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A/C Gauge Reading For Your Analysis

A/C Gauge Reading For Your Analysis

Two A/C systems both blowing warm air.

Car #1: '97 Taurus GL wagon, 3.0l OHV, 181.5k miles

Car #2: '02 Impala 3.4l 131.5k miles


Taurus cycles ON (1-2 secs) and OFF (6 secs) for an 8 sec cycle time

Impala runs constantly.

Hooked up the A/C manifold gauges and here’s what I saw


Taurus = 48/49 Impala = 50/51.x


Taurus = (20 ramps to 40, falls back to 20 cyclical)/(70 to 65 cyclical). Compressor cycles ON & OFF

Impala = (27 ramps to 32, falls back to 27 cyclical)/(79 to 76 cyclical)


#1: About 2 minutes after removing the manifold set from the high side port of the Taurus, the port suddenly started spraying refrigerant out like a volcano! It probably leaked for about 30 seconds before I was able to reattach the manifold’s high-side hose back onto it (then re-attach the plastic cap). Might this be the source of a leak?

#2: After removing the gauge set from the Taurus, the low side gauge stuck at the last reading while the high side went back to zero.


#1: The FORD shop manual clearly states NOT to refill the system using “small cans”. Instead a charging station or cylinder should be used (because the claim is you cannot charge the system properly with the small cans). Any truth to this?

#2: The FORD manual also says do NOT charge liquid refrigerant into any place other than the accumulator or the compressor could be damaged. Are the 12oz cans I’ve bought (Quest R134a w/UV from AutoZOne $12.99) liquid or gas? They FEEL like liquid. Is there a gas product that I should be using? The manual also shows a sketch with the LOW service port in between the accumulator and compressor, but I think this is incorrect. It appears the port is actually between the evaporator and accumulator. If I am using a LIQUID refrigerant, wouldn’t I be injecting liquid into a gas stream???

You should seek instruction from someone who knows what they are doing before you hurt yourself or some bystander intrigued by what you are doing.

Well, I certainly appreciate the potential harm here - that’s why I’m asking these questions. I have actually READ parts of the FORD shop manual A/C sections. It’s not like I’m just hooking up cans of refrigerant to see what happens. I’m sorry if you think the questions I ask make me look stupid, but I’m just trying to be careful here - like I am in my everyday life and line of work.

And no - nobody else is in the vicinity of this project. If you knew me better, you’d know not to make such a statement. I’m about as detail-oriented a person as you’ll ever meet. I go to great lengths to try to think of EVERYTHING before executing almost ANYTHING. It can be maddening sometimes - for me, AND my family, but I can’t help it. I need to know enough about something up-front in order to feel confident enough to proceed (and do a good job).

All of those pressures are way too low. With the engine and A/C off and with the systems settled down you should have about 115-120 PSI on both the high and low sides. With the compressors engaged you should have about 35-45 on the low and 225-275 on the high. (varies a lot due to a bunch of factors)

The Ford manual is wrong. You certainly can charge a system with small cans. The reason they state use the large cylinder is they’re assuming that you will not purge the lines on the gauge set between each can and will allow a small amount of air into the system.

If you hold the cans upright as you charge the system you will be charging gas only. If you turn the cap upside down then you will be charging liquid into the system.
Liguid charging is faster but you really need to develop a feel for this. If you flip the can upside down and leave it there for too long it is possible to damage a compressor as the compressor is unable to handle the large load of liquid that it’s being fed.

If the charging process seems to be going very slow try palming the can of opened refrigerant. Body heat will help to evacuate the can faster. You can also set a pan of very hot water up there and sit the can it it. That speeds it up even more. And wear goggles.
Hope that helps.

All the advice OK gives is great. I’d add that on a sticker somewhere under the hood, the total capacity of the system should be listed. Since there is already some refrigerant in the system, you can use the total capacity as a common sense guide for how much to add. If it’s really low, I wouldn’t add more than 75% of what the sticker says. Don’t just keep dumping can after can into it. I’d keep the cans upright when charging, and put them in a pan of hot water. (don’t immerse them) Also, let some escape when connecting them to the charging port–this will purge air out of the line so you won’t add air to your system.

OK4450 and Oblivion:

Thanks for your clear and concise replies. Much appreciated. I think I feel good enough about this now to proceed. Most of the time, I only need a few questions answered to do what I want to do (because there’s always holes in every explanation), but then it’s still a matter of reaching the right people on these forums, then hoping they’re able understand my posts - which are always filled with every little detail I know in order to help prevent partial replies (necessitating follow-ups from me).

