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A/C Gauge Reading For Your Analysis

Colt Hero
04-10-2010, 12:44 PM
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???

04-10-2010, 03:53 PM
You apparently need to define the units for your pressure readings because if you're talking psi for both high and low sides, those numbers are WAY off.

Weird stuff #1: Not so weird, either the pressure was extremely high on that side or the schrader valve stuck open. Did you have any adapters installed at this point? Was the EPA watching?

Weird stuff #2: Sounds like you had the adapter for the high pressure side open but remembered to close the low side adapter before disconnecting the lines. Or you have a loose connection in the high side somewhere and the low side is properly sealed.

Question #1: The comment is probably more aimed at the DIYer that doesn't use a manifold gauge set to monitor the high and low side pressures while charging. It is possible to properly charge the system using the small cans if you know what you're doing and have the proper equipment. Most of the people that own an evacuation and recharging station understand how to use it and to tell from the gauges if there are other problems that should be fixed before attempting to charge the system. It probably would have been more appropriate to just say that service much be performed by licensed technicians.

Question #2: The cans you have are both liquid and gas. It depends on the orientation of the can as to if it comes out as a liquid or a gas. This kind of relates back to the "licensed technician" comment.


Colt Hero
04-10-2010, 04:59 PM
I think I read the gauges properly. Those numbers are the outermost band on the scale - in black, so it's PSI. The next inner "thick" band (in blue or red) is temperature in F (for R134a). The next (and last) inner "thin" band (in blue or red) is also temperature in F (For R12).

The low side numbers are a bit low, but they're not THAT far off. The high side numbers are VERY low, but this might be one indication of a low charge, right? LOW/LOW = Low charge, no?

As for the "volcano" effect - I didn't use any adapters and the MasterCool gauge set does not have any secondary valves at the ends of the hoses where the "quick-connect" fittings are, either. The valve just suddenly opened on its own. You say I have a "loose connection on the high side somewhere" - what do you mean? The leak came out the top of the test port - like maybe the center pin in the port didn't fully pop out until I re-connected the gauges a 2nd time and pushed it back in? This was the first time in the life of this car that either port was excercised, so maybe this is common? I was thinking that maybe there has been a very slow leak from this test port that I've never noticed and that's why the system is low. Do these ports ever need to be replaced (or tightened somehow)?

While filling from the cans, should I follow the directions on the can that say: [paraphrase] ..."shake can, rotate between 12 and 3-o'clock every 2-3 seconds while continually shaking the can back and forth. Continue until empty (5-15 mins) or until correct amount is charged. If can feels empty, hold upside down for one minute to remove all refrigerant and additives". Is this how I get more of a gas injected into the line (versus liquid)?

What is the low-side PSI I'm supposed to be charging to? According to the Temp/Press chart for R134a, ambient F of 75 degrees equates to 35 to 45 PSI low side gauge and 150 to 170 high side gauge. Are these the numbers (stopping based on the LOW side gauge)?

04-11-2010, 09:13 PM
Readings indicate low charge for both systems. GM car may not use a clutch cycling switch, but an expansion valve to control refrigerant flow through the evaporator (compressor stays on). Use liquid (inverted can) charge, interior fan on high to keep cycling to a minimum. Low side should cycle between 22-45 psi (approx), high side psi should be ambient temp plus 100 (approx).

04-12-2010, 06:30 AM
Part of the reason I still question the readings is because on both cars the compressor engages, and on the Impala it runs constantly. The low pressure switch should be causing the compressor clutch to kick out.

The loose connector comment is in reference to the low pressure gauge staying "up" while the high side doesn't when the gauge is disconnected. Something is allowing the pressure to vent on the high side gauge. Maybe the same reason the high pressure reading on the Impala is reading low. Maybe the system is working fine on the Impala but the heater control valve or blend door is allowing air to blow across the heater core even when A/C is selected. [edit: Plymouthsrock obviously knows more about this than I do - I'm not familiar with the functional difference if the system has an expansion valve. Another thing for me to keep in mind in the future.]

I wouldn't be too concerned about following the directions on the can. Rotating between 12 and 3 o'clock shouldn't cause the R-134a to come out as a liquid. And the last trickle going in as liquid, along with the dye and oil, etc. also won't be a problem.

