So you want to recharge the AC on your car?

Reposted from my forum post on rennlist.

I am going to start with re-charging an otherwise working A/C system.
– It has started to short cycle, meaning you can hear the A/C compressor under the hood clicking on for 3 seconds, and then off for 3 or 4 seconds repeatedly.
– It is not cooling well at all. I have to leave the A/C on full blast for a good 5 minutes on an 80 degree day to get any real cooling in the cabin.
– It is NOT my Porsche, but the process and theory are exactly the same. Repair and certain diagnostics are a different matter, as this has a capilary tube and accumulator where our cars have an expansion valve and receiver/dryer.
– It is R134a. If you re still on R-12, things aren’t much different other than the refrigerant costing 3x as much and you need an EPA cert ($20 online) to purchase it.
It is 83 degrees F outside, with 70% relative humidity.

I am going to start with re-charging an otherwise working A/C system.

– It has started to short cycle, meaning you can hear the A/C compressor under the hood clicking on for 3 seconds, and then off for 3 or 4 seconds repeatedly.

– It is not cooling well at all. I have to leave the A/C on full blast for a good 5 minutes on an 80 degree day to get any real cooling in the cabin.

– It is NOT my 944, but the process and theory are exactly the same. Repair and certain diagnostics are a different matter, as this has a capillary tube and accumulator where our cars have an expansion valve and receiver/dryer.

– It is R134a. If you re still on R-12, things aren’t much different other than the refrigerant costing 3x as much and you need an EPA cert ($20 online) to take the test.

It is 83 degrees F outside, with 70% relative humidity.

The Equipment








#1 – A manifold gauge set and hoses. Let’s not get fancy here – we’ll just say that your manifold gauges MUST say they are made for the kind of refrigerant your car uses (you can get away with others if you know what you are doing – we’re going to cover basics here only). This is an R-134a/R404a/R507 gauge set. It does NOT have R134a hoses on it. If it did, they would be the same colors, but have black stripes down them.

#2 – Hose adapter set. If you have R134a hoses, you do not need this. If you do, the set will come with 3 pieces, one for each hose. All 3 are different (we’ll get to that).

#3 – Refrigerant valve for the type of refrigerant you are using (all refrigerants have different thread types to keep things from getting mixed up)

#4 – Refrigerant!


First you need to determine whether you need adapters or not. R134a fittings look like this:

They SHOULD have a plastic cover on them that screws on like on a tire valve stem. The threads are actually INSIDE the R134a fitting, and are not used for service, only to retain the cap. The outside of the fitting is a snap lock. The hose end that fits on it looks like this:

There’s only a few more things you need to know to hook up. First of all: MAKE SURE THE VALVES ON YOUR MANIFOLD GAUGE ARE CLOSED

The blue hose should be on the left side of your gauge set. It is attached to the lower pressure “compound” gauge which reads both pressure in PSI and vacuum. The red hose should be attached to the right side of your gauge set. It reads only PSI, and goes much higher than the low side gauge. The yellow hose is for charging and vacuuming the system, and will not be used yet.

You need to find the high side service port of your car. This is easy – it’s right in front of your face when you open the hood.  (edit for the repost: this is the service port location on an 85/2 and ’86 model Porsche 944)

Optimally, your car is off, and has been sitting long enough to be completely cooled off. Remove the caps, and attach the red line to the high port, and the blue line to the low port. For R134a, this is just a matter of pushing them on until they snap, just like an air tool fitting. If you still have R12, you will be screwing them on. Make sure they are hand tight, but don’t overdo it.

With R134a, you CAN’T screw this up. The high side only fits on the high side, and the low side only fits on the low side. This is NOT the case with R12, so be sure. It is also possible to put R134a adapters on the wrong hoses, so if you are using them take care that the high adapter goes on the red hose, and the low adapter goes on the blue hose.



Go check the temperature. As previously mentioned, it’s 83 degrees F today. So that’s what we’ll be using.

Both gauges should be showing some kind of pressure if your system was doing any cooling or cycling at all. If you see nothing, you are totally empty. There is no need to open either valve on the manifold gauge. They are not in line between the high pressure host and gauge nor the low pressure hose and gauge.

Here’s what mine looked like:

That’s pretty ugly, but at least both the high and low sides show about the same pressure. If they’re not close to each other, just stop. You have another issue and won’t be re-charging today.

In my case they are both showing about 80 PSI, and this explains why I’m not cooling much and why the compressor cycles on and off. Here’s why: at 83 degrees, liquid R134a has a static pressure of about 88 PSI. If I have less than that for a static pressure, it means that there is NO LIQUID REFRIGERANT in the system. Just enough to be in there as a gas, which is NOT enough to do its job.

How do I know R134a should be at about 88 PSI if its 83 degrees out? I could look it up on a chart, or I could just look here:

Yeah…THAT’s why there’s all that stuff on the gauge. The best part is you can use the temperatures for even more stuff than just that.

