Adding Freon to Car AC – Gauge Readings Explained
April 17, 2009 7:29 am Air Conditioning, DIY, How To Auto Repair
Get your A/C checked at a local shop or check it yourself? Freon will need to be added by someone.
Summary ©DenLorsTools.com: Adding Freon or topping off a cars A/C system is the most common task performed to restore performance and get cold air blowing again. However, adding refrigerant isn’t always the solution for car air conditioning problems and troubleshooting may be required before just charging a car AC system.
There can be many other things wrong besides a system being low on refrigerant. To diagnose/troubleshoot problems, an A/C manifold gauge set is needed to read high and low side pressure readings. Avoid adding refrigerant with a simple charging kit like the ones sold at parts stores. Don’t add any stop leak, this can cause problems in the compressor, expansion valve or condenser.
The image above was taken at Moog and US Hwy 19 in Tarpon Springs Florida, it made me laugh. The sign says “Your wife is hot! She wants her A/C fixed, let us check it for free”. For some help taking care of your AC problems yourself continue reading.
This is where it starts to get serious. Keep in mind that using an A/C gauge set and seeing BOTH high and low side readings can help in diagnosing the problem when you know what to look for. First, on a 134A system the high and low side service ports are different sizes. AC gauge sets have color coded hoses, the blue color coded hose has a connection that fits on the low side service port and the red hose has a connection that will only fit onto the high side. The yellow hose won’t hook up to anything if just checking the readings; it can be used to connect to a vacuum pump or attached to a refrigerant can or tank.
*Make sure the condenser fan comes on when the readings are being checked. Below are normal car AC pressure readings with 134A.
* Normal readings on high and low side with AC OFF (static pressure) Depends on outside temperature, but normally is between 80-105 PSI * Normal low side reading with AC on high-speed and MAX & engine at 800-1000 RPM’s Ranges from 25-35 PSI Note that on many Chrysler products a normal reading on the low side may be 15-25 PSI
* Normal high side reading ranges from 200-350 PSI Don’t assume that if adding little Freon is good that adding allot is better! Overcharging just a little can decrease the performance of the system and possibly damage the compressor. Additional Car A/C Info Troubleshooting Gauge Readings
With the AC on the coldest setting, use a thermometer in a middle vent. Normal vent temperature readings will vary depending on the (ambient) outside temp. The vent temperature should range from around 42-55 degrees in my experience. If normal gauge readings are obtained and the vent air is cold – STOP don’t overcharge the system. The only proper way to remove refrigerant is with a AC recovery machine so if this is being done at home I can’t emphasize enough NOT to over-charge the system. And actually the best way to insure the proper charge is in a system, is to use an AC machine to recover the Freon and then evacuate and recharge the system with the correct amount. Most cars have the factory specified amount on a decal under the hood. See the next page for gauge readings and images. Continued on Page 2
dennisb - Auto Tool Sales :
Date: June 8, 2014 @ 3:46 pm
If your gauge’s are reading correctly, looks like the compressor is not pumping. BTW, we sell professional quality AC Manifold Gauge Sets. The low side should go down to about 30psi and the high side should be around 250psi. If it was just low, the low side would go down to near vacuum and the high side would start to climb but would cycle on/off, indicating not enough refrigerant. Good Luck.
drhill :
Date: June 9, 2014 @ 9:44 pm
Thanks for the tips on my 99 Navigator (my post was on 6/6/14). I did replace the accumulator as well. I did smell the flush after I opened the system up after recovering the freon the first time. I will leave it on vacuum a long time this time. How do I tell if there’s too much oil in the system by looking into a line? What should it look like? What would too much oil look like? I can easily open up the liquid line where the orifice tube goes or would you recommend another line? Thanks for the help!! Looks like I’ll be buying future tools from you now! 🙂
dennisb - Auto Tool Sales :
Date: June 10, 2014 @ 6:50 am
If there’s excess oil in the system it may be pouring out once you disconnect a line. The accumulator/drier may be saturated with oil. Sometimes you may be able to blow some of the excess out or take it off and turn it upside down and rain some excess oil out. Some people don’t know that the replacement compressors will sometimes have oil in them already (most do). And therefore add too much oil. That along with not taking into account how much oil may still be remaining already in the system. I would always notice if a system appeared dry when I opened a system up or if oil would spew out freely when a line was removed. Either extreme is not good. So you have to use judgement along with knowing the specified amount of recomended oil to add for the system components being replaced. Not enough oil and the compressor may fail due to lack of oiling/lubrication. Too much oil and the concentration of refrigerant is lessened and the cooling efficiency will be reduced. Remember the refrigerant is what transfers the heat and cold in the system, not the oil. Also too much oil can harm the compressor because it can’t pump it as well when too much is pooled in excess.
