Defroster turns on A/C??

BABrede · 02-14-2010, 10:23 PM

When I turn on the defroster the air conditioner light (next to the a/c button) goes on and the discharged air drops in temprature. When I switch back to floor or bi-level the light goes out and I have heat again. Is this normal?? Seems odd to me, but then again I'm learning that RV'S are a different breed of vehicle. LOL

Our coach is a 2000 Fleetwood American Eagle on a Spartan chassis.

ernieh · 02-14-2010, 10:45 PM

Most of todays vehicles run the AC compressor in defrost mode to help remove moisture.

Teach · 02-15-2010, 08:12 AM

Quote:

Originally Posted by BABrede

Is this normal??

Perfectly normal.

RustyJC · 02-15-2010, 08:20 AM

Modern vehicles run the A/C in defrost mode to condense the moisture out of the air passing over the A/C evaporator coil, then reheat the air over the heater coil. This lowers the dew point of the air so that the moisture in the air won't condense when the air hits the cold windshield.

Rusty

hdossett · 02-15-2010, 09:02 AM

Quote:

Originally Posted by BABrede

Is this normal?? Seems odd to me,. LOL

Most times I'd take odd over normal, I mean, who wants to be normal?

But on this, normal is good. GM has been doing this for decades! My '68 Chevy did it, my '56 didn't have A/C. Sometimes, in cool damp weather, I use the A/C just to clear the fog from all windows. Works great.

Way back then I always heard it was to exercise the compressor and keep all the seal in shape over the winter, but I like the dehumidifier reason better!

Half of the job of air conditioning is dehumidifying. Just ask people in FL, we even need it about 5 days a year in AZ.

Harold
In Quartzsite right now!

wa8yxm · 02-15-2010, 10:58 AM

As someone said, the reason they do that is the defroster condenses moisture out of the air.. In older cars it was not uncommon to turn on the defroster/defogger and the windows would FOG UP from moisture in the air blowing across them (at least till the engine warmed up)

If this bothers you. as it bothers me, there is a simple fix

Locate the air condiioner compressor. IT's a big metal thing with a pulley on one end, two hoses or pipes, and the main fan belt or serpitine belt goes across it... With the engine ideling and the HEVAL (Heater/Defroster/A-C) turned OFF, the pulley should turn but the end (Face?) won't.. turning on the AC should cause the thing to make a CLUNK sound and the face to turn.

This will have a wire connected to it,, Usually just one (though some vehicles have two, in a single plug)

Disconnect this wire. Label and tie off in a safe place.

Elkhartjim · 02-15-2010, 11:11 AM

Quote:

Originally Posted by RustyJC

Modern vehicles run the A/C in defrost mode to condense the moisture out of the air passing over the A/C evaporator coil, then reheat the air over the heater coil. This lowers the dew point of the air so that the moisture in the air won't condense when the air hits the cold windshield.

Rusty

I love it when Rusty talks about dew points and such. And just what constitutes a "modern" vehicle Rusty?

RustyJC · 02-15-2010, 11:12 AM

Quote:

Originally Posted by Elkhartjim

And just what constitutes a "modern" vehicle Rusty?

Anything later than a Model T??

Rusty

FlyGuys · 02-15-2010, 12:07 PM

yeah, one of my pet peeves.

"air conditioning" does NOT mean "cold air". it means conditioning the air by removing the moisture (humidity). the defroster uses "conditioned" air to help remove the fog on the windshield. once the engine is warmed up, this conditioned air is hot--or whatever temp you set it to on the dash temp selector. while the engine is cold, the defroster air is going to be cold, too. once the engine is warm, you get the full range of temps to select--from hot to cold. and nice and dry, too.

Pete & Linda · 02-15-2010, 02:57 PM

The AC compressor will run on defrost regardless where the temp selector is set.

Bob (WA0MQE) · 02-16-2010, 07:49 AM

A second effect for this feature is it exercises and lubricates the AC Compressor. On older vehicles that didn't automatically run the AC in Defrost it was recommended by manufacturer's to run your AC a few times during the long winter to prevent it from drying out and jamming up due to lack of use.