Now, some other possibilities that could have happened here (most of which would make me angry):

– If the static pressure was 88 PSI: All that tells me is that there is SOME amount of liquid refrigerant in the system. Not how much. A 3000 gallon tanker truck full of R134a will read 88 PSI at 83 degrees, as will a 14 ounce can that you’ve taken all but 1/2 an ounce or so out of. As long as there is still LIQUID refrigerant, the PSI will have a linear relationship with its temperature.

– If the low side pressure was significantly higher than the high side: I’d be angry, and not making this post. There are plenty of things that can do that. On the truck I’m charging, it would likely be a clogged capillary tube. For it to get clogged, there obviously must be something in there to clog it. There’s the fun part. Figuring out what ELSE is broken besides needing to replace the capillary tube.

– The static pressure was the same on both sides, but significantly HIGHER than 88 PSI: Again, mad. This means your refrigerant is contaminated with something else, generically referred to as “uncompressible gas”. This likely means your in for a full system evacuation and a receiver dryer replacement at a minimum.



OK, here’s the fun part. Make sure you hoses, tools, body parts, cans of refrigerant, etc are all out of the way of anything that will be moving. Start the car. Listen for bad noises (like hissing). None? OK…good. Roll up the windows, close the sunroof, put the A/C on high and the blower on high. Close the doors.

Go look at your gauges. In my case, they are swinging wildly as the compressor kicks on and off – but it all makes sense when you look at it.

– When the compressor kicks on, the low side starts dropping (the compressor is drawing refrigerant into it, compressing it and pushing it out the other side), and the high side goes up.

As you can see, our 80 PSI static pressure is now 23 PSI on the low side and over 100 on the high side. This drop on the low side and increase on the high side continues until the low pressure cut off switch decides its had enough. The the compressor turns off, and the gauges swing back in the other direction. Then it turns back on, and this repeats and repeats every 3 to 4 seconds.

Why? Because without enough refrigerant in the system, it doesn’t make it back to the compressor fast enough to keep it fed full of refrigerant. And to keep the compressor for burning itself up do to lack of lubrication (there is oil mixed with the refrigerant) and cooling (the refrigerant carries heat away from the compressor), the low pressure switch turns it off to protect it.


So this system is a good candidate for charging, and we’re going to do that. I’m not going to use a pure R134a, as it just wasn’t convenient to pick up. I grabbed the NAPA brand stuff that includes oil, which is fine by me and pretty convenient for a quickie recharge. It just changes the process a bit. So we’ll go through what needs to be done for this, and discuss the difference later.

– First thing first. Shake the hell out of the can. You need to mix the refrigerant and oil.

– Then, get your refrigerant valve. When it is screwed all the way in, it will be closed, BUT it has a pin sticking down that is used to break the seal on the can, so you need to OPEN the valve almost all of the way. You do NOT want this pin getting anywhere close to that can seal right now.

– Connect your refrigerant valve to the service line (yellow hose) of your manifold gauge set. If you needed hose adapters for R134a, you will have one that goes on the service line.

– Screw the can of refrigerant onto the valve. At this point, the valve you are screwing the can on should be open, and both valves on your manifold gauges should still be closed.

– Once everything is all set, CLOSE the valve you put on the can. All the way. You will feel some resistance, and you will hear it hiss. Once it’s been closed all the way, the seal is all the way punctured, and you can re-open that valve all the way.

At this point, your service line is charged from the can, your high and low lines are charged from your system, and the only thing between the two are the valves on your manifold gauge.


So with our refrigerant so low (no liquid refrigerant in the system) and the types of mix I’m using (oil and R134a), we need to be slow and careful to start. The car should still be running, and you should still have the AC on high, fan on high, and the car closed. Why? Just do it. Stop asking so many questions.

Now SLOWLY open the low side manifold gauge valve. You’ll see the pressure shoot up to 80 something, the compressor will kick on, it will get dragged back down, it will happen again, etc. Let it go SLOWLY (valve barely open) for 30 seconds or so. Now close the valve.

What do you hear? Probably still a compressor that is short cycling, but it’s cycling longer now. Open up the low side valve a little bit again. Let in some more. Slowly. I like to stop every 20 or 30 seconds and just let it go for 30 seconds or so at this point in the process. If you don’t, you might freeze your evaporator.

Also, while I’m doing this, I’m rocking the refrigerant can between 12 o’clock and 3 o’clock, favoring the 12 o’clock position. When I tun off the valve, I’m giving it a couple of good shakes. I am NEVER putting the can in a position where liquid refrigerant is going into the line consistently. You can’t compress liquids very much, and your compressor will hate you for making it try to do that. It’s called “slugging” and it’s bad, mmmmkay? (there will be exceptions to this later, of course)

Keep on going. Pretty soon – magic – the compressor will turn on and stay on. You’re getting there. Keep on going slowly and watch your low side pressure gauge. It’s no longer spiking up to 80 diving back down….it should be steady in the high 20s or so (depending on where you low pressure cut off switch is set).

We’re going to keep on going slowly, shaking the can (you’ll be feeling it get really cold by now), watching the gauges. You might even need more than one can. If you do, just close the manifold gauge, close the refrigerant valve, remove the refrigerant can and valve from the service hose, CAREFULLY open the refrigerant valve to release the residual pressure, and follow the original procedure for attaching a can of refrigerant.

