A/C Question . . .

[Ronnie's.930]

Yes, I LOVE ice cold A/C !!!!

My system has been converted to r134 and has an additional, "in the left rear fender" condensor installed . . . this system works great except I have a leak, or leaks, that I need to find/repair . . . I will be adding refrigerant myself in order to chase the leaks and also to keep the system working here in the sick, Texas summer heat.

My question: what are typical low and high side pressures for my system (say if the outside air temp is 95)?

Thanks!

Ron

[JFairman]

This is the best R134 temprature~high side chart I have seen.
It works great for me.

When it's 95 degrees out and the engine is at idle speed the high side pressure should be about 258psi if everything is working correctly.

This is if all the moisture has been evacuated out which takes at least 3 hours with a good vacuum pump. It takes at least that long because of the 40+ feet of hose and 3 condensors you have.
This is at idle speed and the high side pressure will rise while the low side pressure drops as you raise engine rpms.

Low side pressure will be between 25 and 35psi and this chart only has high side pressures.
Alot of that depends on your expansion valve and how healthy your compressor is.

That is with the the rear lid down so air is being drawn in to cool the rear condensor and both the front and left rear fender well AC condensor fans are running.
If everything is working right with your 3 condensors and you have a good reciever dryer, expansion valve, and evaporator then you should get low 40's out of the dash vents and mid to low 30's from the bowtie vent under the dash after the system has been running and the interior of the car has cooled down into the low 70's... while it is 95 outside.

You won't get those really low vent temps until the system has run a few minutes or so to cool everything down... If the car has sat in the sun in that kind of heat it could take 5 minutes to chill it off.

Here is a copy and paste from Griffiths with the high side pressure chart:

"To find a better P&T working relationship with R134a there is a rather good "rule of thumb" stated by the manufacturers of R134a and the auto industry in general and that is "with R134a pressures tend to be 10% greater". However this rule of thumb is specific to an ambient temperature range. What I mean by specific range is that starting near 80F ambient R134a tends to run at a higher pressure than R12, and that difference is is more like an upward curve moving away from the R12 diagonal line. Once you start moving toward 95F ambients the curve shoots upward and the higher you go the straighter it climbs. So "we" have new factors or multipliers which we can safely apply to R134a or given "normal ranges of operation", say anywhere from 85F to 110F (desert people don't use refrigeration systems anyway, they use swap coolers). So this is how you estimate to a better finite target with R134a:
you recall the relationship with R12 was "2.4" (2.4 times the ambient should be the mean target for the high side pressure). The ranges for R134a "we" work with look something like this:

80-85F = ambient times 2.4 X 1.05 or the + 5% rule
86-90F = (ambient times 2.4) X 1.1 or the +10% rule
91-95F = ambient times 2.4 X 1.13 or the +13% rule
96-100F = ambient times 2.4 X 1.15 or the +15% rule
101-105 = ambient times 2.4 X 1.17 or the +17% rule
106-110 = ambient times 2.4 X 1.20 or the + 20% rule"


If you can't evacuate the system for at least 3 hours to boil out any moisture that may be 20+ feet away from the compressor fittings in a hose or front condensor, than charge it and run it for a day or so it all gets mixed up and then let a bunch of the regrigerant out of the system without letting any air in and recharge it again.
As the R134 blows out it will take most of the remaining moisture with it and then you recharge it again.
Always unscrew the hose fittings a little on the manifold gauges to purge air out of the hoses before charging.
This is called sweeping the sytem.

hope that helps..

[Ronnie's.930]

WOW - thank you for the excellent a/c tutorial - this is VERY helpful! Thank you so much for taking the time to write it up!

R