The pressures for the high and low side that you think you should be charging to sound about right for R-134a. I've seen a reference to high side pressures of ambient x 2 plus 50.


Colt Hero
04-15-2010, 07:22 PM
Didn't get to this last weekend - family obligations. Will do it this weekend.

I'm thinking the Impala might still be just above the cut-off pressure for the compressor - keeping it running. It's low is still 7 PSI higher than the Taurus, which cuts off at around 20, I believe.

Plymouthsrock says I should charge liquid, but isn't the can-shaking technique used to charge liquid and gas alternately (so that you don't overcharge too much liquid - which can damage the compressor ... or is this just an issue with the Taurus)? At least with the Taurus, which has the low-side service port between the evaporator and accumulator (which is a gas line), you really want to charge all gas (no?), but that would be too slow (right?) - so that's why they tell you to shake the can to charge some liquid and speed up the process a bit.

Am I right about this?

Regarding Weird #2: maybe the low-side gauge stuck after removal from the car due to a problem with the gauges (the connector at the end of the hose didn't release)?

Also - as I charge the refrigerant, am I going to see the cycling stop and level out so that I am charging to a stable number before turning off the low-side gauge? Right now, for example, the Taurus' low-side is bouncing between 20 and 40. 40 is actually a pretty good number for the low side. As I charge the line, am I going to see the 20 slowly creep up, closing the "window" until the needle stabilizes at 40?

04-15-2010, 08:43 PM
I agree with you about alternating the can between the 12 and 3 o'clock positions. While I'm hesitant to suggest anyone charge as liquid, it is much faster so I usually find my self rolling the can a bit while charging as well.

Can you tell if the gauges are bouncing because of the compressor cycling versus bouncing even during the short time the compressor clutch is engaged? If they are bouncing even while the clutch is engaged, there may be a system problem rather than just the charge level. You should have pretty steady gauges when the compressor is running, certainly once the charge level approaches where it should be.


04-15-2010, 09:44 PM
Cycling times are dependent on ambient temps, mostly. On warmer days, the compressor stays on longer, and may not cycle off at all when idling in hot weather. On cooler days, with the fan on low, the system may cycle frequently even with a full charge.

Colt Hero
04-16-2010, 12:24 PM
Can't tell. The compressor only engages for about a second. That's a pretty short window. The high side gauge is pretty stable - for whatever that's worth.

04-16-2010, 09:29 PM
If you are having trouble keeping the compressor on long enough to complete the charge, you can disconnect the cycling switch and jump the connector- the compressor should stay on. If after a full charge the system still cycles too frequently, it's possible you may have blockage, usually occurs at the orifice tube. Icing of the lines is another symptom of that.

Colt Hero
04-17-2010, 10:21 PM
Had one "incident":

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 cracked 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.

04-18-2010, 09:04 AM
What is the R-134a capacity of each system? If you put 1.75 12-oz cans in the Taurus, that sounds to be pretty near capacity. If that's the case, you likely have a leak somewhere. Granted you lost some of that when the valve came apart, or is that your estimate considering the valve came apart? Before throwing more R-134a in I'd suggest you drive it awhile and see how quickly the refrigerant leaks out. You should find yourself a leak detector and/or black light to determine where the biggest leak is. Since it is also possible to overcharge these systems and loose effectiveness, you might also consider having the system evacuated, then start over from scratch so you know you have the correct amount of R-134a in it. The evacuation process will also help you identify if the system has a leak or not.


Colt Hero
04-18-2010, 04:55 PM

Impala refrigerant capacity is 2.2lb (35 oz) with 9 oz of oil.

Taurus is 34 oz with 7 oz of oil.

The 1.75 is a guess for the Taurus (and it DOES include the valve "incident", so the Taurus actually has LESS than that). I KNOW I emptied the first can completely into the Taurus. After that, it's debatable how much remained in the second can (that I ending up dumping into the Impala). As it turned out, I don't think I fully emptied the 2nd can because when I pulled the tap off, it seemed like there was hissing for 30 seconds!

I bought a vent thermometer this morning and measured the temperatures in both cars. Taurus is actually better than I thought: 48 degrees with the fan on high and full cooling, but as I turned the fan down a few notches on the 15-minute drive home, it dropped to 42 degrees. I think that's pretty good and I'm going to leave this one alone for now.