You want to be charging at about 30 PSI on the low side at this point. If you get over that, slow down. I mentioned the relative humidity before, because here’s where it matters – I’m charging slowly because the humidity is so high. Go faster than this, and you’ll start getting your evaporator down to 32 degrees F. When its humid, it’s going to freeze pretty easily. This is also why we closed up the doors and windows – we want to let the condensate drain do it job and get as much water out of the air inside the car as possible to avoid freezing up the evaporator. If either of these things happens, it’s not the ned of the word, but you’re going to have to stop and wait for things to thaw before your proceed.

You’re going to keep on going until you see your low pressure gauge right around 35 PSI, and the high side right around 235. For R12, you’re looking at about the same on the low side, and about 220 on the high side.

These aren’t exact numbers, but they will get you close. It’s beyond the scope of what I can reasonably explain here how you arrive at these and how they change, but, suffice it to say, if its between 70 and 90 degrees outside, this is about right on.

– If your low side pressure goes above 40 (once you have enough refrigerant in the system to stop cycling) STOP

– If your high side goes about 250 STOP

– If compressor doesn’t stop cycling after putting in about a half a can STOP


Refrigerants are different. The come packaged pure, or with different things in them. Read the container. It will tell you how to use it, but here are some basics you’ll find.

Pure R134a or R12: No need for shaking. No need for tilting. Can upright at all times. It gets charged as a gas.

R134a or R12 with oil/sealers/etc: I don’t like sealers. It’s like putting the gunk in your cooling system when your radiator leaks. It’s a quick fix, but it’s not a good idea. Refrigerant with oil pre-mixed I’m OK with. Most tell you to shake the can frequently, and rock is back and forth. This allows it to be charged primarily as a gas, but with little bits of liquid (to make sure the oil and sealer gets in there). Go slow with these. No need to give your compressor a big gulp all at once.

Non-azeotropic blends, mostly R-12 replacements: Please just don’t use these. Yes, I’m talking about Freeze-12 and the like. Freeze 12 has absolutely no R-12 in it at all. “Non-azeotropic” basically means that its a blend of different gases that all have different boiling points. They somewhat mimic what they are supposed to be replacing, in this case R-12, but they have things in them like propane. Yes, propane. They are charged as a liquid, with the can upside down. If you charge them as a gas the ratios will be off. So, you have to be VERY CAREFUL not to slug your compressor while you do this. And the biggest problem is that they are a one-shot deal. You can NOT top them off like you can with real R134a or R-12. Because they are a mix of gases with different boiling points, they leak out of the system at different rates. If you tried to top the system off after using one of these blends, your gas ratios would be all off. You need to vac the system and start all over again. So, please….just don’t do this. It’s a waste of time, a waste of resources, and you won’t be happy with the results.


9 responses to “So you want to recharge the AC on your car?”

  1. Bob Maio Avatar
    Bob Maio

    This is the best writeup I have seen on the subject. Better even than the service manual I have for my 99 ram. Better even than the haynes manual I bought. Thanks for taking the time to pull all the information together. One question: is the procedure the same if it is an acumulator versus a dryer system?

    1. daryl Avatar

      Sure is. The only reason I mention a receiver/dryer is because it was originally a post on a Porsche forum for 944s (which have a receiver/dryer). Most of the pictures of the procedure are on my ’97 F150, which has an accumulator.

  2. brian Avatar

    thanks for your great instruction i can now do by myself:)

  3. Joel Avatar

    Regarding A/C fitting locations on the 944. Up to 85/1, they are both on the compressor. The locations shown in your photo are applicable only to 85/2 and 86. For 87, Porsche made the HIGH side fitting up on the strut tower, and the low side on the compressor. FSM and parts diagrams all show the 85/2 and 86 setup.

    1. daryl Avatar

      Thanks, I wasn’t thinking about that when I wrote the original entry. I’ve updated it to reflect the correct model years in the picture.

      One other thing to note for those looking for earlier or later 944 charging – you really need a 90 degree adapter for the low side port on the compressor or your life will be miserable.

  4. zaxter Morgan Avatar
    zaxter Morgan

    I have a 2000 chevy prizm. I’ve located the low pressure valve with the black dust cover on the side of my cars engine compartment. its the only valve that fits the hose i bought. so i put the r134 can on hose and everything fits. running the car/ac. at full blast. car just runs and runs and r134 never leaves the can. any ideas?

  5. zaxter Morgan Avatar
    zaxter Morgan

    I read the post above thoroughly and i believe im doing all as described. my mechanic did it before for me for 30 bucks just last month. its already hot again. Last time i had it charged it lasted 2 yrs. same slow leak in compressor.

  6. Lucas Avatar

    I’ll have to try this out, I’ve been having compressor problems all winter.

  7. Ron Hull Avatar
    Ron Hull

    Great explanation of the drop-ins never understood the leak rate issue before, if you ever write a book I would surely buy it

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