When recharging an AC system and monitoring the gauges, the engine idle should be at 1800-2000 RPM’s and a large fan should be blowing at the condenser. This simulates the vehicle being driven down the road and gives realistic gauge readings. Also monitoring the duct outlet temperature indicates how the AC system is performing relative to the gauge readings.


It has always been the policy of this Forum to impact upon the OP how the Forum has reached a concensus that AC work is not a DIYer job.

This one is on you guys for encouraging actual hands on to a person who does not even have the basics down.

Normally, I might agree but at least Colt Hero is aware of gauge pressures, has a gauge set, and is asking questions up front.

The odds of a can explosion are pretty much nil if one rule is followed; NEVER open the high side valve on the gauge set when a can of refrigerant has been tapped and opened.
If this rule is followed, along with wearing goggles and maybe a pair of gloves, it’s a pretty safe process.

If he were asking where the low side pressure port was or what an accumulator is then I’d be hesitant about dishing out any advice.

Often I get involved in wording a post here and trash it when I realize that my advice could lead to someone getting injured. Air conditioning problems is likey the subject of the most often cancelled posts. There have been so many injuries that I have seen the results of first hand and a few explosions that I have witnessed. The regulations requiring high side ports to be differentiated from the low side was likely a great benefit to the DTYer.The OP here sounds capable and I hope he finds a happy conclusion to his efforts… But with regard to oldschool’s post, I have asked here before and ask again now, how do we who feel qualified to post answers here determine if the DIYer is capable of safely following our suggestions. And is there a source that a DIYer can access to give them basic insight into their problems.

Safety aside, I feel I am doing the OP a financial favor by steering him to a pro.

Didn’t get to this last weekend (family obligations). Should be clear for takeoff this weekend.

ok4450’s comment about NEVER opening the high side valve on the gauge set when the can of refrigerant has been tapped and opened makes me wonder if it’s actually SAFER to charge with only one of those Low-Side hose/gauges they sell. I’m sure the gauges on those units are inferior, but at least this way there’s no way you could accidentally open the high side and cause an explosion. You could do the measurement with the quality gauges first, then use the low-side-only unit to charge.

Any comments on this? Maybe it’s still a good idea to see the high-side gauge as you’re charging to make sure you don’t overcharge? But you’re charging by the low-side gauge, right?

And regarding charging - right now the Taurus (for example) is cycling between 20 and 40 on it’s low-side gauge. 40 is actually a pretty good number (depending on ambient). As I charge, what am I going to see? Will the needle start to dip less and less, closing the “window” and inching upward until it finally settles out (stabilizes) at 40-45 PSI (at which time I’ll close the low-side gauge knob)?

Then, let’s say the 12 oz can is empty and I want to switch to the next can on the current car (I will be doing both cars in series, by the way). What’s the sequence to prevent trapping air in the line before I connect the next tapped can? Is this it (or is this too complicated)?

1.) Close the low-side valve on the gauge set.
2.) Turn OFF the car.
3.) Disconnect the low-side hose at the service port
4.) Open the low-side valve on the gauge set to bleed out any residual empty can pressure
5.) Close the low-side valve on the gauge set
6.) Remove the empty refrigerant can
7.) Attach the next full refrigerant can to the can valve (with needle receded)
8.) Turn can valve to puncture can and allow refrigerant flow to gauge set
9.) Slightly crack open low-side valve on gauge set to push out any air in gauge set line, then close the valve.
10.) Re-attach low side hose to service port and continue charging (if necessary)

The full gauge set is the only way to go as the low side charging hose only gives you half the story.
Charging with a full gauge set is perfectly safe as long as you never open the high side valve when the cap is tapped and opened. If the high side pressure is way up there what happens is that the high pressure in the system is applied to the can and usually what happens is the bottom of the can blows out.
In other words, never open the high side valve at all except when a vacuum (or evacuation) is being pulled on the system.

I’ve seen these go off a few times by accident (not by me but in the hands of A/C qualified people who just slipped up and opened that high side while letting the mind wander) and believe me; it generates a lot of excitement along with a massive steam cloud.

At elevated RPMs I shoot for 35-40 PSI and get a bit antsy over 45 and up. Evaporator temperature is roughly equivalent to the low side pressure so the lower the pressure the colder the air. (I home-brewed my old Sable once upon a time and got 25 degree air out of that one on a 100 degree day while at highway speeds! Freeze a load of beef.)

You don’t have to turn the car off at all to purge the air. Before charging the first can into the system open the can tap. Now slightly loosen the center hose at the gauge manifold. You will hear a faint hiss. This is the air being purged out of the hose. Snug the hose up, open the low side valve, and away you go.
When the can is empty, close the low side valve, and repeat the process.

