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Testing Subaru Pressure Switch

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  • #16
    Sounds like a plan. Ill see what I can find.

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    • #17
      Originally posted by Gnarsinski View Post
      So here is what I think I've figure out so far. With gauges connected, system has 80psi on hi and low side. When AC compressor is jumped and running low side is about 20psi and high side is 110psi.


      Both the high and low pressures are too low. Looks to be under charged. A leak some where is a virtual certainty. Under charged and the compressor protections are keeping it from running, and doing damage. Don't jumper it any more.
      Find and fix the leak, and put a proper charge in it....

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      • #18
        Pressure can only tell you temp at the point measured! static pressure of 80 psi equals a temp around 77 deg, running pressure of 110 (high side) equal a condensing temp of around 94, or 17 deg above ambient, not bad, remember, the condenser fans are running. 20 psi low side is 24 deg F, a bit cool, but if the evaporator itself hasn't cooled yet (system run long enough stabilize temps and pressures), that can throw off things a bit. Low, yes but not out of the ballpark..
        It may be a little light on refrigerant, but we don't have enough run time or info to say that for sure, At the pressures indicated I know of no safety that would shut the system down with those readings. It is classic proof to why state of charge can't be determined by pressure alone.
        1st and foremost solve the electrical feed issue that you observed, once that is solved, then move on to any remaining issue. Without a copy of the wiring, I can not give more advice.

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        • #19
          When low side is used for a cut-off the psi setting is quite low, IHC, for example, uses a 7 psi switch setting in the low side to shut the system down. High side shut downs a much higher but still well below the 80 psi range, and these would not trip with the compressor running until it has expelled virtually all the refrigerant. High side cut-out often are as low as 35 psi, anything above that and the compressor runs.
          The one exception is one CCOT systems the clutch cycling switch is set around 28 psi, When one of these systems are low, the clutch will cycle more and more frequently until it reaches a point where the static pressure falls below the switch setting and the compressor never comes on. On these systems there is no separate "low refrigerant" cut-out, rather they let the system cycle shorter and shorter until it stops all together.
          Last edited by Cornbinder89; 05-28-2017, 07:38 PM.

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          • #20
            Todays computers changed everything in the automobile world. Many systems will not allow the vehicle to hurt itself... Past systems that allowed a compressor to run as low as 7 PSI can hardly be called a protection system.

            Is that the case in this instance? Don't know. I do know that nobody yet claims to know how it works for this car. And nobody has any specicfics on just what the suspect switch does...It cannot even be found.

            I also know that 110/20 pressures are too low. Running the compressor longer will not improve that... It could wreck the system worse than it already is... Perhaps the computer is stopping it from hurting itself.

            At any rate, I would find a shop that can evacuate and measure what is reclaimed.... It will most assuradly be less than it should be. If it holds a vacuum, then put the proper charge in it. If the system then works, chasing all these electrical gremlins is not necessary.

            If it was low, then a slow leak must be found. The most likely places are the schrader valves, or the front compressor seal.

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            • #21
              Actually, 7 psi on the low side will shut the compressor off well before any high side limit switch will and while there is still enough refrigerant to move oil. It controls a latching relay which will not re set until the problem is solved. A high side pressure switch will keep the compressor on until it has pumped all the refrigerant out of the system, a low side will shut the system down when there is way more in the system, well above 35 psi static pressure. The low side safety doesn't prevent the initial start of the compressor (unless there is no refrigerant) it shuts the system down and locks off when an operating system is below 7 psi on the low side, a 4 lb system will trip the lock-out with something slightly less than 2.5 lbs, well before a typical high side switch of less than 1 lb in the system. On these systems it can be difficult to charge without pumping liquid into the high side, because the low side charging will trip the safety and prevent pulling the charge into the system, esp if small cans are used.
              My point is the pressures are well within operating range, and evacuating and attempting to charge before solving the electrical issue is putting the cart before the horse. I haven't the wiring diagram in front of me, but according to the O/P who has, he doesn't have the correct 12 volts to the limit switch from the fuse, so any attempt to correct the charge would be a moot point. It would be impossible without a tools that can pump the complete charge in, and you don't know it is low to begin with! 120 psi is not low if the outlet of the condenser is at 94 degs rather it is spot on! same for the low side. Remember, a Tx doesn't limit the temp of the evaporator, it limits the amount of refrigerant so that it all boils by the outlet, by comparing the inlet temp/pressure to that of the outlet. If it limited temp you wouldn't need a frost switch. SO if the metal core of the evaporator hasn't cooled enough to trip the frost switch, and low pressure reading of 20 could be an accurate read of the refrigerant temp. You can't assume charge by pressure without taking temp into consideration.
              Quite often I will see low side pressures in the low 20's and high side around 100 when I 1st start my system, if the weather is coolish and the condenser has air being drawn over it Once the evaporator cools and the system runs longer the pressure stabilizes around 28-32 for low and high depends on the air temp across the condenser. If the condenser is larger enough and doing its job, your high side TEMP (and therefore pressure) can be in the range of 5-10 deg above the air passing thru it. You don't need nor want pressures in the 180-200 psi range, that just shows the condenser isn't shedding the heat!
              yes, it may be a little undercharged, it probably is , but not to the point any system would shut down, those pressure could be seen in a fully charged system under the right conditions. A very least a properly working system would start under that static pressure, so that is not what is keep the compressor off. Even ECM controlled systems have to have the system operating to determine if something is out of range, that static pressure is within operating range. Solving what is preventing the compressor from starting HAS to come 1st. then move on to the charge amount.
              Last edited by Cornbinder89; 05-29-2017, 10:21 AM. Reason: added info

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              • #22
                Let me try and explain another way. On initial start, the Tx valve is trying to meter vapor as all the liquid has settled to the low spots in the system. The Tx can't flow enough volume of vapor thru (not designed to meter vapor), and the low side pressure drops below what you would expect to see, once liquid reaches the Tx it expands 10x the volume when it turns to vapor in the evaporator and now the low side pressure begins to rise. It is why you need to run the system until it stabilizes before making any judgements based on pressure readings.

