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Best way to determine proper charge on a TXV system

pettaw on Mon August 30, 2004 1:25 PM User is offline

When my system was converted the guy installed a very slightly smaller, but parallel flow condensor, but also quite a bit larger receiver/drier. So I reckon the factory weight is a bit misleading now.

I've heard about feeling the evaporator lines until their the same temperature going into and out of the evap then adding about 10 ounces more, but that works on CCOT systems only.

The 2.2x ambient charge rule is great but so dependent on how powerful my fan is. So is there any equivalent method of measuring the charge. There isn't even a sight glass either!

Thanks and sorry for all the dumb questions.

DetroitAC on Mon August 30, 2004 2:11 PM User is offline

That's not a dumb question at all pettaw! I always get a chuckle out of people that charge by high side pressure.

Here's how it's done on a production OEM vehicle in a dynamometer climatic wind tunnel. You'll have to take whatever shortcuts, since I doubt you have a dyno wind tunnel.

Start with an evacuated, dry, leak free, evrything working, oil in it, ROOM TEMPERATURE system.

Guess a starting charge. Guess way low, like maybe 2/3 of your expected charge level. Use either a heated bottle like a dial-a-charge, or if in the field set the bottle out in the sun to get the refrigerant warmer than the A/C components. Use a scale to measure how much refrigerant mass goes in, and put the guessed initial charge in the system.

Get the wind tunnel up to conditions (Varies by oem, but generally a very hot high load condition) (Chrysler 110F, 19%RH)(Ford 110F, 50%RH)(GM can't remember, but close to the others)

Run the vehicle to get everything up to normal operating temperatures.

Run the vehicle in recirculation mode, A/C on, full blower, all windows open, driving at (varies with oem) 25mph to 30mph, for 15 minutes to stabilize conditions and operation, record all temperatures and pressures in the refrigeration loop, and air handling systems.

If any "bogey values" are exceeded, immediatelystop the test and add 25g more refrigerant (or just add it quickly while running). Typical bogey values are compressor discharge temp>250F, Low side pressure
Add 25g every 15 minutes, recording total charge amount and all refrigeration loop and air handling parameters every ten seconds.

Stop the test when the condenser out subcool reaches the "target" value (varies by type of system and OEM, but 20F subcool is typical)

End of test

For you pettaw, the challenge is to keep the conditions constant enough so that you can compare each charge level. I suggest you stop charging right after the end of the "flat spot" or "charge window", let me explain.

As you are adding charge you will intially be undercharged, the low side pressure will be too low, the high side pressure will be too low, the vent temps will be too high, compressor will be running too hot.

Then things will get better, and then you will see a "window" during which additional charge really doesn't seem to change anything. This window is the process of filling up the receiver. Changing the level of refrigerant in the receiver has only a barely perceptible change on refrigeration loop operation.

Once the receiver is full, additional refrigerant will be stored in the bottom passes of the condenser. Subcooling will increase very quickly and high side pressure will increase very quickly (when additional refrigerant increments are added). This is the point at which the system is operating the best, the most efficient and is fully charged.

Does that make sense?

There are 10 kinds of people in the world, those who understand binary and those who do not.

pettaw on Mon August 30, 2004 2:22 PM User is offline

I've got a Robinair recovery/recycling machine. So I can charge an exact weight, if I knew what it was!! I've got the hang of dialling in 50g increments and just tweaking the low side valve to dribble the gas in without slugging the compressor so I shall experiment.

pettaw on Sun September 12, 2004 7:25 PM User is offline

OK, so I did it, I guessed way low, and added about 500g to start with, and I got bubbles in the sight glass. I added 50g at a time and at about 600g, the sight glass cleared. Then exactly as you described, I continued to add increments of 50g and not much changed, the pressures went up very slowly, and then at 900g the high side started to climb rather faster, so I stopped and recovered about 50g. Leaving me with 850g.

I used the late great Mitch's method of determining superheat: jump evap freeze thermostat to stop the comp cutting out, run car at certain revs and use blower to just freeze and unfreeze the evap slooowly, and discovered that I was only running a superheat of 2C, which would explain why the suction line was freezing before the evaporator outlet.

I screwed in until the superheat was about 6-7C. I then adjusted the evap thermostat so that on high fan it cycled just at the point of the outside tubing freezing, (it doesn't freeze at all lower blower speeds, but I guess on high fan its warmer where the probe is than where I can see the evap on the outside lines)

My only slight worry is the lower low side readings, it wasn't that hot this weekend whilst I was fiddling about (65-68F) so the car was cycling even on maximum blower, but at 1500 RPM the high side was about 160 and the low side got down to 15 PSI right before it cycled.

But it cools great, and that's the main thing, much better than our much newer car, whose A/C system could be best described as weak, even though its a TXV system, the manufacturers just seem to have set the cycling temperature very high.

I would never have got it this way without the help of this forum so thanks.

NickD on Mon September 13, 2004 7:55 AM User is offline

Charging by the sight glass is quite accurate provided you are running your engine at 2,000 rpm and the ambient temperature is at least 85*F, you already learned that when your glass cleared you no longer experienced a decrease in vent temperature as at this point you are feeding your expansion valve with pure liquid. When the ambient temperature gets cooler, even at 2,000 rpm, AC on, blower at max, you may see some bubbles in your glass. Note that Detroit AC is charging at 110-115*F, you may never see those temperatures in the UK.

I like to stop charging when the sight glass clears, for this reason, the extra charge does not add to the cooling capacity, but only acts as a reserve, with a sight glass, it's easier to check your charge than to check your engine oil and if I see bubbles under the same conditions that I charged it at, this tells me immediately that I have a leak, if I add 3-4 ounces for a reserve, that must leak out first before the bubbles will appear, and especially with R-12, that is 3-4 ounces gone forever. This extra liquid is just stored in the receiver and does not add to the cooling of the vehicle. What would be nice is if they had a second sight glass on the side of the receiver so you could see the level, but under certain humidity conditions, you will see condensation above the liquid line on the receiver that shows a line as to how high the liquid refrigerant level is, but around here, I cannot depend on that.

If you added another 250 grams at 68*F after your sight glass cleared, you may be overcharged, much depends on the capacity of the receiver, this would compound and may kick out the dual function switch if the ambient get above say 90*F.

I have noticed that with CCOT systems in particular that the acceptable vent and pressures listed in the service manuals is a very wide range of values, I wonder if this is due to production variations in the system. But I prefer to charge by pressures noting the ambient and RH conditions and charge to the center of these values, seems to work well for me, but have to wait for a warm day before doing so. Would be nice to have a heated wind tunnel for charging.

Ha, when the weather is ideal for charging, its too hot to go outside to do it, rather stay in the AC cooled house.

toddk63 on Tue September 14, 2004 3:05 PM User is offline

This is great info. The moderators should make it sticky.

Todd K.

TRB on Tue September 14, 2004 3:21 PM User is offlineView users profile

I sent an email asking if it would be all right to place this in the FAQ forum. No response yet but maybe in a day or so.


When considering your next auto A/C purchase, please consider the site that supports you:

pettaw on Thu September 16, 2004 2:09 PM User is offline

did you send it to me? If so I didn't receive it but the answer is yes, no probs.

TRB on Thu September 16, 2004 2:13 PM User is offlineView users profile

Email was sent to DetroitAC. He also approved me adding his comments to the FAQ forum.


When considering your next auto A/C purchase, please consider the site that supports you:

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