A/C Pressure

lane620 · June 8, 2006

How much pressure should be in a coolant system.I have a 1987 suburban thats running 60 lbs of pressure and cooling at 56 deg. I have a 1995 chevy 2500 light duty thats about 40 lbs and is cooling at 45 deg. does the suburban have too much pressure causing it not to cool propely? I charged them both yesterday.

bobcat5544 · June 8, 2006

How much pressure should be in a coolant system.I have a 1987 suburban thats running 60 lbs of pressure and cooling at 56 deg. I have a 1995 chevy 2500 light duty thats about 40 lbs and is cooling at 45 deg. does the suburban have too much pressure causing it not to cool propely? I charged them both yesterday.

<{POST_SNAPBACK}>

First of all had the 87 suburban been retro fitted to r134a

snoman · June 8, 2006

How much pressure should be in a coolant system.I have a 1987 suburban thats running 60 lbs of pressure and cooling at 56 deg. I have a 1995 chevy 2500 light duty thats about 40 lbs and is cooling at 45 deg. does the suburban have too much pressure causing it not to cool propely? I charged them both yesterday.

<{POST_SNAPBACK}>

This is a tuff question to asnswer correctly because pressure is related to tempature and humidity and type of refrigerant. YOur 87 may just have a tired comprssor that is leaking some and carring a bit high of a pressure on low side. Try checking it at about 1200 to 1500 RPM and I will bet it drops some. Also, it will be lower on a low blower speed than on a high blower speed. Not like checking engine il pressure as there is a lot of varibles here.

bobcat5544 · June 8, 2006

here are the specs on r134a systems

ken1mod · June 8, 2006

Gentlemen,

In general, while pressure is important, very important, cooling is what we are after. In the fixed orifice system used in our vehicles, the charge is critical.

The more ref added to a system after the ideal charge, the warmer the air will get.

What we are looking for is the minimum charge that fills the evaporator with refrigerant.

This is how I charge a system. From a vacuum, add refrigerant. the first thing you will notice is that the compressor will start because the pressure switch will activate. I rarely use gauges, they look impressive but cold is what we want. Gauges can definitely help diagnose a system but we are assuming a properly operating system here.

Set fan on high, and the outside temp should be summery, maybe 80 or 90 degrees. Continue adding refrigerant slowly until the tube coming from the evaporator into the accumulator gets cold. Use your hands to judge this. let the system stabilize a few minutes, the compressor will be cycling. Now add a bit more, until the accumulator gets cold. wea re talking maybe a quarter can at most. pressures will rise slightly as the accumulator begins to fill. Ideally the system operates with a full evaporator and a little refrigerant in the accumulator.

What we are looking for is the minimum charge that gets the accumulator cold. the more you add at this point the worse the system will run. Cycling is how the system works. if the compressor does not cycle, it is overcharged and maybe being damaged through liquid floodback to compressor.

Ken

dewfpo · June 8, 2006

I perform a similar but slightly different procedure. If I think the vehicle is a little low, I turn the A/C to max, fan to high, (hot temps outside, preferrably above 80F), I put a thermometer in the center dash vent. If the temps are about 54F or higher I connect an R134 can to the low side and see if the temp starts to drop. If it doesn't, I disconnect and don't add any more, If the temps get cooler I leave the can open until the temps out of the center vent no longer go down after 2-3 minutes.

It's not the best way to charge (a complete vac./test/recharge is the best) but it seems to work well for me.

Yes, these R134a systems with the fixed orifice valve are very sensitive to too much pressure. That's why they seem to work their best just before they stop cooling well (as they cross thru that 'sweet spot' of pressure.)

DEWFPO