Just a reminder that venting (the act of just releasing the Freon into the atmosphere) is illegal and harmful to the ozone. The only proper way of removing the refrigerant is by the use of a recovery machine which can recycle the refrigerant. Most repair shops have this equipment. For everyone reading this, that may be posed with this decision – please, let’s try to take care of the planet for our future generations rather than saving a few bucks by venting.
j :
Date: June 11, 2014 @ 11:34 pm
Help please….90 Mustang installed new compressor new oiler line new accumulator took to shop to be chargedhe tried charging twice said it won’t go past the oiler line said it goes through compressor through the hose through the accumulator through the firewall back out oiler line frost up and goes no farther
dennisb - Auto Tool Sales :
Date: June 12, 2014 @ 12:00 pm
If there’s contamination from the old compressor failing, it can clog up the orifice tube which is usaully located in the inlet of the evaporator. The liquid line, which is what I think you mean when you say “oiler line” is the small diameter line that goes from the condenser to the evaporator. Some Ford’s have the orifice built-into that line and some, the orifice tube just is inserted into the inlet of the evaporator. Look for a clamp type mark that prevents the orifice from going further. So check the orifice tube. You should have already replaced it. But even if you did it could be clogged with trash from the old compressor. It should look like the one pictured below.
Mattd :
Date: June 13, 2014 @ 2:00 am
Ac doesn’t blow cold. Hooked up to a gauge and it reads 300lbs in the red and wont move. What could be the problem?
dennisb - Auto Tool Sales :
Date: June 13, 2014 @ 7:27 am
Sorry Matt, not enough information to go on. I’d check the high and low readings, check the condenser fan and go from there depending on what I found. Good Luck.
Paul S. :
Date: June 13, 2014 @ 10:33 pm
2004 Toyota Corolla. Ac working great until about 1 week ago. While driving the air would be cold, then slowly warm up (30 secs), then continue blowing cold. It seems to keep doing this back and forth – maybe a few minutes of cold, then 30sec warm, followed by cold again. I bought a can of freon and added it to the car. Using the guage on the can it read a little low. I went though maybe 3/4 can and it still reads about the same. Not really sure if its over or under charged, but the cold, warm, cold cycle keeps happening. Any ideas where/what to look for or replace. Thanks!
dennisb - Auto Tool Sales :
Date: June 15, 2014 @ 4:37 pm
I would start by checking the high and low readings…
y2k :
Date: June 17, 2014 @ 8:24 am
Vehicle: 2000 Nissan Xterra
Once vent temperature reaches 38-40 deg, compressor cuts out. When vent temperature goes up to 43-45 deg, compressor kicks back in.
Pressures at idle: Low = 30-35 psi and High = 250 psi. Outside temp. = 93 deg.
I am trying to figure out why the compressor is cycling when the vent temperature reaches the 40 deg mark.
Thanks.
dennisb - Auto Tool Sales :
Date: June 17, 2014 @ 9:01 am
Seems like your car’s AC system working perfectly. Any cooler than that means the evaporator would be near (32 degrees) freezing, causing the system to have problems. Nothing needs to be done.
y2k :
Date: June 17, 2014 @ 9:31 am
Thanks Dennis for the response.
Would it harm the compressor/clutch if it cuts in and out constantly. With the recirculate on, it reaches 40 deg and almost every 2 minutes there is a cut-in/cut-out cycle.
