Questions on charging A/C with R134A

[rnln]

Hi guys,
I have read many threads on this, but still have couple questions to confirm, please input.

1- Pulled vacuum, high -30 PSI, low -28 PSI. OK?
2- Charge, only charge on low side with gas (up side up), engine running, A/C and fan on max. At a certain PSI, expansion valve (evaporator) will open up and transfer to high side. My question is at which PSI it will open? So that if my expansion valve is bad and never open, I know when to stop.

Thank you.

[Pazuzu]

You probably need one gauge to be adjusted, but if it's 28-30 that's OK.

First charge with a static charge, a/c compressor not running. You'll get 80, maybe 100 psi in based on the temperature. Then close the gauge valve, fire up the compressor, read low side and high side, open valve for a while, watch high side go up, close valve to get proper reading. When the valve is open, you're getting 25-50psi from the system but 80-100 psi from the can, so it will read wrong.

I kept filling, watching the high side creep up, every 10psi or so I'd close the valve, let the system settle for a minute, then get a low/high/temp reading.



Edit: Which doens't answer your question. My system instantly went to a high pressure differential after the compressor kicked on, with 100ish psi static charge. I think that with a bad expansion valve, you'd get low/high pressure that is very similar, not a factor of 10x difference.

[T77911S]

what he said

after you pull a vacuum you need to turn off the valves and then the pump and let it sit to see if it holds.


the expansion valve does not open based on pressure. it opens based on temp. the bulb that is clamped to the low side out of the evap monitors the temp of the line.
so it is open even when empty.

[GH85Carrera]

The vacuum pressure should read the same. The lower your vacuum pump can get it the better.

[rnln]

thanks guys

[rnln]

Originally Posted by T77911S

what he said

after you pull a vacuum you need to turn off the valves and then the pump and let it sit to see if it holds.

the expansion valve does not open based on pressure. it opens based on temp. the bulb that is clamped to the low side out of the evap monitors the temp of the line.
so it is open even when empty.

I have heard there are situations the expansion valve doesn't open, which makes the refrigerant doesn't go to high side. Is there a situation the expansion valve is always open, which makes refrigerant keep going to high side to the point high side keep going up very high and low side is always too low?

[T77911S]

if the TXV is stuck CLOSED, the low side is too low and hi side is too hi.
if there is a blockage the pressure will always be hi before the blockage and you will usually see the line freeze up after the blockage.

you kind of have it backwords.
the hi side is BEFORE the TXV. refridg travels from comp-cond-TXV-evap-dryer-comp.
the comp builds up pressure, the TXV then regulates how much refridgerant goes into the evap. because of this the OTHER side of the TXV is the low side.

the TXV can stay open if the bulb is not clamped to the low side line out of the evap or properly insulated.
as the refridgerant inside the bulb heats up it incereases pressure. this increase inpressure pushes down on the spring that keeps the valve closed thus opening it to let in more refridg.

[billybek]

Pressure element = Pressure evaporator + Spring Pressure. TX valve in balance
Element or bulb is an opening force
Evaporator pressure is a closing force
Spring pressure (superheat adjustment) is a closing force.

So with a high vacuum in the system the valve should be in the open position and your system should be equalized. Are you using one gauge on high and low to determine vacuum level?

Your vacuum level is in inches of mercury gauge.

You could do a standing vacuum test to see if it holds.
Pressure testing and using a good electronic leak detector and soap is a better way to find leaks.

Non condensables in the system from poor evacuation or errors while charging will drive the discharge pressure very high.

Over charge will drive up discharge pressure too.

A plugged expansion valve will give you low suction and high temperatures (for the suction side) returning to the compressor (high superheat).

A valve that fails in the full open position will flood the evaporator and bring liquid back to the compressor.

[rnln]

I use the vacuum rental from autozone. After vacuum for a few mins, my high side is -30 PSI and low side is roughly -28 PSI. I tried to read it with 2 diff sets of gauges and result are the same. Does this say anything about my expansion valve?

Also, similar to what T77911S is saying, after I Charged the system with R134A (I forgot and left the engine lid open when charging), the high is way too high and low side is way too low.

Originally Posted by billybek

Pressure element = Pressure evaporator + Spring Pressure. TX valve in balance
Element or bulb is an opening force
Evaporator pressure is a closing force
Spring pressure (superheat adjustment) is a closing force.

So with a high vacuum in the system the valve should be in the open position and your system should be equalized. Are you using one gauge on high and low to determine vacuum level?

Your vacuum level is in inches of mercury gauge.

You could do a standing vacuum test to see if it holds.
Pressure testing and using a good electronic leak detector and soap is a better way to find leaks.

Non condensables in the system from poor evacuation or errors while charging will drive the discharge pressure very high.

Over charge will drive up discharge pressure too.

A plugged expansion valve will give you low suction and high temperatures (for the suction side) returning to the compressor (high superheat).

A valve that fails in the full open position will flood the evaporator and bring liquid back to the compressor.

[T77911S]

the vac is ok. exact numbers are not that important. what you want is to pull the vac for several hours then it should hold with pump off and valves closed. (do the hold test before pulling a long vac). before i could acually measure the microns, i would pull a vac, then shut it all off and let it sit overnight.

what are the prssures.
did you replace the TXV. (dont remember if you said)
did you flush the lines and the coils

did you put fans on the condensors. i usually put a box fan on the deck lid and one is front.

with the inside fans on hi, monitor the inside temps, how much you have put in and pressures.

[rnln]

For vacuum, I pulled it for around an hour or so, then rest for around an hour, then pull again for several hours, then leave it for days. I came back and hooked gauges to check on it twice and found PSI stay around -28 on low side and -30 on high side.