As I mentioned previously, wear safety goggles and since you’re relatively inexperienced at this a pair of gloves is not a bad idea either. The odds of something blowing up on you is close to zero (as long as that high side is never opened) but you never push your luck. When it comes to cars Murphy’s Law is always present.
Hope that helps.

(Back decades ago some friends and myself used to go target shooting out in the boondocks. One of my friends ran an electric motor shop and they used R12 (cheap back then) to freeze motor parts during service and he used to bring along a dozen cans for additional plinking purposes. We would set those cans up at 50-100 yards and you should have seen them when hit by a bullet. Kind of like the space shuttle blasting off! Trying to even hit one of those cans at that range was a real challenge and that’s why we did it.)

OK - gotcha - NEVER open the high side valve. Maybe they ought to make the knob easily removeable, so you can’t use it (another idea)…

1.) The lower the low-side pressure, the lower the A/C vent temp.

2.) Don’t charge over 40 PSI (or the pressure/temp table figure for ambient).

3.) Slightly loosen the center hose after can tapping to purge the yellow hose of air, then snug back up.

4.) Safety goggles and gloves

Thanks again - and some pretty funny reading, too!

I don’t think you speak for the rest of us, certainly not for me and where is this “policy” written? I don’t usually answer to AC posts simply because it is clear that most posters do not have the prerequisite skills for this task, no matter how good the instructions are.

I didn’t see this post when it was first posted or I would have given him about the same information that ok4450 and Tester gave. BTW. Colt Hero have been posting here for years and some of us that have been around for awhile are aware of his skill level.

I would not worry about that insignificant amount of air. What I would worry about is the type of oil that may be in the can you bought. Most do not have any oil in them, but if there is, there are two types of oil, PAG and ester. They are not compatible with each other so find out what is in your system and what, if any, is in the can you bought.

BTW, I bought the ID $89 set at O’riellys along with a can of the new synthetic R-134 without oil.

Next time your at the auto parts store, pick up a tool for the schrader valve for AC systems. It’s only $2, but if you need it and don’t have it, it will cost you a lot more.

First off - thanks to keith (and ok4450) for the vote of confidence for Colt Hero, however, I must report an “incident” during the charging “marathon” which might downgrade my implied competence score somewhat (although maybe it wasn’t totally my fault):

At one point, while charging the 2nd can into the Taurus (the 1st car), I turned everything off to take a short 5-10 minute break when it seemed that the gauges just weren’t going anywhere anymore. When I returned, I re-opened the low-side valve to start charging again, but to my horror, while turning the thumbwheel counter-clockwise, the entire valve popped out of the valve body (leaking refrigerant in a very messy pattern)! Fortunately, I was able to quickly get the valve back in against the back pressure, but subsequent static testing on the Impala proved that the valve was leaking. Removing the valve, I noticed the O-Ring was history. So I ended up swapping the set at AutoZone to continue with the Impala. Now, was that my fault or was the nut holding the valve in simply not tightened enough against the valve body? I’m guessing it was the latter because it wasn’t like I exerted any undue pressure on the thumbwheel as I fully opened it (unlike I had been doing when CLOSING the thumbwheels). The nut must’ve been thumb-tight and when the thumbwheel reached “fully open” it took the nut with it! Anyway, after cleaning up the refrigerant spew, I DID try tightening the nut against the valve body, but after doing this, the thumbwheel wouldn’t fully seat against the nut anymore (like the red thumbwheel did). This makes me wonder if there are gauge sets out there where this cannot happen. Or - maybe these valve nuts need to be checked for tightness before using the gauge set. Of course, the laminated instruction card hanging from the low-side thumbwheel makes no mention of this.

So what were the results (at 80 degrees ambient)?

Well, both cars are now blowing colder than coolish. I didn’t have a thermometer in the vent so I can’t give an exact number, but I can say that with the temp knob at full cold and the fan on high, both cars feel comfortable inside (but are probably still very short of where they could be).

While charging the 2nd can on the '97 Taurus, it seemed like I could never get above 30-33 PSI on the low-side, while the high-side seemed to max out around 200 PSI. Because of the high-side reading, I felt like I had to stop before getting closer to 40 PSI. One nice thing was the increased ON time of the compressor as the system charged. It went from one second ON to 10 seconds ON, to ultimately ON all the time (I think).

With the '02 Impala, it was pretty much the same thing. After one can, all I got was 32 PSI aand 180-210 PSI. One thing about the Impala that might be troubling is the needle never really went all upward or all downward. Instead it would suspend, go up a bit, then continue moving downward. Or suspend, go down a bit, then continue going upward.