dennisb - Auto Tool Sales :
Date: June 17, 2014 @ 9:41 am
I see your concern. However, the system is designed to cycle, keeping the low side at around a 30 psi average. This also keeps the evaporator from freezing up. The cycle times will vary depending on what the outside temp is and what settings you have the controls adjusted to. There’s nothing you can do to change the way the system is designed to work. I wouldn’t be concerned as long as the air conditioning is working good, blowing cold air out of the vents. When a system blows cold air that is the best indication that it is working properly…
Amopower :
Date: June 17, 2014 @ 7:42 pm
Hi Dennis – I am trying to find out once and for all whether the pressures should be checked at idle, or at 2000 RPM’s. I look thru all these posts and see people list idle pressures, and they are NOT scolded to re-measure at 2000rpm. Then I see other posts about making sure you have 2000 RPM’s. Even the P/T charts don’t seem to state one way or the other. When I measure at idle versus 2000 RPM’s there is a *significant* difference, enough to make me misdiagnosis. So once and for all – can you confirm that pressures should be measured at 2000 RPM’s? Thanks alot!
dennisb - Auto Tool Sales :
Date: June 18, 2014 @ 7:06 am
Good point. I check at both, but as far as the pressure readings for diagnosis the readings you get at higher RPM’s are more accurate. If you have a weak compressor there usually is a lot bigger difference between idle and at 1500-2000 RPM’s. The compressor is turning much faster at the higher RPM’s so it pumps more even when the compressor is good and is functioning as it’s designed. On a worn compressor at idle it may not pump enough to cool well. At higher RPM’s it may be less noticeable that there’s a problem. So, when you are checking the readings record them at higher RPM’s usually 1500 – 2000 RPM’s is good. You can also check mechanical clutches to make sure it’s pulling a lot of air. If you have an electric cooling fan, it can be checked also. Good Luck.
smokey :
Date: June 18, 2014 @ 4:30 pm
Hi Dennis: I have a 1994 Ford Explorer which the air condition was leaking freon but now has been repaired by an Automotive Air condition shop. My question is what should the Vent temperature read? Or what is the temperature range from the Vent? The ambient temperature is around 85 to 90 degrees and the output temperature from the vent is 60 degrees. Thank you
dennisb - Auto Tool Sales :
Date: June 18, 2014 @ 5:26 pm
The vent temperature on the middle or right side vents, closer to the evaporator, should be cooler than 60 degrees when the ambient (outside) temp is 85-90 depending upon the settings. The control settings should be on Max (recirculate), the blower speed on low and the RPM’s 1500-2000. If checked at these optimum settings most cars will cool 40 to 50 degrees cooler than the outside temperature. Remember 32 degrees is freezing temperature and the system will not allow the evaporator to freeze, it will cycle the compressor off, so take that into consideration when using this rulle of thumb. So if your Ford doesn’t have a vent temp of 45-55 there could be a problem. Maybe low refrigerant or possibly a broken recirculate door. Black vehicles are also harder to cool because of the sun heating the cabin due to the darker color. I used to hate when customer’s had black vehicle’s and complained of the AC not being cool enough. There’s only so much I could do and changing the exterior color wasn’t one of them. Good Luck.
Melo :
Date: June 21, 2014 @ 8:12 pm
Hi Dennis I have a 98 Rav 4 and my ac’s not cold. It’s fully charged with proper amount and had good vacuum. It’s not cold, even though the compressor clutch does stay engaged. But when you turn the ac on the low side gauge stays at 0 PSI and high side at 110 PSI, even if I raise the RPM’s to 2000 there’s no change.
dennisb - Auto Tool Sales :
Date: June 22, 2014 @ 8:54 am
Inaccurate gauge reading. Even with the AC turned off, the static pressure should be more than zero. More like 80-100psi depending on the outside temperature. So, I’d say the gauge reading is not accurate for the low side. If the compressor is pumping the high side should be around 200-300 psi. The high side reading of 110 psi (if that’s an accurate reading) indicates it’s not pumping which means the compressor is bad. Good Luck.
Les :
Date: June 22, 2014 @ 8:54 am
Click the image for a larger view.
I’m working on a 72′ Pontiac LeMans Sport of which I’m the original owner. I’ve replaced all major components and installed a new (not rebuilt) Sanden compressor in place of the A6 Harrison. I’m going back with R-12 and noticed after putting in 4 – 12 OZ cans that my low side reading is falling into the negative area (Vacuum). High side is only around 80 PSI. My thoughts are that the compressor is pulling against a restriction on low side area (TEX Valve, evaporator, Liquid side Receiver Dryer to TEX Valve. I realize the system needs 4 1/2 LBS of this liquid gold to operate properly but I was expecting the pressures to at least be positive on the low side and the high side to be above 125. Outside temperature at the time was 88 degrees. My thoughts are that I have a problem with the TEX Valve.