What is TXV? Do you mean the expantion valve? No, I have not replaced that. I would if I can tell it is bad.

I did not flush the line and coils. What I did was open up all hoses and blow in there with compressor air.

I didn't put extra fan on condenser

I use tank, so I can't tell how much I put in. When low side went up, high side also went up. Low side stop raising at around 20-25 PSI. High side went up all the way high. When it was around 25/170 PSI (75 degrees day. Inside car temp was around 65 degrees before charge), I got 40 degrees at vent, which is good. But high side keep coming up and low side coming down after several mins, then temp at vent went up to 60-65 degrees.

I turned off the engine, and start it up after an hour or so, turned A/C on. It does the same thing again, which means refrigerant come from low to high. Car get cold when it's around 25-30/170. But it keep comming. After several mins, high side went up pass 200 PSI and low came down as low as 10 PSI. To this point, vent temp was back up to around 60-65

[T77911S]

TXV= expansion valve

you need air blowing across all condensors.

my brothers did that, 10-15 low, hi side was very hi.

was this an R12 ssytem before
how much have you put in

i would put more in.

[rnln]

For the record, I replaced expansion valve, then vacuumed. After less than a min, they both went straight to -30/-30.

Originally Posted by rnln

I use the vacuum rental from autozone. After vacuum for a few mins, my high side is -30 PSI and low side is roughly -28 PSI. I tried to read it with 2 diff sets of gauges and result are the same. Does this say anything about my expansion valve?

Also, similar to what T77911S is saying, after I Charged the system with R134A (I forgot and left the engine lid open when charging), the high is way too high and low side is way too low.

[billybek]

Cool. (See what I did there?)

How is the system working with the valve replacement?
Did you replace the receiver/drier at the same time or was that new from the beginning of your repairs?

[rnln]

I am looking for the prestite tape while waiting couple days and check vacuum again, then replace the dryer again. Then test. Then charge. I want to play safe this time

[kuehl]

Originally Posted by rnln

1- Pulled vacuum, high -30 PSI, low -28 PSI. OK?

In a 'perfect world', if your pump pulled down as best it could, and numbers you are reporting are after that with the pumped off and the service set gauges closed, the difference of -2 inches of mercury (inHg on your gauge, not psi) between the two gauges could be either accuracy of the gauges or how your are viewing (seeing) the needle against its background (the incremental lines printed on the face of the gauge). Ideally, you want the low to be closer to -30 inHg. The reason for wanting a lower reading is to insure there are not residual ambient gases (air) or moisture in the system. Water or moisture 'boils off' at a particular vacuum level, at a particular temperature. Refer to this chart:
Boiling Point of Water at Various Vacuum Levels
On a 911 you can pull down a vacuum rather quickly however the time to do it right is usually over an hour and you will need to purge the system a few times with refrigerant gas (you could use nitrogen but for a DIY you'd need to make adapters).
If you have residual gases mixing with the refrigerant your system pressures will be higher than normal, and residual moisture in the system can freeze internally in the TEV.... causing the TEV to stick closed, stick partially open, stick fully open or block its port.

Originally Posted by rnln

2- TEV .....My question is at which PSI it will open? Thank you.

TEV's (expansion valves) never fully close. 911 TEV's seldom fail and when they show signs of failure it is either debris in the system or moisture in the system.

R134a Basic charge: after you pull your vacuum inject approximately 80% of the original R12 weight the vehicle used to start. For example, if the vehicle previously used 47 oz of R12 you will need to put in about 37 oz of R134a and adjust the final charge amount (+/- x oz) based on ambient outside air temperature and high side gauge reading. For DIY, you can charge by 'gas method', meaning refrigerant can upright, charging through the low side, keeping the high side valve closed. Start your charging process with the engine off, allow the system to pull in as much as it will; usually this will be 1 to 2 small cans (cans can be either 12 oz or 16 oz so keep track of how much you are using). After the system pulls in as much as it can then you start the engine keeping the high side service set valve closed. At idle the system will take a while to pull in refrigerant, if you raise rpm's it will pull it in quicker, if you warm the refrigerant can (hot water) it will move things along sooner. Be cautious when you put on a new can of refrigerant to the service set, you don't want to introduce air into the system. This is just a basic overview of the charging procedure, I'd suggest you get yourself a good Auto AC book and read it over.

AC Help

[Pazuzu]

Originally Posted by rnln

I am looking for the prestite tape while waiting couple days and check vacuum again, then replace the dryer again. Then test. Then charge. I want to play safe this time

I got a roll (25 ft, maybe $10?) at the local A/C supply house, we have thousands of them here in Southern Texas I picked one on the way to work.

They called it cork tape, didn't know what "prestitite tape" was.

[kuehl]

Originally Posted by rnln

I am looking for the prestite tape

If you can't find the black prestite insulation you can use anything that
provides some form of insulation to outside air, that will not harden, and that is removable. Example.... Plumber's putty

You only have to cover the TEV's sensor tube (the copper pigtail mounted
under a clip on the top of the evaporator outlet pipe).

911 Evap

[bob deluke]

Make sure sensing bulb is clean, as well as evap line you are attaching to. Bulb should be tight against evap line for good sensing. Attach bulb with strap, wrap with a layer of prestite tape. You don't want ambient air affecting operation of txv. If evap line is in horizontal plane, attach bulb at about 2o'clock position so you get a vapor/liquid mix. If attached top of line, you will flood evap. with liquid. If evap line is vertical, doesn't matter where bulb is placed. Just make sure bulb is clean, tight to line and insulated.

[kuehl]

The sensing bulb should be mounted on the top, at 12 o'clock; so you don't flood the evap core; and that seldom happens on the 912/911/930.