So, having screwed up my wifes Volvo a/c, I've gotten it back to working again. But I ended up doing it via gauges and a Toyota manual.
I have the Volvo factory computerized repair manual, VIDA, for the repairs. It lists the pounds of R134 refrigerant the system is supposed to hold, 35 ounces as I recall. But it does not list anything for gauge pressures. Nothing. According to the VIDA, you're really supposed to just suck it completely dry, and then recharge it with the exact weight of refrigerant.
Is there really any reason for Volvo to not list any gauge readings for repairs? It just seems weird to me for them not to give anything for repairing a cars air conditioning other than the exact weight of the oem or perfect charge.
The high pressure side varies considerably based on ambient temps and humidity. You can find generic charts to give you ballpark figures, but there is no set pressures.
Most vehicles I work on run 120-200psi on the high side and about 30psi(give or take 5psi) on the low side with the compressor running.
I have always just started with an empty system and put back in the recommended quantity of refrigerant per the manual or the sticker on the car and all but ignored the pressures. Once I started doing it this way (many years ago) it made fixing my own AC almost fool proof.
I only look at pressures as a sort of safety. You don't want to be forcing to much refrigerant in to fast and over pressure the system during filling and blow a compressor or a seal.
From this thread:
https://grassrootsmotorsports.com/forum/grm/nolow-ac-cooling-at-idle/119488/page1/
WildScotsRacing wrote:
Check the charge with a full gauge set, use this rule-o'-thumb. Low side PSI should roughly 1/2 of ambient air temp F degrees number, high side (w/ compressor running) should be roughly 3X ambient air temp F degrees number.
In reply to foxtrapper:
Ya. I have done many many cars this way for friends and family and never had an issue. I have a simple low pressure side connector with a quality gauge on it that uses those 12oz cans (way cheaper than the ones you can now got with the hose and the gauge on them)
I know almost 0 about AC systems, but everything I've read indicates R-134a is much more dependent on the quantity. Just like dean1484 says.
Yah, I never really look at the pressures as a firm figure. Low side should get down to 30-40 and high side 150-250ish. Low side should never get below 30ish because the evaporator will ice over, but you do want to get right on down there. If they are both high then the condensor isn't cooling (bad fan? clogged airflow?) and if they are both low then the system is insufficiently charged, and if they are really close together then the compressor is crapping out.
One of those things that is more knack/feel than science. If you want science, you use a temp probe (not an IR gun, a probe) and measure temperatures at different points in the plumbing.
Doing it by weight makes it a bit hard with a custom system. You could always watch pressure as a safety and go by outlet temp right? I do know too cold can be bad and can freeze up the evaporator.
(I am eventually going to install a custom front mounted condenser with Sanden compressor in my late Corvair coupe so I will have to wing it a bit)
If I can't go by weight, I usually judge by a combo of vent temp, pressure and the temp of the lines in and out of the evap.
On an orifice tube system, the line coming out of the evap should be about the same or slightly colder than the line going in. If it's warmer, it's under-charged.
For vent temp, when you get the coldest temp, you're usually pretty close (sometimes a hair under-charged). However, make sure that it's producing condensation, as if you over-charge, you can end up with an almost as cold vent temp but much warmer evap (so it's not condensing water out of the air). This is more noticeable on a humid day, of course.
Generally speaking, it should be impossible for the low side to get too low. A basic orifice tube/cycling system will use either a low side pressure switch or an evaporator temp probe to shut off the compressor when the low side gets to about 30psi, which is around 32degF. (I don't have my R-134a temp/pressure chart handy, but in the region of temps comfy for humans, PSI is nearly the same as degrees Fahrenheit, just like R-12)
Systems with an expansion valve have an internal bulb that ensures the low side never goes below that point, and variable displacement compressor systems vary the compressor displacement based off of low side pressure.
If the pressure ever does get that low, then there's a fault somewhere... stuck low-side switch, stuck expansion valve, orifice tube plugged solid so the compressor is sucking it down to nothing, something's wrong.
Gauges will be unreliable unless you've vacuumed the system down first.
I can't count the number of times I get a car with pressures that should be OK... But blowing warm to "meh."
Its air/moisture in the system. It gets sucked in with low side leaks or through the compressor end seal in my experience. I can't explain it because even 30psi is more than atmospheric pressure, but that's what I see a LOT of.
