Air Condition Problem

Johnboy · 07-21-2008, 08:25 AM

I have a 1990 Model 4X4 1/2 Ton Truck, the Air Condition works fine at the start but after it is on for a few minutes it starts blowing hot air. If you press the accelarator down then it starts blowing cold air again, must be a vacuum problem somewhere. I think on a steady run down the road it finally loses vacuum and when you hit passing gear it starts making cold air again, is there some kind of vacuum valve under the dash that may be bad? The Freon is full in the unit, compressor and all that is okay, has anyone ever seen this kind of problem before? Any help will be greatly appreciated.

Johnboy

bradrhale · 07-21-2008, 11:49 AM

if your blend door is vaccuum operated, you might have a leak that wont let it operate. when you have high rpms, she will build up enough to actuate it. that or your unit is overcharged.

leolkfrm · 07-21-2008, 02:21 PM

How do you know the charge status what kind of pressure on the low side, was it recently evaped and recharged? or just added to? did the problem start all at once or gradually?

flyballman · 07-21-2008, 02:28 PM

make sure your condenser isn't clogged or that your fanclutch is working properly, you might be building up to much heat

Johnboy · 07-22-2008, 05:41 PM

Well I had the Compressor Checked, it's okay, clutch is fine, plenty of Freon and Oil in Unit. As for Vacuum hoses there are none, it's all Electronic, there is one down on the left on lower heater box next to gas pedal, and one behind the glove box, have to take glove box out to get to it. Dealer tells me ther is no vacuum hoses on 90 chevy 4x4, it all is electronic, may be losing voltage, sounds strange to me though, anyway I will look at them and see if it appears to have a problem, will check volts when running. Thanks so far for the replys.

Johnboy

leolkfrm · 07-22-2008, 05:43 PM

That is correct they are servo operated, the one on the lower box changes from das to def etc. the one behind the glove box is the one to check

Johnboy · 07-22-2008, 05:48 PM

If I look at it and see nothing Visual, could it still be bad, I am sure it is going to be a sealed unit?

Johnboy

leolkfrm · 07-22-2008, 05:51 PM

manually move the damper and see if the air works correctly

gmctrucks · 07-22-2008, 06:29 PM

My 93 F/S GMC would start blowing hot air then it would start blowing cold air after a while, well it turned out that I was low on Freon and it was freezing up and when it would thaw out it would work good again till it would do the same thing again, again, and again.

Johnboy · 07-22-2008, 07:43 PM

Well everything I have checked is working properly, servos are good. Now I think I have figured it out, when the compressor kicks off I still have power on the hot and ground where the wires plug into the top of the compressor. If you tap on the side with a small hammer it goes again, I guess it must be heating up and kicks out, but I don't understand why I still have power. I checked the diode in the clutch part of the compressor and it is good, any ideas from anybody? Can there be a fix wiothout changing compressors? Looks like to change just the clutch parts and wiring may be a problem, not always a good fix, looks like I may have to change compressor, oh well. Maybe someone else may have a option, Thanks for the replys.

07-21-2008, 08:25 AM	#1
Johnboy Full Member Join Date: Jul 2008 Location: Tennessee Posts: 19	Air Condition Problem I have a 1990 Model 4X4 1/2 Ton Truck, the Air Condition works fine at the start but after it is on for a few minutes it starts blowing hot air. If you press the accelarator down then it starts blowing cold air again, must be a vacuum problem somewhere. I think on a steady run down the road it finally loses vacuum and when you hit passing gear it starts making cold air again, is there some kind of vacuum valve under the dash that may be bad? The Freon is full in the unit, compressor and all that is okay, has anyone ever seen this kind of problem before? Any help will be greatly appreciated. Johnboy

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07-21-2008, 11:49 AM	#2
bradrhale Moderator Join Date: Apr 2006 Location: Texas Posts: 2,968	if your blend door is vaccuum operated, you might have a leak that wont let it operate. when you have high rpms, she will build up enough to actuate it. that or your unit is overcharged.

07-21-2008, 02:21 PM	#3
leolkfrm Senior Member Join Date: Mar 2008 Location: N.Y. Posts: 1,074	How do you know the charge status what kind of pressure on the low side, was it recently evaped and recharged? or just added to? did the problem start all at once or gradually?

07-21-2008, 02:28 PM	#4
flyballman Junior Member Join Date: Jul 2008 Posts: 1	make sure your condenser isn't clogged or that your fanclutch is working properly, you might be building up to much heat

07-22-2008, 05:41 PM	#5
Johnboy Full Member Join Date: Jul 2008 Location: Tennessee Posts: 19	Well I had the Compressor Checked, it's okay, clutch is fine, plenty of Freon and Oil in Unit. As for Vacuum hoses there are none, it's all Electronic, there is one down on the left on lower heater box next to gas pedal, and one behind the glove box, have to take glove box out to get to it. Dealer tells me ther is no vacuum hoses on 90 chevy 4x4, it all is electronic, may be losing voltage, sounds strange to me though, anyway I will look at them and see if it appears to have a problem, will check volts when running. Thanks so far for the replys. Johnboy

07-22-2008, 05:43 PM	#6
leolkfrm Senior Member Join Date: Mar 2008 Location: N.Y. Posts: 1,074	That is correct they are servo operated, the one on the lower box changes from das to def etc. the one behind the glove box is the one to check

07-22-2008, 05:48 PM	#7
Johnboy Full Member Join Date: Jul 2008 Location: Tennessee Posts: 19	If I look at it and see nothing Visual, could it still be bad, I am sure it is going to be a sealed unit? Johnboy

07-22-2008, 05:51 PM	#8
leolkfrm Senior Member Join Date: Mar 2008 Location: N.Y. Posts: 1,074	manually move the damper and see if the air works correctly

07-22-2008, 06:29 PM	#9
gmctrucks Senior Member Join Date: Nov 2006 Location: Houston,Texas Posts: 1,722	My 93 F/S GMC would start blowing hot air then it would start blowing cold air after a while, well it turned out that I was low on Freon and it was freezing up and when it would thaw out it would work good again till it would do the same thing again, again, and again.

07-22-2008, 07:43 PM	#10
Johnboy Full Member Join Date: Jul 2008 Location: Tennessee Posts: 19	Well everything I have checked is working properly, servos are good. Now I think I have figured it out, when the compressor kicks off I still have power on the hot and ground where the wires plug into the top of the compressor. If you tap on the side with a small hammer it goes again, I guess it must be heating up and kicks out, but I don't understand why I still have power. I checked the diode in the clutch part of the compressor and it is good, any ideas from anybody? Can there be a fix wiothout changing compressors? Looks like to change just the clutch parts and wiring may be a problem, not always a good fix, looks like I may have to change compressor, oh well. Maybe someone else may have a option, Thanks for the replys.