Air conditioning - pressure switch bad? Reply #30 – July 02, 2009, 04:35:06 PM In all the years I was wrenching, I never set an R-12 cycle switch to cycle off below 32* (about 30psi), and I adjusted many of them. If the evaporator surface temps drop below freezing, the water in the air will freeze to the evaporator and clog it. Shiny Side Up!Bill Quote Selected
Air conditioning - pressure switch bad? Reply #31 – July 03, 2009, 11:55:47 PM Okay, so when I went out there again to try to finish charging it, the static was at 90psi. I can only fill it to about 29-30/220-225 before it won't build anymore - I blame the orifice (more easily clogged if smaller) and/or the compressor itself (better to replace) for the inability to separate the two sides of the system more. While I DID have some icing of the liquid line before, it just builds moisture slowly now.So going off of this whole temperature thing, does that mean that this system is now slightly overcharged (maybe ideal)? I should be shooting for 26-28 psi for r134 in order to get near freezing? I haven't seen the actual concept of phase change but I assume the high pressure is built to better cool the liquid (and turn it to liquid period)? Please help me understand if this is correct - now that a big project at work is complete, my mind is much more relaxed and these things now make sense :pWith our engine bays and headers right next to the liquid line, is ~30 degrees/26psi ideal since it is so hot in the area? Without a larger evaporator or more efficient refrigerant, there isn't a whole lot more to gain I would assume - just stick it on max ac and wait. What affects what exactly to improve a/c performance on newer cars? Everything?Edit:This was only at ~1k rpm's - I assume the temperature would be lower at higher so perhaps I am in a safe spot. I will mess with it tomorrow at higher rpm's after the system has some time to settle and I can get another static reading. I'm sure this may help the compressor a bit more and let me dial in the pressure switch a little better. Quote Selected
Air conditioning - pressure switch bad? Reply #32 – July 04, 2009, 05:58:31 AM The way it works:The compressor turns the refrigerant into a High Pressure Gas, as when you compress anything, it becomes hot. The High Pressure Gas then goes through the Condenser where the High Pressure Gas is cooled to become a High Pressure Liquid. The High Pressure Liquid is then passed through a metered orifice releasing the pressure. When you release the pressure the High Pressure Liquid expands becoming quite cold, entering the Evaporator as a Low Pressure Liquid. This Low Pressure Liquid is then passed through the Evaporator where it Absorbs the the heat in the air passed through the atmosphere side of the evaporator causing the refrigerant to become a Low Pressure Gas which then makes its way back to the compressor. Note that the A/C system Absorbs Heat from inside the car and releases is outside the car. Shiny Side Up!Bill Quote Selected
Air conditioning - pressure switch bad? Reply #33 – July 04, 2009, 06:11:17 AM Quote from: Seek;280920Okay, so when I went out there again to try to finish charging it, the static was at 90psi. I can only fill it to about 29-30/220-225 before it won't build anymore - I blame the orifice (more easily clogged if smaller) and/or the compressor itself (better to replace) for the inability to separate the two sides of the system more. While I DID have some icing of the liquid line before, it just builds moisture slowly now.So going off of this whole temperature thing, does that mean that this system is now slightly overcharged (maybe ideal)? I should be shooting for 26-28 psi for r134 in order to get near freezing? I haven't seen the actual concept of phase change but I assume the high pressure is built to better cool the liquid (and turn it to liquid period)? Please help me understand if this is correct - now that a big project at work is complete, my mind is much more relaxed and these things now make sense :pWith our engine bays and headers right next to the liquid line, is ~30 degrees/26psi ideal since it is so hot in the area? Without a larger evaporator or more efficient refrigerant, there isn't a whole lot more to gain I would assume - just stick it on max ac and wait. What affects what exactly to improve a/c performance on newer cars? Everything?Edit:This was only at ~1k rpm's - I assume the temperature would be lower at higher so perhaps I am in a safe spot. I will mess with it tomorrow at higher rpm's after the system has some time to settle and I can get another static reading. I'm sure this may help the compressor a bit more and let me dial in the pressure switch a little better.What was the outside temperature? What was your vent temperature with he system set at Low Fan?On a 90* day, with the system set on Normal, Low Fan, you should see outlet vent temps around 50*. A 40* drop in temperature is quite good. With the system fan set on High and the engine running around 2000 rpms, the outlet temps should stay around the same, but your high side readings will go up a bit as you will be pulling the heat from a greater quantity of air. Ideally, in an R-134A system, the inlet line to the evaporator will be a couple of degrees cooler than the outlet line, and your High Side readings should fall somewhere around 2.5 times outside air temperatures. Shiny Side Up!Bill Quote Selected
Air conditioning - pressure switch bad? Reply #34 – July 04, 2009, 11:54:54 AM Low pressure on the low side = cooler temperatures but what exactly does higher pressures on the high side do? Just allow more heat transfer in the condenser? Quote Selected
