Compressor cycles with defrost? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

chevymotocross · 11-14-2007, 08:27 PM

With cold weather here in New England, I have been using my defrost and heat. I never noticed this on other cars/trucks but when I have my defrost on, and I think even the heat, the ac compressor cycles on/off. What is up with this?

BLE 'BURBAN · 11-14-2007, 08:43 PM

Vehicles from the early 80's and newer are designed to cycle the A.C. compressor every now and then to keep them from siezing up from non-use. All it's doing is circulating the oil in the compressor to keep everything lubed inside. No worries no biggie.

C-10volvo · 11-14-2007, 09:06 PM

An air conditioning system is mainly used to cool the air that’s going through your vents and defroster. A useful side effect of air conditioning is that it reduces humidity levels. Less humid air being pumped through your defroster equals faster defogging on your windscreen.

spudz · 11-14-2007, 09:10 PM

It does that on defrost because the a/c drys out the air.. like fog on the windshield for example..

spudz · 11-14-2007, 09:11 PM

Quote:

Originally Posted by C-10volvo

An air conditioning system is mainly used to cool the air that’s going through your vents and defroster. A useful side effect of air conditioning is that it reduces humidity levels. Less humid air being pumped through your defroster equals faster defogging on your windscreen.

ya beat me to it lol

**old Rusty C10** · 11-14-2007, 09:17 PM

i was gonna say i havent had a truck that hasnt done it in years

crawdad · 11-15-2007, 06:42 AM

Mine cycles on even when I have the "vent" on.

75gmck25 · 11-22-2007, 07:10 PM

According to the repair manual for my '75 GMC the compressor will always be on in defrost mode. It will cycle off when the evaporator temp sensor (or outside air) reaches 35 degrees.

My original A/C controls were marked in one section as Economy Mode, since the compressor or will not cycle on in those positions (I think it was the positions for Floor only or Vent only).

Bruce

jchav62 · 11-23-2007, 11:14 AM

Quote:

Originally Posted by crawdad

Mine cycles on even when I have the "vent" on.

Sounds like something's wrong....

11-14-2007, 08:27 PM	#1
chevymotocross Registered User Join Date: May 2007 Location: East Haven, CT Posts: 882	Compressor cycles with defrost? With cold weather here in New England, I have been using my defrost and heat. I never noticed this on other cars/trucks but when I have my defrost on, and I think even the heat, the ac compressor cycles on/off. What is up with this?

11-14-2007, 08:43 PM	#2
BLE 'BURBAN In the Forgotten far North. Join Date: Jul 2007 Location: Anchorage, Alaska Posts: 2,627	Re: Compressor cycles with defrost? Vehicles from the early 80's and newer are designed to cycle the A.C. compressor every now and then to keep them from siezing up from non-use. All it's doing is circulating the oil in the compressor to keep everything lubed inside. No worries no biggie. __________________ 1987 R3500 CREW CAB DUALLY (BIG RED)Acquired 06/12/2015 1990 chevy suburban V2500 5.7L My cluster Mods-Nov 2007 overhead console Stereo install Round 2 Aug 2009 Heated/turn signal mirror upgrade

11-14-2007, 09:06 PM	#3
C-10volvo Registered User Join Date: May 2003 Location: Lake Oswego, Oregon Posts: 1,028	Re: Compressor cycles with defrost? An air conditioning system is mainly used to cool the air that’s going through your vents and defroster. A useful side effect of air conditioning is that it reduces humidity levels. Less humid air being pumped through your defroster equals faster defogging on your windscreen. __________________ Ken Morgan For fun: 1964 Volvo 122 For slow: 1984 Chevrolet C-10 For everything else: 1997 BMW M3

11-14-2007, 09:10 PM	#4
spudz Registered User Join Date: Oct 2005 Location: Branchville SC Posts: 1,647	Re: Compressor cycles with defrost? It does that on defrost because the a/c drys out the air.. like fog on the windshield for example.. __________________ 2004 z71, 96 z71 on 38s, 88 Chevy k5 blazer, 4in lift. 84 swb project, 82 SWB project , 84 chevy 4x4 lwb project, 88 Jimmy beater/highschool ride. 79 c30. 99 tahoe 4x4 project. and a bunch of parts trucks..

11-14-2007, 09:17 PM	#6
old Rusty C10 Robert Olson Transport Join Date: Oct 2002 Location: recent transplant to NC USA Posts: 20,313	Re: Compressor cycles with defrost? i was gonna say i havent had a truck that hasnt done it in years __________________ Bob 1951 International running on a squarebody chassis "If a man's worth is judged by the people he associates himself with, then i am the richest man in the world knowing some of the fine people of this board" http://67-72chevytrucks.com/vboard/a...t.php?f=25&a=9 (you can review the site rules here!) PM Me for your vehicle/parts hauling needs in the North East US or see my Facebook page Robert Olson Transport Live each day to the fullest.. you never know when fate is going to pull the rug out from under you... I hate cancer!!

11-15-2007, 06:42 AM	#7
crawdad Senior Member Join Date: Sep 2004 Location: Georgia Posts: 614	Re: Compressor cycles with defrost? Mine cycles on even when I have the "vent" on. __________________ 2012 Silverado LT 1500, 5.3, 6 speed auto '86 SWB C-10 Silverado

11-22-2007, 07:10 PM	#8
75gmck25 Registered User Join Date: Nov 2007 Location: San Antonio, TX Posts: 28	Re: Compressor cycles with defrost? According to the repair manual for my '75 GMC the compressor will always be on in defrost mode. It will cycle off when the evaporator temp sensor (or outside air) reaches 35 degrees. My original A/C controls were marked in one section as Economy Mode, since the compressor or will not cycle on in those positions (I think it was the positions for Floor only or Vent only). Bruce



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