Defroster problems 1972 Chevrolet PU - The 1947 - Present Chevrolet & GMC Truck Message Board Network

w6tractor · 04-18-2009, 02:19 PM

I am confused/frustrated wth my A/C-Heater. Everything works perfectly, except the de icer. When I move the controls to the position to de ice, the A/C compressor does not come on. I have had all the A/C/Heat ducting etc. off and replaced. The cables go to the correct places. The switch on the underdash heater box that controls the compressor works, but not when set for de ice. Everything else switches from vents to floor to windshield outlets. How do I get the compressor to come on in the de ice position? What have I missed? Thanks

Camper Power · 04-18-2009, 02:25 PM

the compressor shouldnt come on when you have it set to heat. it should only come on when you set it to A/C. the heater uses heat from the engine coolant to warm the air.

**leddzepp** · 04-18-2009, 02:26 PM

The A/C compressor is not supposed to be on in the de-ice position. You are using heat from the heater core to de-ice. Do you mean that the fan does not come on?

red71c10 · 04-18-2009, 02:31 PM

Modern cars and trucks do have the A/C kick in when the controls are set to defrost (deice, defog), but older rigs do not. The A/C acts as a dehumidifier and helps remove the excess moisture from the windshield.

w6tractor · 04-18-2009, 02:36 PM

I didn't mention that this, but the truck is factory a/c. It has a position for both defrost and de ice. I believe that on this model the compressor is supposed to run to dehumidify the air as you stated. Just can't figure out what's wrong to keep it from working. Thanks for the comments.

**leddzepp** · 04-18-2009, 03:50 PM

I have factory A/C on both my 72's. When you slide the control lever over, it hits the button on the back of the box to shut off the compressor. It is not supposed to run in our trucks to dehumidify the air, unlike newer model cars.

w6tractor · 04-18-2009, 05:24 PM

That's exactly what it does. That seems strange to have that capability and not be able to use it. Thanks for clearing that up. Thanks to all of you that responded. On to the next challenge.

04-18-2009, 02:19 PM	#1
w6tractor Registered User Join Date: Mar 2008 Location: Bynum, Tx Posts: 21	Defroster problems 1972 Chevrolet PU I am confused/frustrated wth my A/C-Heater. Everything works perfectly, except the de icer. When I move the controls to the position to de ice, the A/C compressor does not come on. I have had all the A/C/Heat ducting etc. off and replaced. The cables go to the correct places. The switch on the underdash heater box that controls the compressor works, but not when set for de ice. Everything else switches from vents to floor to windshield outlets. How do I get the compressor to come on in the de ice position? What have I missed? Thanks

04-18-2009, 02:25 PM	#2
Camper Power Registered User Join Date: Feb 2008 Location: Grande Prairie Alberta Posts: 261	Re: Defroster problems 1972 Chevrolet PU the compressor shouldnt come on when you have it set to heat. it should only come on when you set it to A/C. the heater uses heat from the engine coolant to warm the air. __________________ eh 1970 Chevy C20 daily driver 1975 GMC Jimmy 4x4 project 1966 GMC C10 Dad's project

04-18-2009, 02:26 PM	#3
leddzepp Moderator Join Date: May 2007 Location: Southern Cal Posts: 20,006	Re: Defroster problems 1972 Chevrolet PU The A/C compressor is not supposed to be on in the de-ice position. You are using heat from the heater core to de-ice. Do you mean that the fan does not come on? __________________ 1972 C/10 Cheyenne Super SWB. Restored, loaded, slammed. 1968 C/10 50th Anniversary LWB. Unrestored, stock, daily driver/work truck. RIP ElJay RIP 67ChevyRedneck RIP Grumpy Old Man

04-18-2009, 02:31 PM	#4
red71c10 The Red Menace Join Date: Feb 2009 Location: Roseburg, OR Posts: 577	Re: Defroster problems 1972 Chevrolet PU Modern cars and trucks do have the A/C kick in when the controls are set to defrost (deice, defog), but older rigs do not. The A/C acts as a dehumidifier and helps remove the excess moisture from the windshield. __________________ 1971 Custom 10 - 350/350TH - Pwr steering, gauges, Chrome front bumper, Chrome hubcaps, Door edge guards, HD rear springs (current) 1968 C-20 Custom Camper - 350/350TH (orig 327), Pwr steering & brakes, gauges w/tach (former) 1968 C-20 - 307/3sp column (Dad's when I was a kid) "Driving is like coloring. Things get messy if you can't stay inside the lines."

04-18-2009, 02:36 PM	#5
w6tractor Registered User Join Date: Mar 2008 Location: Bynum, Tx Posts: 21	Re: Defroster problems 1972 Chevrolet PU I didn't mention that this, but the truck is factory a/c. It has a position for both defrost and de ice. I believe that on this model the compressor is supposed to run to dehumidify the air as you stated. Just can't figure out what's wrong to keep it from working. Thanks for the comments.

04-18-2009, 03:50 PM	#6
leddzepp Moderator Join Date: May 2007 Location: Southern Cal Posts: 20,006	Re: Defroster problems 1972 Chevrolet PU I have factory A/C on both my 72's. When you slide the control lever over, it hits the button on the back of the box to shut off the compressor. It is not supposed to run in our trucks to dehumidify the air, unlike newer model cars.

04-18-2009, 05:24 PM	#7
w6tractor Registered User Join Date: Mar 2008 Location: Bynum, Tx Posts: 21	Re: Defroster problems 1972 Chevrolet PU That's exactly what it does. That seems strange to have that capability and not be able to use it. Thanks for clearing that up. Thanks to all of you that responded. On to the next challenge.



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