Air conditioning - pressure switch bad? Reply #35 – July 04, 2009, 03:50:48 PM Quote from: Seek;280982Low pressure on the low side = cooler temperatures but what exactly does higher pressures on the high side do? Just allow more heat transfer in the condenser?High side pressures are, more or less, simply a function of low side heat absorption coupled with how full the system is.Shiny Side Up!Bill Quote Selected
Air conditioning - pressure switch bad? Reply #36 – July 04, 2009, 10:26:33 PM Wouldn't more refrigerant with the proper compressor and expansion valve/orifice allow more cooling? I see people talking about r134a pressures of 300+ on the high side and figure it just allows better heat transfer to the condenser. Quote Selected
Air conditioning - pressure switch bad? Reply #37 – July 06, 2009, 09:16:54 AM The import cars I have that use expansion valves tend to have higher high side liquid pressures. I believe this is because the valve dynamically changes the amount of freon being let into the evaporator. The ford system (and gm and most domestics) use the orifice tube which is about as dumb as they get and will always allow the same amount of freon through. For example, the 95 pathfinder cools best with a 300+ high side, my 03 Jetta likes 260-275 with a low side of around 30-35. These results obtained using a thermometer in the vents. I typically will vent the system till I start to see the temperature rise a little. Then charge in small increments checking the thermometer until it hits the low "flat spot".If you are really worried about the orifice tube, change it out. I just replaced my liquid line with one from advance for ~$30. I cut it open and it was full of crud (from the last compressor barfing). I would watch the compressor charge up hit 350 and shut down. The low side would drag down to 10 be very slow to recover. I think I am going to replace the condenser as well as it seems to have restriction of some sort. I also took this opportunity to flush the system with mineral oil. Quote Selected
Air conditioning - pressure switch bad? Reply #38 – July 06, 2009, 01:04:47 PM Yeah, I don't have any obstruction issues and besides the compressor and discharge line, all the rest of the parts are new/cleaned (which aren't old to begin with). My orifice tube is the stock r12 one - it lets more refrigerant past to the evaporator than I can get put into the high side - continuously runs the compressor at around the 30/225 mark.I have to mess with it some more as my cooling doesn't seem any different at 30/225 than when I was only at 180 on the high side/low charge with the compressor cycling. There was no "sweet spot" in between here either but not I don't have to hear the compressor continuously kick on and off. Quote Selected
Air conditioning - pressure switch bad? Reply #39 – July 06, 2009, 01:45:50 PM Take a deep breath :)Measure pressures at 1500 rpmOn Max A/CDoors open or windows downAre you charging with R134a 12oz cans? If so, put them in a bowl/can of hot water to make charging easier. The temperature increase will increase the pressure in the can and increase the flow rate from the can to the low-side of the system. If you shake the can and you feel any liquid swishing about, you haven't gotten the full 12oz of refrigerant out yet. ***Keep in mind if you get the water too hot, you could blow the can up***Our systems take ~40oz of R12. R134a conversions generally take ~85% of the R12 capacity. ~34 oz in our case. The PF condeser throws a wrench in the works as it has a different volume than the tube/fin condenser it replaced. I would hazard the PF condenser has less volume, but that is a WAG. Regarding static pressures. When you have a saturated mixture, the pressure is simply a function of temperature. You have a saturated mixture when there is both liquid/gas present. If you slowly filled the system on a 90 degree day, you'd see the pressure go from -29.xx in Hg to ~105 psi and then level off. It will stay at 105 psi until the system is completely filled with R134a. That is why you can't judge charge by pressure. It could have .5lb or 10 lb and the same pressure would register. A clogged orifice tube would cause short cycling because it tends to create a vacuum after the orifice. To charge the system:Use the hot water method to dump as much refrigerant from two cans while the system is static. Don't worry about the residual refrigerant in the first can as we'll get to that later. (You need two can taps to do it this way) Crank the car, run at 1500 rpm, max ac, high fan, doors open/windows downNote hi/low pressure and center vent temperatureSuck the rest of the refrigerant out of the second can using hot water againNote hi/low pressure and center vent temperatureAdd the rest of the first canNote hi/low pressure and center vent temperatureStart on third canNote hi/low pressure and center vent temperatureWhat you'll find is pressure will keep going up and then they'll plateau. This is the point where you now have liquid exiting the evaporator. You can add 4-6 more to get to the correct charge.If you don't want to pay attention to evaporator inlet/exit temperatures, just keep adding until you see the vent temps start to rise and then remove 2-4 ounces. Keep an eye on the pressure the whole time to make sure the high side isn't running away. In general, the high sides will run 2.2-2.9 times the ambient temperature. So the high side will be ~200-260 psi on a 90 degree day. Quote Selected