So what should be my next course of action? I could try charging another can (maybe a different brand). These 12 oz cans from Quest (with UV dye, but no mention of oil) seemed to empty out pretty quick (and as I said, they never really got cold - only coolish at the very top). Maybe a different brand would work better? Or I could just drive it a while and observe. It DID seem like at higher engine RPM as I drove the Taurus around, that the air got a LITTLE cooler (but not dramatically). Probably should get a thermometer so I can get a numerical reading (any recommended models??)

On a general note, the cans - although seemingly emptied out each time, never really got cold. I could feel some coolness through my gloves, but nothing like what I was expecting. I ended up using the constant agitation technique, rotating the can between 12 snd 3 o’clock every 3 seconds like the can suggested. I also used Ok4450’s suggestion to crack the yellow hose at the gauges to push out the air with each can. I would say the Taurus took about 1.75 cans and the Impala got 1.25 cans.

OK, Oblivion, I know you were only trying to help this guy out, but at some point here he is just going to hurt himself…Read post #1 then post #16 and maybe you will see where Old School and I are coming from…

Hero, the low side will take care of itself. WATCH THE HIGH SIDE PRESSURE AS YOU ARE FEEDING IN THE FREON!! It’s the DIFFERENCE between the high and low sides that produce cold air out the vents! 275-300 tops on the high-side…

The reason Ford does not like small cans is because what are you going to do with any freon left in the can when the system is full??? With a large gas bottle, you CAN open that high-side port and allow the excess charge back into the cylinder…The most common mistake DIY’s make is over-charging…They don’t know what else to do with the 1/2 can of freon, so they dump it in. They have no clue what the high-side pressure is…

The problem with the valve sounds like a set of gauges that was not properly assembled and things like this are really not that rare.
A couple of years ago a neighbor bought a set of gauges from O’Reillys and wanted me to show him a few basics about their use. The high side connector on the gauge set would not fit any of the high side service ports on any of his cars or any of mine.
After taking the set back and getting another it was discovered the second set would simply not fit. Close, but no cigar.
Finally the third set would fit; sort of. It was still a bit iffy and on a few cars would not fit anyway. It fit his, so he called it good.
(At O’Reillys the high side connectors on the defective gauge set would actually fit the aftermarket service ports sold by O’Reillys with no problem. Fitting on the cars was another matter altogether.)

Just offhand, it does sound like both systems could use a bit more refrigerant. At 80 degrees (and there are still other factors at work) what you would like is about 225 PSI at elevated RPMs and during hotter weather maybe even 250 or so. This will bring the low side up a bit too. Going to 45 won’t hurt anthing; it’s just that I consider that not low enough or cold enough. Forty-five degree air on an 80 degree day is fine but it won’t go far when the temps are 110 like it gets here in OK.

Pressures of around 200-210 are a bit low. If pressures on the high side start heading towards 275-300 on an 80 degree day then it’s time to back up a step.
If the air is humid and the lines around the accumulator are sweating profusely then you know things are on the right track.

Caddyman, I wasn’t doing anything reckless or ignorant here. I was being very careful. As far as the valve coming out of the valve body - while this “incident” SEEMS to support your argument that A/C work is not for the DIY’er, to be truthful, that could’ve happened to anyone. As far as charging the system, my plan was to charge to the Temp/Pressure table numbers. At 80 degrees ambient, the low side would be 40-50 PSI and the high side would be 175-210. When I got into the 200+ vicinity on the high-side, even though the low side was still LOW, I stopped everything because I didn’t want to overchage the system. So I was playing it safe. As far as your 275-300 “tops” number, isn’t that only valid for 95 degrees ambient, and if so - aren’t you providing misleading information that could be harmful?

So ok4450 - you think I should charge a bit more? 225 PSI equates to 85 degrees on the T/P chart. Tomorrow it’s going to be cooler - probably mid 70’s at most. One other thing I should mention - while doing the charging on both cars, I only revved the engine to 1500 rpm for short periods because I didn’t have a helper - although I had to laugh because my grass is about 6-7" tall right now because my mower hasn’t started yet this season and two young boys actually approached the car while I was charging it (well after the “incident”, I might add) to offer to cut the grass. I immediately thought of what Caddyman said, breaking out in an ear to ear grin as I sent the boys on their way (with a confused look on their faces). Sometimes I think the forces of Evil are FAR GREATER than the forces of Good in this world. We call it Murphy’s Law, but I think it deserves a far more sinister name.