Defrost engages AC compressor

Driving around last night, I noticed that every time I turned the heat on, I felt a slight but sudden loss of power. I couldn't figure it out. The AC light was still off, but something felt wrong.

Then I tried turning the heat on and off on different settings and realized it only happened when air was being sent to the windshield. It then occurred to me that conditioned air is dry and helps with defrosting... Pretty clever.

Maybe this is how all cars are these days, but I spent about twenty minutes thinking I was going to have to fix some gremlin with the AC...Just figured I'd mention it, in case someone is getting bad gas mileage from having the heater set on feet/windshield all the time...

 

this is pretty much how vehicle have been for sometime...take the humidity out of the air..thus no fog...

 

This was on a 2009... The next newest car I've ever had was 1996. The world, she is a-changin'...

 

correct me if im wrong but i think there are write-ups in the diy forum on how you can make the defrost run without having the compressor run. this makes it nice because most of the time i need it i dont need the compressor to run and kill my mpg. then if you do need the cool dry air you just puch the ac button and it comes on. i believe there are writ-ups for both 1st and 2nd gens too.

 

Nice. I may just do that. This truck is thinking too much.

 

Does anyone know how to deactivate the ac compressor running with the defrost on a 2nd gen (2013) 4 cyl.

 

Search is your friend. There are issues getting it to work on 2009+ models though.

http://www.tacomaworld.com/forum/2nd-gen-tacomas/7160-c-defrost.html

 

i cant remember the source, but i was given the information that automakers have the AC come on while defrost is selected in order to ensure the AC pumps/systems get a little use during winter months when they are turned off for long periods - in order to keep the seals from drying out.

my coworkers dodge diesel does the same thing

I personally feel the defrost setting is the devil and I keep my dial away from it as much as possible - so I don't encounter this AC pump kicking on problem much

 

Unfortunately I must use my defrost daily up here in Central NY and really would rather not run the A/C. I realize that it is good to turn it on a few times during the winter to keep the seals conditioned but the fact that it automatically engages with defrost as well as with turning the temp dial all the way to cold really bugs me. The 2.7 bogs down every time I turn on the fan as defrost is my default setting...

 

You need the ac to dry the air so it actually de fogs the window. Otherwise you are blowing humid air onto the window which doesn't help

 

If running the AC is too much for your engine, then you probably should have gotten the V6...

If running the AC is too much for your wallet, then you probably should have gotten a Corolla...

 

THE AC compressor engaging in defrost has been that way since the defrost option was available. I don't know about the 60s, but I know in the 70s that was how it worked. So yes OP, your 1996 did the same thing. You just didn't notice it. You cannot defog or defrost a windshield with humid air. It has to be dehumidified, which is what your AC system does to the air. It first dries it, then the heater core will heat it.

 

How does the defrost/defogging work on Cars or Trucks without AC? My first 2 vehicle didn't have a compressor, and the defroster setting worked.

 

This.

I don't understand some of the "issues" people come up with here.

 

By blowing heat on the window. AC is just more effective, because it removes moisture.

 

Ok just making the point that you don't "need" the compressor to run, in order for defrost to work.

 

I would prefer it to run when the defrost is on. It keeps the "musty" smell down.

AC/dehumidification is a good thing, and it's not a big parasitic loss.

 

I agree, but having control on when it runs and when it doesn't when blowing air on the windshield isn't necessarily a bad thing.

 

Normal

 

If you do not want the AC compressor to run when in defrost mode but still want it to cycle some to keep the compressor exercised try running a wire from your back up lights to a relay to kick it on while in reverse. It will get exercised every day a little unless you never back up and you will be able to run in defrost all the time without running the compressor. I always like to run split between floor and windshield.