Automotive Air Conditioning Information Forum (Archives)

Area1509 on Wed May 29, 2013 4:58 PM

Year: 1998
Make: GMC
Model: 1500
Engine Size: 350/5.7
Refrigerant Type: 134a
Ambient Temp: 100
Pressure Low: 05
Pressure High: 200
Country of Origin: United States

Hi all, I have a 98 GMC 1500, The A/C blows cool but not cold it doesn't seem to have any leaks.
When I connect the gauges it shows 200 on the high side and 5-8 on the low side.
The compressor never cycles it just stays on, if I unplug the low pressure switch it will turn off
but if I unplug the high pressure switch there is no change.

Any ideas welcome.
Phoenix AZ about 90 to 100 outside
Thanks A.J.

WyrTwister on Wed May 29, 2013 5:37 PM

The low side pressure is TOO low . The high side sounds too high to go with the low side pressure . But I may be wrong ?

Did you say if the radiator cooling fan is working ( mechanical or electrical ? ) ?

Is the condensor stopped up with leabes , bugs , trash ?

Is the engine over heating ?

Do not know if you have an expansion valve or orifice tube system ? Guessing , orifice tube . And wondering if it is stopped up ?

Were it me , I would try to trace the high pressure line / liquid line to the point where it is goung into the evaporator / cab of the truck . Is it cool or cold at all ? If it is , try working your way back the other way , towards the condensor coil . Does it transition from cool / cold to warm / hot ?

If so , at that spot , are there a couple of fittings there ? If so , thet is most likely where the orifice tube is located .

If it transitions where there is no fitting , you may / probably have a restriction there ?

Not being there , I would have to take baby steps . And proceed , acording to the information that was discovered .

God bless
Wyrr

GM Tech on Wed May 29, 2013 10:19 PM

better get a new cycling switch-- yours is stuck shut- should go off at 21-22 psi and on at 42 psi.....continued running at low suction decreases oil flow and burns up compressors...

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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

MrBillPro on Sun June 02, 2013 12:25 AM

Quote
Originally posted by: GM Tech
better get a new cycling switch-- yours is stuck shut- should go off at 21-22 psi and on at 42 psi.....continued running at low suction decreases oil flow and burns up compressors...

I have the same truck, factory charge is 2 lbs. when I weighed in 2 lbs it brought my suction up to around 35 psi. on high fan, sometimes i wonder if I have the wrong orifice. It will freeze you out, but at 35 psi. the suction drops at high rpm when the cabin cools down, then the compressor cycles, sometimes this charging is like splitting hairs. Heck I am a commercial and residential a/c tech, the suction don't fluctuate, and if it does, you have some serious issues.

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Don't take life seriously... Its not permanent.

WyrTwister on Sun June 02, 2013 2:32 PM

In a car , the engine RPM is constantly changing , while you are driving . This constantly changes the compressor RPM .

The air flow over the condenser changes from what the fans can provide , to much more than that when you are driving at highway cruising speeds .

With all the glass surrounding a car interior , the heat load varies a lot . And there is not a tremendous amount of thermal insulation separating the driver and passengers from the outside environment .

None of these situations apply to stationary HVAC . Or , at least not as much . Not at all , when considering the variable RPM and condenser air flow .

God bless
Wyr