I would like to get your option on this before proceeding further. Thanks
dennisb - Auto Tool Sales :
Date: June 22, 2014 @ 9:40 am
Les, I think you’re on the right track. You’re only 8 oz away from a full charge, but with low side going negative I’d suspect the expansion valve also. Kinda surprised the expansion valve wouldn’t have been replaced during the compressor and dryer installation especially on such an old car. If there’s no cold air coming out of the vents at this point, I wouldn’t put the last 8 oz. in. Best to recover it and install the new expansion valve and dryer if you haven’t already. You could blow through other components like the evaporator and condenser when you have the system down just to make sure nothing else is restricted. Dryer desiccant can come apart in the dryer and clog components very quickly. Good Luck.
Les :
Date: June 22, 2014 @ 7:59 pm
Thanks Dennis for getting back so quickly.
The reason the TEX Valve was replaced was do to leakage coming from the pressure line going into the top cap where the diaphragm is located. I wasn’t counting on this valve going belly up, quality of parts vary as years past as you know. I did replace the dryer as part of the process with out a doubt it’s good practice as you know. I hope it didn’t come apart! I hadn’t thought about that!!
Thanks again for getting back with me and I’ll keep you informed on my progress.
Les.
zzmike :
Date: June 28, 2014 @ 9:51 am
I have a 98 Mustang that wasn’t cooling well. Hooked up the gauges and found the high side was only going up to about 70. Charged up to 210 with low at 30. Worked fine for a couple of days, then quit cooling. Gauges showed high side low again. Evacuated system and vacuumed down to 30 inches. Shut off pump and left gauge open. Held vacuum for two hours with no leak down. Recharged with a small can of leak stop (which is 90% R-134a) and added additional R-134a until high side was up to 210 again. Worked well for 2 days then started warming up. Gauges show Freon low again. Is it possible that my compressor front seal is leaking during operation but won’t leak during vacuum test? Any other ideas on where the leak is? Leak stop had a dye and I can’t find any at any fittings. Thanks, zzmike
dennisb - Auto Tool Sales :
Date: June 28, 2014 @ 10:07 am
Checking the gauges after using a vacuum pump, to see if there’s a loss of vacuum, is only one way to check to see if there’s a leak. I never relied on that very heavily. It’s not a real dependable test contrary to what you may have read. I only use that, as a quick preliminary check in case there was a huge leak (one that wouldn’t even hold Freon for a couple of days). I don’t recommend stop leak, it can cause other problems like stopping up condensers and expansion valves etc. It is also damaging to recovery and recycle machines. With that being said you obviously have a leak. It could be from the compressor seal while running as you said, but there would most likely be UV dye residue there. On a close visual inspection, if I don’t see dye I would consider the possibility of the evaporator leaking. It can’t be checked visually because it is in the dash concealed. On fast leaking evaporators, I’ve seen oil/dye in the drain tube of the evaporator. I’d use a good electronic leak detector and sniff the whole system paying close attention to the evaporator. A good place to check the evaporator is at the drain tube being careful not to wet the sensor of the electronic AC leak detector. Good Luck.
Gene :
Date: June 29, 2014 @ 12:35 am
Dennis:
Been helping a family member with their AC Problem…. it would cool nicely for 2 or 3 days, then stop….obvious leak… put some dye in system, and never saw anywhere it was coming out…. then I used my “sniffer” to see what was going on…. I put the probe in the Evaporator “Box” and occasionally it would alarm….so I opened he firewall cover (Older Pontiac) and though I did not see any dye, I went ahead and replaced the Evaporator when I was is in there…evacuated the system and went to dinner…. an hour and a half later the gauges were at “0”…. Therefore, I got a “guaranteed” used compressor…evacuated again…. an hour later, gauges at “0” again….. the only thing I have not replaced now is the condenser, but I have seen no UV Die from anywhere on the condenser. BTW…even used different hoses on the manifold gauges in case there was a problem with the hoses…… still no luck and not holding pressures steady for an hour or so…… any ideas here….. OR maybe I’m relying on the gauges after evacuation too much? I have not filled with R134A since I did the evaporator, as not wanting to waste the expensive stuff….. should I go ahead and charge and see what happens??