The Impala wasn't as good. 50 degrees (maybe 48 or 49) was the best it could do, and it seemed like it took longer than the Taurus to get there (and it felt warmer). This car might need an additional charge, but before I do that I was reading my Impala shop manual this morning and I ran across some interesting stuff:

1.) The A/C System Performance Test table says that with an ambient temp of 75 degrees, the STATIC pressure should be 70 PSI. I didn't record a STATIC PSI for the Impala AFTER charging, but I did for the Taurus and it was 78-79 after charging the first can (or very near ambient). I'm going to check the Impala again. I'm guessing it's going to be in the proper range.

2.) The A/C Performance Table (Temperature based) says that at ambient temp of 76-85 degrees F and Relative Humidity of 35-60% (my Weather Channel electronic indicated 47%, but I doubted it's ambient temp, so who knows?), the Low Side should be 31-42 PSI and the high side between 148-195 PSI (with a maximum left center discharge air temp of 60.8 F). I'm in the ballpark of these numbers right now.

3.) The A/C System Pressure - Zone Classification graph seems to indicate that at a Low Side Pressure of less than 35 PSI and a High Side Pressure of about 200 PSI, I'm in Zone B on the graph - which leads to a Diagnostic chart with a LOT of steps and verbiage. Maybe I should try to plow through this to see where it leads me. The problem with these tables is, whenever I've tried to use them, I usually hit a dead end because I don't have the proper equipment (eg: "hook up the Rotunda", or whatever it's called ...)

So the plan now is to leave the Taurus alone for now, run it, try to find the leak, and see what happens. For the Impala, I'm going to try to run through the diagnostic table to see where it leads me. If it turns out I need to charge some more, that's what I'll do. Just wish I could've gotten some refrigerant with the PAG oil in it this time. Had to buy the same stuff I'd been using: Quest 12oz w/UV dye (and no oil) because AutoZone managment didn't realize this is A/C servicing season.

Also ran across some interesting verbiage in the Impala manual that mentioned the possibility of dye incompatability: "Not all of the R-134a dyes are compatible with PAG oil. Some dyes decrease the oil viscosity or may chemically react with the oil". It also went on to say, "GM passenger cars are now manufactured with flourescent dye installed in the A/C system ...[use GM dye J41447] and follow the procedure for vehicles that do not already contain the dye". Funny - this manual is for the 2002 Impala/Monte Carlo. You'd think they'd be able to give a definitive statement as to whether these cars have the dye already in them from the factory or not (instead of the generic statement that they undoubtedly cut/pasted into ALL the manuals).

Colt Hero
04-24-2010, 07:17 PM
Finally got to the Impala today. Here's what I found:

1.) STATIC Pressures: Low = 79/80, High = 79/80. These numbers seemed to correlate to ambient temperature, which turned out to be debatable. The Weather Channel said it was 72 degrees here with 52% humidity. My WC electronic thermometer had numbers 10+ degrees higher (that thing must need calibration of some kind), and the vent thermometer read 80 degrees. I went with the 80 degree number.

2.) RUNNING PRESSURES: I ran the test THREE times, third time manually holding the linkage under the hood so I could see the gauges real-time (A/C full blast, driver's window open 6", 2000 rpm (guesstimate, no tach) for 5 minutes). Each time I got the same result: Vent Temp = 60 degrees, Low = 31 (28-34 range), High = 150 (145-205 range).

According to the Shop Manual Table, assuming ambient of 80 degrees F and GT 60% Humidity, the numbers should be Low = 32-46 and High = 155-212 PSI.

So I charged a can in warm water while watching the gauges. The warm water idea was truly magical - the can discharged completely in about a minute+! After charging, the numbers were: Vent Temp = 42-43 F (pretty good!), Low = 33 (fairly steady now, no real range), 165-173 (very slow cycling). New STATIC pressure after charging = 66 PSI (both sides). Could STATIC = AMBIENT be an indication of a low charge??? Before charging, both cars' STATICS were right at ambient. Conincidence?

So I'm inside the proper ranges now with the Impala, but maybe still on the low side. It could probably take another partial can, but I'm going to stop for now and see what happens. I'm thinking, so far, I've added about 1.5 cans (maybe less) to the Impala (or about 18 oz). Now I'll just sit back and see how long it lasts (while looking for obvious leaks).

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