Charge by weight is the way to go.
Fwiw most of the time in a 70-75F shop near every car will show 30-35psi low side, and 150-170 high side at its lowest reading while cycling with Max A/C and recirc.
Air in the system can only be from either improper servicing (recharging without first pulling down to hard vacuum) or air-contaminated A/C machines, which is technically also improper servicing. If a shop evacuates a system with air in it... guess what? Their machine is now air-contaminated.
Yes, technically and legally, you're not supposed to recharge vehicle with recovered refrigerant. You're supposed to send recovered refrigerant to recycling centers and only ever install "virgin" refrigerant. In practice, exactly nobody does this.
My s2000 AC has been acting up for a while and I'm not sure where to turn to now. It works great for a few minutes while the interior is warm and then it starts to cut in/out frequently. When it does cut on it cuts on pretty hard, enough to feel in the seat of your pants. There is sometimes a noise from around the compressor area when it cuts in, sometimes like a belt slipping noise or like a venting noise. It also cools much less while this happens. I did install a new condenser and drier but this happened before that too. It was vacuumed for thirtyish minutes before refilling. I don't have a scale to weigh in the refrigerant so I went by pressures. The first few minutes it runs the pressures seem to be right on with vent temps in the low to mid 40s. After that it almost seems to be bouncing back and forth between the low and high pressure switches, but I'm not totally sure. I'm thinking it might be some kind of clog in the high side or an expansion valve issue, again I'm not sure, I'm really hoping it's not a compressor issue. Any more expertly opinions?
flatlander937 wrote:
I can't count the number of times I get a car with pressures that should be OK... But blowing warm to "meh."
Its air/moisture in the system. It gets sucked in with low side leaks or through the compressor end seal in my experience. I can't explain it because even 30psi is more than atmospheric pressure, but that's what I see a LOT of.
That's also basically what hers was doing, and now is acting like it's got air in the system, and like you I cannot explain it.
Well, the cans are cheap, and I've a vacuum pump. Probably buy a replacement drier as well. By weight it is.
Knurled wrote:
Yes, technically and legally, you're not supposed to recharge vehicle with recovered refrigerant. You're supposed to send recovered refrigerant to recycling centers and only ever install "virgin" refrigerant. In practice, exactly nobody does this.
I was going the say the E36 M3s that redid my AC put my refrigerant back into the system once they had flushed it and put in a new dryer.
44Dwarf
UltraDork
6/29/16 8:29 a.m.
Find a "temp glide chart" and several thermometers to clip in to the vents and an infrared gun to check the condenser. then you can work the math to find approx. gauge readings. Yup sort of a pain in the butt so they give you LBS of charge.
Knurled wrote:
Air in the system can only be from either improper servicing (recharging without first pulling down to hard vacuum) or air-contaminated A/C machines, which is technically also improper servicing. If a shop evacuates a system with air in it... guess what? Their machine is now air-contaminated.
Yes, technically and legally, you're not supposed to recharge vehicle with recovered refrigerant. You're supposed to send recovered refrigerant to recycling centers and only ever install "virgin" refrigerant. In practice, exactly nobody does this.
I can't speak for others but our machine at work purges air after every time you recover stuff.
I've had a few cars where I was in disbelief, recovered and charged 2x to be sure. It gets air back in it somehow(weeks/months later).
One that I recall vividly was a Mazda3 that made a weird sucky/squealy noise when A/C was on. No it was not the clutch making noise. And no it was not mechanically inside. It was sucking in air from the center shaft seal. Sounded sort of like a tea kettle.
So, how does a home mechanic know what weight of refrigerant he has put in a system ? I'm assuming you DIYers have vacuum systems and scales at home?
What is a good way to cost effectively vacuum a system that has been opened up for an engine change?
On another note do I need to worry about replacing the green fluid I am assuming is oil that dripped out?
pappatho wrote:
What is a good way to cost effectively vacuum a system that has been opened up for an engine change?
On another note do I need to worry about replacing the green fluid I am assuming is oil that dripped out?
Yes, the lost oil needs to be replaced. IIRC, I paid a bit over $100 for a vacuum pump when I decided to get the gear to do my own charges.