zzmike :
Date: June 29, 2014 @ 7:20 am
Thanks very much for your quick response. I’ll find someplace that has an electronic leak detector, do another charge and see what I can find. I’ll let you know what finally fixes it. zzmike
dennisb - Auto Tool Sales :
Date: June 29, 2014 @ 9:30 am
Gene, sometimes leaks can be very hard to find. A zero reading on the gauge after letting it sit awhile after vacuuming the system down does indicate a leak. However, the leak could be somewhere from the gauge set or connections also. The leak would have to be large to make the gauge go to zero so quickly. Not that a refrigerant leak that takes only 2-3 days to affect the system isn’t large but it may not be enough to cause the gauge to go to zero so quickly. On older Pontiacs (and other GM’s) evaporators are not that common of a problem. Usually it’s the compressor, although anything’s possible. I wouldn’t try a used compressor. After the compressor sits awhile it’s much more likely to have dry seals that tend to leak after it’s put back into use (especially the front shaft seal). I’d put enough refrigerant in to get at least 50 psi and check the system with a good electronic leak detector. Don’t trust the leak detector every time it beeps even with a good one. Read the following if in the market for an excellent Freon leak detector. Good Luck
Gene :
Date: June 29, 2014 @ 11:51 pm
Thank you, I worked on it this afternoon, and over a period of over 4 hours the gauge needle only decreased by just over a point or two, so I am unsure if it’s in the hoses, or the system.
dennisb - Auto Tool Sales :
Date: June 30, 2014 @ 7:08 am
Exactly, could be either.
Gene :
Date: June 30, 2014 @ 10:25 pm
Went ahead and charged system today…benn blowing at @ 42 degrees all day….. I will take a couple days and see what happens….
doug :
Date: July 1, 2014 @ 1:28 am
Hi Dennis, thanks for your great article. I should have found it before I used the autozone r134a and low side hose/gage I borrowed from my bro in law….
My 2008 Taurus was not very cold so I added the can of 134a and stopped when pressure was about 40 psi. I also noticed radiator fan got faster as it filled. When i was done, Ac didn’t work, so I vented some out and now ac is cold, but fan is really screaming! What should i do now? Leave it alone? Buy high and low pressure gages? Bring it to a pro? Thanks very much!
Doug
dennisb - Auto Tool Sales :
Date: July 1, 2014 @ 7:48 am
Doug, thanks. But I really wish you wouldn’t let refrigerant vent into the atmosphere; it’s illegal to vent Freon into the atmosphere because it damages the Ozone. Refrigerant from our air conditioners escaping or being vented intentionally is the primary cause of our Ozone layer being diminished. Anyone needing to remove refrigerant because of an overcharge or to open a system up to be worked on, must (should) take the vehicle to a repair shop so it can be handled properly with a RRR (Recover Recycle & Recharge) machine. The refrigerant will be recovered, recycled and reused.
I would check both high and low side readings. But at this point, if it’s cooling well, that’s one of the best indications that the system has the correct amount of refrigerant. With too little or too much Freon, the air conditioning will not function properly. The cooling fan works on different speeds as the high side pressure readings dictate. Unless you think the fan is excessively load like it may have a worn motor, I wouldn’t be concerned.
Mms :
Date: July 1, 2014 @ 3:51 pm
Hi, I just changed evaporator and now radiator fan and compressor run continuously. Plz advise. I have corolla 2007
dennisb - Auto Tool Sales :
Date: July 1, 2014 @ 3:59 pm
Changing an evaporator has nothing to do with the compressor and radiator fan staying on all the time. I would check each circuit separately using a wiring diagram for your specific vehicle. Keep in mind that the radiator fan will stay on if the temperature of the engine is high or if the high side reading for the AC is on the high end. The compressor may run even with the AC off to help remove moisture in the passenger compartment. Good Luck.
