Automotive Air Conditioning Information Forum (Archives)

kingoffixin on Mon June 27, 2011 9:49 PM

Year: 1999
Make: Chevy
Model: Silverado
Engine Size: 4.3L
Refrigerant Type: R-134a

My AC has been acting up lately, runs for a while super cold and then stops cooling. I'm thinking maybe the charge is low and it's kicking out on low pressure. The problem is I can't find the port on the low pressure line to charge it through. Weird...any suggestions?

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Gary Nelson

JJM on Mon June 27, 2011 10:13 PM

Both the high and low side ports would need to be connected to the gauges, then you would open the low side valve and charge through the center port.

Joe

mk378 on Mon June 27, 2011 10:46 PM

As it gets cold once, you must have enough refrigerant. There aren't gremlins under the hood taking it out and putting it back in when you're not looking.

When it stops cooling, pull over and open the hood to see if the compressor is still engaged. If it is not, a couple of things to consider are the clutch gap being too wide, or a flakey cycling switch. If the compressor does stay engaged, check if the accumulator and lines are covered with ice. That would be overcooling because the switch is stuck closed. If the lines are not cold but the compressor is engaged you will need pressure readings.

Edited: Mon June 27, 2011 at 10:52 PM by mk378

GM Tech on Tue June 28, 2011 7:19 AM

on top of compressor last I looked.

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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......

kingoffixin on Tue June 28, 2011 10:36 PM

Given the age of the truck I think the charge could be low. On the commercial R22 systems I've worked on it seems like there is a point of low charge where things get super cold and then kick out the low pressure switch. I assume my truck could be doing the same thing?? Any ways...where is the suction line port that I would try to charge through? I have two ports on the high pressure side put I can't find any on the low pressure side.

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Gary Nelson

bromodragonfly on Wed June 29, 2011 1:12 AM

Since you've worked on commercial R22 systems, you've probably observed that even though you have a coil and suction lines that are icing up, it doesn't mean that it's effectively cooling the space. With larger commercial rack systems or even larger tonnage rooftop units, you still have a recommended charge, but have much more leeway than a car system due to larger liquid receivers, and headmasters or other capacity controls that are designed to hold back liquid and maintain a steady head, in low ambient conditions.

With cars, a low charge will give you lower operating pressures right off the bat. In short, lower discharge pressure = lower condensing temperature = less subcooling = less ability for latent heat absorption = less net refrigeration in your evap. The refrigerant boils instantly at the condenser inlet and only becomes superheated as it moves through the rest of the evap coil. It is still cold, most likely freezing temps, but it does not absorb as much heat as it would if it were consistently boiling in the evap. Keep in mind in a regular AC evap, there's about 20% flash gas entering. Combine that with lower suction pressure = lower evaporating temperature = higher superheat = TXV wide open. With high return air temp and low evap temp, humidity condenses rapidly on the coil, it ices up. Eventually the ice impedes the airflow = zero load = superheat = TXV closes = compressor starves and trips on low pressure.

With R22 units, it is much easier to ice up your components, since you're looking at much lower temperatures in relation to pressures, when compared to R12 or R134A... something like a 40psi suction will land you close to 16F evap. Also keep in mind the R22 systems you've observed may use a liquid line solenoid that de-energizes on thermostat break, which is designed to pump down the system until the low pressure switch cuts out.

Check the compressor for the suction port, or just follow the line from the evap back to the compressor. There's gotta be one. Good luck


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There is no knowledge that is not power