Air conditioning - pressure switch bad? Reply #40 – July 06, 2009, 01:49:01 PM Quote from: Seek;281179I have to mess with it some more as my cooling doesn't seem any different at 30/225 than when I was only at 180 on the high side/low charge with the compressor cycling.What fan speed and what rpm?What is the center vent temperature? Quote Selected
Air conditioning - pressure switch bad? Reply #41 – July 06, 2009, 04:13:16 PM HVAC on max a/c/"high" speed (position 4 only gives me position 3 now - I need to put rotary controls in. I used to be able to push the switch sideways to make high come on), mechanical fan with new standard output thermal clutch NOT at 1500 rpm's (haven't gotten around to rechecking with higher rpm's quite yet), vent temperature is only ~25 degrees cooler than ambient with the two passenger side vents closed - there's some wire in the way of their doors right now so they can't be opened). I have only tested this so far with 90+ degree ambient. The condenser doesn't get warm/hot enough to be uncomfortable to place my hand on the front. Liquid line and accumulator tubes coming out have light moisture buildup after ~10 minutes. Return line is cool at the compressor - discharge is scalding.I do not get better cooling going down the road than idle. Quote Selected
Air conditioning - pressure switch bad? Reply #42 – July 06, 2009, 05:02:03 PM I'm guessing you're undercharged. As ProTouring said, you want the evaporator outlet/inlet to be close to the same temperature. If the outlet is noticably warmer, then you don't have enough refrigerant in the sytem. The refrigerant is boiling off before it comes out of the evaporator. Ideally the evaporator is 'flooded' and liquid exits into the accumulator. A system with an expansion valve will have have an outlet ~6-12 degrees warmer than the inlet. The valve senses the temperature difference between the inlet/outet and changes the orifice size to achieve the desired superheat*. If the superheat is too low (outlet 2 degrees warmer than inlet), the orifice will get smaller. If the superheat is too high (outlet 20 degrees warmer than inlet), the orifice will get bigger. *Superheat is how much hotter the vapor is over it's saturated state. If you have a pot of water at 212 degrees at sea level. It would be a saturated liquid. If you boil the water, it is a saturated vapor. If you raise the vapor to 222 degrees, you have a superheat of 10 degrees. Quote Selected
Air conditioning - pressure switch bad? Reply #43 – July 06, 2009, 10:31:05 PM Okay, so perhaps I am understanding this a little better. I went to mess with the a/c and found that at "1400" (stock gauge - probably closer to 1300 if I datalog), if I wait for the lines coming out of the evaporator to just begin to freeze before cycling the compressor off (23psi on the gauges), I get the coolest temperatures. I am not sure if this is fine or if I should raise it 1psi on the low side (lines don't even get close to freezing) but I figure the frozen fins should quickly thaw when it cycles off, a bit slower than the lines in the engine bay with all the heat.At idle, the system can barely get below 30psi on the low side before the compressor can't keep up any longer. Of course, temperatures are much warmer but both idle never cycles off and 1400 rpm's cycles at about 10 minute intervals. Should I charge it just a tad more to always prevent freezing of the return line? It freezes at the compressor at about 24psi and on the accumulator side at around 23psi. I assume that as long as there is basically no moisture in the system ("20 micron" pump) and the evaporator isn't clogging up, shouldn't the system be fine with the above numbers? Quote Selected
Air conditioning - pressure switch bad? Reply #44 – July 07, 2009, 08:41:40 AM Quote from: Seek;281240Okay, so perhaps I am understanding this a little better. I went to mess with the a/c and found that at "1400" (stock gauge - probably closer to 1300 if I datalog), if I wait for the lines coming out of the evaporator to just begin to freeze before cycling the compressor off (23psi on the gauges), I get the coolest temperatures. I am not sure if this is fine or if I should raise it 1psi on the low side (lines don't even get close to freezing) but I figure the frozen fins should quickly thaw when it cycles off, a bit slower than the lines in the engine bay with all the heat.At idle, the system can barely get below 30psi on the low side before the compressor can't keep up any longer. Of course, temperatures are much warmer but both idle never cycles off and 1400 rpm's cycles at about 10 minute intervals. Should I charge it just a tad more to always prevent freezing of the return line? It freezes at the compressor at about 24psi and on the accumulator side at around 23psi. I assume that as long as there is basically no moisture in the system ("20 micron" pump) and the evaporator isn't clogging up, shouldn't the system be fine with the above numbers?I wouldn't add refrigerant at this point because your icing at 1400 rpms is a function of your cycle switch. On a hot day, it is likely the compressor will not cycle at idle, so as long as your high side readings aren't out of sight, I think you're OK.Drive it and see what happens. If on a highway cruise you loose a/c due to the evaporator icing over, then I would bum your cycle switch up one psi/*, otherwise, you are probably in good shape.Shiny Side Up!Bill Quote Selected
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