Mms :
Date: July 1, 2014 @ 4:15 pm
Thanks for quick reply indeed. I want to say that the technician may have some connection with the problem during this evaporator fix, where he might of forgotten something. I talked to him but he doesn’t understand wiring very well. Before this repair the fan was turning on only when the ac needs it. Now it is turning on before starting. Appreciate your time and reply.
dennisb - Auto Tool Sales :
Date: July 2, 2014 @ 7:33 am
OK, sometimes when there’s a poor connection or ground not properly made after doing a repair and having many connections undone. He should retrace his steps and make sure that ALL connections are plugged in all the way or ground screws are tight. Once I had a problem with the power seats not working properly after removing a dash and it ended up being an electrical plug that wasn’t plugged all the way into a module. Good Luck.
bert :
Date: July 2, 2014 @ 10:24 am
Hi, I checked a 2001 Toyota Avalon yesterday, the AC clutch engages and the high pressure goes to 400 psi in 2 – 3 seconds then shuts the compressor off, I sprayed the condenser coil with a water hose and no drop in high pressure, I shut the car off and it holds the high pressure and does not drop for at least 10 minutes? No bubbles in the sight glass. Help
Thanks
dennisb - Auto Tool Sales :
Date: July 2, 2014 @ 10:36 am
Possibly over charged or a restriction in the system like a stuck expansion valve. I’d check the low side reading also and see what the static pressures are on both sides after the pressures equalize. Good Luck.
Blue :
Date: July 4, 2014 @ 2:23 am
Borrowed my sons 2003 Chevy Trailblazer for a 4th of July Camping Trip. When he drops off the vehicle he lets me know the AC needs freon. I check and its blowing hot air. Too late, to have, it serviced the afternoon before I leave for my trip. So a go get some R134a, a fill gauge & tube set at my local autoparts store. But when I attempt to fill no freon will enter system. So I run over to HF & pick up
a full gauge set. When I put it on I see ~50 psi on both hi & lo. With motor idling & AC on I see low side range from 45 down to 0 and high side goes to 75-100psi. Actually, low side sucks down to 0 in about 4-8 seconds when compressor kicks on & rises a bit slower to 45 after it kicks off. Hi side climbs to 100 as compressor runs & falls to 75 while its off. With engine at 2000 rpm it cycles faster and low side actually draws down to about -20 psi vacuum. But when I try to add freon, the system continues to cycle but no freon will flow in. I know I’ve got flow on low side gauge because I can see pressure cycling and I know I have freon flow into gauge because when I close valve at low side AC connection I can produce pressure in the inlet line by shaking the freon can. Is this symptomatic of a clogged orifice tube?
dennisb - Auto Tool Sales :
Date: July 4, 2014 @ 9:48 am
I would say you probably have several issues. Low Freon is number one which means there’s most likely a leak. Since it’s not environmentally friendly to just add. I’d check for a leak with a leak detector. After any leaks are fixed I would vacuum the system down and add the proper amount of refrigerant (you could take a look at the orifice tube if it is equipped with one). After the correct system charge has been put in, I’d check both high and low readings. If the system is still going down into a vacuum there’s a restriction which could be a clogged orifice tube or a stuck expansion valve, whichever it has. If the high side doesn’t ever go up to at least 200 psi with a full charge the compressor is not pumping enough. Good Luck.
99 Ranger :
Date: July 4, 2014 @ 5:32 pm
Dennis: I am having issues with too much high side pressure, system kicks out on high pressure on ranger at 400-450 and the high side line is extremely HOT, low side seems fine 35-45. The system has been recovered and the compressor dryer and orifice tube replace ( the issue mentioned above was before repairs had been made ) After replacement of parts mentioned above, dropped in the recommended oil half in compressor and other half in dryer and pulled vacuum on system. Then dropped in recommended amount of r134a on ranger is 30 oz. when you start to get close to filling the system and it starts to cool a little the high side goes up again and causing the compressor kick off and on due to high pressure and the line is still very HOT. Had the system broken down again and had a feeling the condenser was a parallel and was stopped up I replaced condenser in front of radiator and started over again and lo and behold the same thing has happened. Im at a loss and im no AC expert but a average home mechanic working on his own vehicles. This issue is baffling to me and I don’t understand how i can have the same issue 3 times with new compressor, drier, orifice tube and condenser, note when I replaced major components the evaporator coil was flushed several times and blowed out with dried air for a while till all moisture was gone. This repair has costed me lots of money even a remanufactured compressor and aftermarket replacement parts also freon WOW, don’t know what else to do other than wishing i had taken it somewhere else to start with. This simple to moderate repair has been a major headache!!
dennisb - Auto Tool Sales :
Date: July 5, 2014 @ 10:09 am
45 on the low side is high and you already know the high side is far above normal. Most of the time two things cause too high of pressure;
1. The system is overcharged (with Freon, flush or oil).
2. There’s not enough air through the condenser to cool it sufficiently.
See the short article on “How to tell if an auto A/C compressor is faulty” for more details. Good Luck.
