Heater/AC question. - The 1947 - Present Chevrolet & GMC Truck Message Board Network

scotts1970 · 10-12-2009, 01:32 PM

I have used this website many times in the past few years to help me figure out various problems and always appreciate yall's help. So today I have another I need help with.

1990 chevrolet 1/2 ton 5.7 automatic

Heater nor defroster would'nt get warm. Was on high and the highest temperature setting. Air was cool/lukewarm. Heard the ac compressor clutch cycling.

Checked under the hood and both heater hoses were hot. Engine was at operating temperature. AC lines were very cold and the compressor was cycling on and off.

I unplugged the clutch cycling switch and the engine cab air turned to very warm like it was supposed to.

Question: What would make the AC run at the same time as the heater? Could it be a defective control assembly? Hope not.

Thanks for the help in advance.

ChevLoRay · 10-12-2009, 01:37 PM

When you select Defrost, as I am sure you already know, the A/C compressor will cycle on and off to provide what the MFR calls "CFC Linked Defrosting".

Now, as to why you have the problem you have....I'm at a loss.

scotts1970 · 10-12-2009, 01:52 PM

Actually I didn't know that so I am glad you told me. Learn something new everyday.

ChevLoRay · 10-12-2009, 07:36 PM

Oh yeah, and if the temp control is set to the coldest setting, then the air that comes out of the vents will be at its coldest. Using the temp control will show you that it is probably working okay. If it is set so that the windshield fogs up on the outside, then it is probably too cold/too humid.

10-12-2009, 01:32 PM	#1
scotts1970 Registered User Join Date: Jul 2006 Location: Texas Posts: 14	Heater/AC question. I have used this website many times in the past few years to help me figure out various problems and always appreciate yall's help. So today I have another I need help with. 1990 chevrolet 1/2 ton 5.7 automatic Heater nor defroster would'nt get warm. Was on high and the highest temperature setting. Air was cool/lukewarm. Heard the ac compressor clutch cycling. Checked under the hood and both heater hoses were hot. Engine was at operating temperature. AC lines were very cold and the compressor was cycling on and off. I unplugged the clutch cycling switch and the engine cab air turned to very warm like it was supposed to. Question: What would make the AC run at the same time as the heater? Could it be a defective control assembly? Hope not. Thanks for the help in advance.

10-12-2009, 01:37 PM	#2
ChevLoRay Old Skool Club Join Date: Jun 2001 Location: Benton, AR "The Heart of Arkansas" Posts: 10,880	Re: Heater/AC question. When you select Defrost, as I am sure you already know, the A/C compressor will cycle on and off to provide what the MFR calls "CFC Linked Defrosting". Now, as to why you have the problem you have....I'm at a loss. __________________ Member Nr. 2770 '96 GMC Sportside; 4.3/SLT - Daily driven....constantly needs washed. '69 C-10 SWB; 350/TH400 - in limbo The older I get, the better I was.

10-12-2009, 01:52 PM	#3
scotts1970 Registered User Join Date: Jul 2006 Location: Texas Posts: 14	Re: Heater/AC question. Actually I didn't know that so I am glad you told me. Learn something new everyday.

10-12-2009, 07:36 PM	#4
ChevLoRay Old Skool Club Join Date: Jun 2001 Location: Benton, AR "The Heart of Arkansas" Posts: 10,880	Re: Heater/AC question. Oh yeah, and if the temp control is set to the coldest setting, then the air that comes out of the vents will be at its coldest. Using the temp control will show you that it is probably working okay. If it is set so that the windshield fogs up on the outside, then it is probably too cold/too humid. __________________ Member Nr. 2770 '96 GMC Sportside; 4.3/SLT - Daily driven....constantly needs washed. '69 C-10 SWB; 350/TH400 - in limbo The older I get, the better I was.



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