temperature.

Air Conditioning removes moisture from the air, a dehumidifier, and blows that dehumidified air through the ducting. If there's moisture or condensation in the ducting, something is amiss. If you have a musty smell, the drain for the moisture may be plugged with debris of some sort (leaves most likely). When running the A/C in a humid climate, you should be able to see water dripping from underneath the vehicle somewhere below the firewall. The drain on the Tundras is just above the transmission bell housing and isn't easily viewed directly.

 

My old (2005) Volvo has the afterblow function, when it decides there will be a moisture buildup after engine shuts off and turns on cabin blower after 15-50 minutes for a few minutes to dry it up. When humans haven't messed up with this function, i.e. not turning it off, the A/C stays odor free for life. Don't know why it's beyond Toyota's imagination and we don't have a similar function.

 

Doesn’t make a difference in modern vehicles.

 

I leave mine on, but living in the high desert everything dries out fast. It doesn't hurt anything on a modern vehicle.

 

The AC would actually dry the air

 

I use my AC almost year round. Summer obviously for cool air and winter stays on feet/defrost which runs AC to dry the air. I've done it on all my vehicles

 

Zachly. The AC is “dry”, its not a swamp cooler. Zero need to shut it off.

 

The evaporator coil is below dew point when running. Moisture in the air condenses on it.
@TundraClayDigger I'd have to imagine you'd need to run it with compressor off at least 10 min to dry out the coil in most US climates. No data, just a guess.

 

I have never turned off the air conditioner in any of my vehicles.
It’s always on. I have only had 2 A/C failures. One was a poorly designed condenser (way to go GM) the other the compressor housing cracked.
That’s less than a 10% failure for me. So I’ll keep cranking that A/C all the time.

 

I always try to shut the AC off and let the fan run before I shut the truck off. I do this with my home air conditioners as well. Its just good practice but not the end all be all if you dont, just helps to prevent condensation. I also run my AC in the winter time periodically as well to help prevent the system from freezing up, running the AC with defrost on actually helps clear your windshield faster as well. For what it's worth even my home air conditioners when set to auto mode will run the fan only before shutting down after achieving the desired temp.

 

This is my situation exactly, bacteria or mold isn't really a concern for me.

This is the confirmation I was looking for, thanks. It's just tedious to keep switching it on and off, especially if I have few errands next to each other, the more stops you have, the more you have to switch on and off.

To everyone else that has responded, thanks.

 

Also….keeping the ac on, hence engaging the compressor actually prolongs the life of the system. The refrigerant also acts as a seal lubricant for any o rings or rubber gaskets in the system. Circulating the refrigerant keeps these parts from drying out. Not using the ac at all is the biggest culprit in ac system leaks as the rubber seals become dry rot.

 

A byproduct of the physics behind the A/C's in all vehicles and most residences is dryer air (A/C's in extremely dry climates may use a different method to cool the air). If you are turning your A/C off to "dry your vents" or anything like that, you are actually being counterproductive. Since the late '80's, I have left the A/C on 100% of the time in all my vehicles for their entire lives and have never had an A/C related problem beyond damage from an external source. And that includes vehicles that I got upwards of 300k miles out of.

 

As a former mobile HVAC product development manager, the idea of shutting off the A/C system (still let the blower operate) prior to shutting off the vehicle was to help remove any moisture left on the evaporator coil. Having moisture collect over time can lead mold, mildew, and an unpleasant odour. Not sure what modern vehicles do with all the electronic controls these days, but the principal would still apply. I’m pretty sure that all A/C systems still shut “OFF” (e.g. disengage the AC compressor) when the evaporator coil approaches temps just above freezing. This is to prevent any moisture on/in the coil from freezing which would block airflow through the evaporator

 

Well come to the vehicle corner of preservation and costly repairs. Imagine starting, stopping, driving, washing, and maintaining your body as your daily vehicle. When staring your car in the morning remember to allow it to warm up before adding additional load to the engine, battery, or alternator. Give the engine time to wake up and disseminate fluids, and electrical current throughout the system before overloading: Radio, air condition, windshield wipers, defrost, computer, power steering, brakes, headlights, and any other system. You wakeup in the morning and remain in bed additional 15 to 30 minutes before getting up: wakeup and immediately run several blocks before showering and brushing your teeth. So, allow your car time to warm up before driving. Remember to turn off everything to allow the car to reset itself as much as possible. Take care of your automobile and it will take care of you. Enough Said!

 

Since 1993, I have logged over one million miles on a total of four pickup trucks, all on the streets/roads/highways of central Florida, running the AC full time, all the time, at least nine months a year.

Ignore the myths.

 

If I read the original post correctly, it’s more about leaving the ac button “on” while shutting off the truck. Not about driving with ac on.

 

This is the definition of believing everything you read online.

Start vehicle with AC left on from night before, immediately put in gear, drive out of neighborhood, proceed immediately to 55mph speed limit. Decades of far worse done to others and not a single failure to date. Just like every other Tom dick and harry out there.

QUIT OVERTHINKING THINGS AND BELIEVING THE INTERNET!!! So many “experts” that don’t have a damn clue but believe they are experts because they read from other online “experts” and continue to spout ridiculousness. Vehicles are not the delicate flower the internet makes them out to be. Don’t believe me? Go talk to a farmer or a cattle rancher.

 

Shouldn’t believe you either. It’s on the internet.

 

To the OP you should be fine. I’ve never had issues leaving the HVAC on both from an electrical load on the starter viewpoint and from a musty vent etc viewpoint.

I should point out that I live in a climate where I’m able to run just the vents without the compressor/condenser on for a decent amount of time of the year.

 

This is a good article. Like I mentioned previously, it is always a good idea to shut off the AC a few minutes earlier before shutting off the vehicles as this helps to remove any moisture left on the evaporator coil. This is not a myth, but a fact. There are people who experience a foul smell coming from their AC unit all the time (me included). This is because of mold/mildew build up. This is a fact and not a myth.
I replaced the evaporator coil years back in my 97 LX450 while I had to take the entire dash out for some rust repair around the windshield. This was before the days of “cabin” air filters. You wouldn’t believe the amount of dirt/debris/leaves/dead insects etc. Parts of the aluminum coil were also corroding. All this crap will attract and retain moisture which will lead to mold/mildew/fowl smells. Even modern cars with cabin filters will eventually have crap/crud make its’ way to coil. So, the advice in the article is solid. Keep the coil clean by changing the filter often. Ensure the condensate drain tubes are free and clear, and “warm” up the coil before shutting off the vehicle.
Going back to the OP’s original question regarding “modem” electronics and “auto” mode as it relates to the operation of the AC system, I’m sure each vehicle is different, and unless it is written somewhere in the owners manual, the only way to understand its function would be to speak to someone who wrote the code on how it operates.

https://www.ultra-fresh.com/why-your-car-ac-smells-bad