Rick :
Date: July 5, 2014 @ 10:39 pm
My Question has to do with the gauge set,in one of your earlier comments you stated that when testing pressures the high side manifold valve should be closed, this seems to make no sense. Does that mean the high and low side pressure is reading from the low side valve? Could you elaborate on this? When I turned both sides on, the high side pressure pushed the low side gauge to max. It seems to me that the 2 sides of the manifold should be separate.
dennisb - Auto Tool Sales :
Date: July 6, 2014 @ 7:52 am
Rick, both high and low valves should be closed when just checking the gauge readings. The manifold allows pressure from the vehicle to be measured. You only open BOTH valves when hooked to the vacuum pump. You typically only open the the low side valve when adding refrigerant. Opening both high and low with a charged system will allow the high side pressure over to the low side gauge as you’ve found out. Since the low side does not go up as high in pressure you may damage the gauge by pegging it out.
Rick :
Date: July 6, 2014 @ 9:27 pm
Thanks Dennis thats what i needed to know, that explains it perfectly,thanks for your time
Patrick :
Date: July 7, 2014 @ 10:39 pm
I just finished repairing system on 98 Rav4. I removed all components and flushed, cleaned debris from condensor and evaporator fins, replaced a sticking expansion valve, replaced drier, changed oil and added new R-134a. On a 100 degree day with the car running I’m getting gauge readings of low 35-40 and hi 180-210. The air temp at center vent on fresh air is 52 degrees. When referring to R-134a temperature pressure charts they say at 100 degrees low pressure should be 50-55 and hi pressure 315-325. I’m confused! My system seems to be cooling great! Can you give me your opinion on these numbers?
dennisb - Auto Tool Sales :
Date: July 8, 2014 @ 7:19 am
The readings are normally higher with increased outside temperatures. It depends upon several things, one of which is how much air is being pulled through the condenser. It also depends upon how many RPM’s the engine is running at when you’re checking the readings. The low side readings are typically higher at idle compared to 1500 to 1800 RPM’s. That’s because the compressor (which is a pump) is turning faster, therefore pumping more, making the suction side (low side) readings lower. The best indication that the system is correctly charged, is that it is working properly. So if I were you, even though you may not understand all the actual readings compared to specs; I wouldn’t be concerned because the most important number is the vent temp, which is good.
Chris :
Date: July 8, 2014 @ 4:07 pm
Dennis, thank you for providing your time and knowledge. It’s greatly appreciated.
I just bought a 92 BMW 325i for my daughter’s first car and the ac is only blowing warm air (72-78 deg). It’s an R12 system and I have not tried adding any freon but here are my initial readings. Static pressure is 100-105 psi, low side pressure is at 35-40 with ac max and car idling but drops to 20-25 when at 1500-2000 rpm. High side is at 310-330 idling and will go up to 350 at 1500-2000 rpm. Here’s what puzzles me the most. The center vent temperature, with all other vents closed, will get down to 72 degrees. If I open all the vents, the temperature rises to 78 degrees. Is this normal? And what would you recommend that I do / check first? Again, thank you for your time.
dennisb - Auto Tool Sales :
Date: July 8, 2014 @ 4:30 pm
Looks like it’s fully charged. Since you just bought it, it’s possible someone charged it with something other than pure R12 refrigerant. If need be , a shop can test it to identify the type of refrigerant. Not all shops have the refrigerant identifier tool, but should. First though, if the temperatures seem right on the AC lines (don’t burn yourself on the high side line). The suction should be cold and the discharge should be hot. This will very some depending upon where you check. It’s best to use an infrared digital thermometer for safety rather than physically touching all the lines. Basically, if the AC evaporator is getting in the 30’s you could have a door issue. Most likely the blend door. BMW’s aren’t always the easiest to work on, so it may take a tech to look into this further. As far as the vent temp being colder when all other vents are closed off that would be normal. But as you already know, 70’s is not cool enough.
Patrick :
Date: July 8, 2014 @ 4:57 pm
Dennis,
Thanks for the assurance on the 98 rav4! I’m so glad I was able to ask a pro. I will quit worrying about the numbers since it is blowing ice cold!