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                • #23
                  ? OIl and when it circulated and mandatory - the subject just that: Not all the same apparently as some stay on well lower than refrigerants could move it alone? I'll just plead IDK for a few some always turn and no clutches so somehow a different concept for me to learn. Old sump, plain pistons ones at least saw one just used to pump air?? Not for A/C at all.

                  It's my observations that by the point you don't notice a respectable pressure high vs low you'll get a noisy compressor for lack of oil but not for too long. Supposed to shut down and notice most do not totally quit soon enough for my liking nor many compressors that fail just because of that,
                  Tom
                  MetroWest, Boston

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                  • #24
                    Remember the "thermal limiter" GM used? it detected low charge by return temp. It was one of the more sensitive shutdowns. The low pressure switch is more complicated and makes the system harder to charge, but mimics or exceeds the sensitivity of the thermal limiter. Unless CCOT, most automotive just use a high pressure switch, which is fine if the system looses charge over the winter and prevents it starting in the spring, but does little to protect a system in operation.
                    I guess the newer PCM controlled A/C (where they are used) sense low pressure side and would be better at shutting down the system if enough perimeters are crossed. As we learned on the old site, the guy with a plugged high pressure line, the system will turn on, then cycle when the pressure gets out of whack. Even there, it didn't shut down the system and lock it off.
                    I have seen more than one York style recip being used as an air compressor, but with zero oil control, and no cooling, they don't last if used more than vey intermittently. I had a service truck that was set up that way, you had to add oil daily and alot got into the air tank. I replaced with a gasoline driven compressor.
                    Last edited by Cornbinder89; 05-29-2017, 11:28 AM.

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                    • #25
                      Yes! Thru much of the 1980s at least into some of 90s didn't use a LPCO on CCOT systems I think the thermal thing was called a "thermistor" was both on time AND system would NOT turn back on until battery was disconnected! Had a GM sticker that said to do that when servicing! 92 Coupe DeVille I recall by the car - it was blue and a compressor clutch had failed bearing and think a rubber type shock absorbing part of it was not available for some reason owner put a whole new compressor on it just for that - twice mind you messing up hose manifold to compressor fixed then by me but wouldn't charge up then noticed the sticker!

                      Another was a 1983 RWD also Coupe De Ville I just put a "T" I think and a LPCO switch on it as it was a freebie job by me just then behaved like an Olds of same year. TMK that idea was Cadillac only but can't prove that just all I saw of those were Cadillacs,

                      Tom
                      MetroWest, Boston

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                      • #26
                        The thermal limiter I'm talking about was common from the 70's thru mid to late 80's The 1st GM compressors had no switch on the them, then they had a thermal switch with a probe in the low pressure cavity in the pump, then they went to a high pressure fan switch in the compressor I am talking in the A-6/R-4 timeframe.. I never had one like you are talking about. The problem or design of the Thermal limiter type was it was a three terminal "fuse" that when the temp probe closed it grounded the wire to the clutch and cause the internal "fuse" to burn. You had to replace the limiter with a new one to reset the system. On the plus side, once tripped the system stayed shut down until the limiter was replaced. Often people would "jump" the terminals to get the compressor to run, and with no repair the compressor would soon fail.
                        The IHC system trips and lock off the system until the key switch is turned all the way off, then re-sets, if the problem is gone, it will run, if the problem is still there, the compressor will kick in until the low pressure line drops to 7 psi and then it locks off again. The switch is placed in the outlet/suction line from the evaporator, but near the evaporator so as not to trip prematurely as it might if the located near the compressor and it was engaged with the engine at a higher rpm. It takes very little run time to reach 7 psi if the system does have a problem. Even at 7PSI low side, enough refrigerant/oil mixture would make it back to the compressor. Add to that a compressor with a large enough oil sump, and you have a robust system. Some of these small compressors today lack a large sump and rely on oil returning to the compressor to both cool and lube it. I think they are much more sensitive to a low charge starving the compressor for cooling and lube.
                        Edit:
                        I should add on the IHC system the relay latches and locks off the system even if the pressure at the switch rises above the reset point (24-32 PSI), which it most certainly would when the compressor shuts down. It will not allow a re-start until all power is shut off, and the engine re-started. As long as the power is on, the relay remains latched. This prevents constant cycling of a defective system, once a defect is detected it locks off. This prevents the short cycling that is so common to CCOT systems, prevents clutch damage/wear as well.
                        Last edited by Cornbinder89; 05-29-2017, 12:35 PM.

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                        • #27
                          OK. I remember those and owned ONE! Never named it just followed why I couldn't get a compressor to engage. I think that fuse thing was square and like me bought a few. No clue that long ago why it blew (probably rusted the thing somehow?) but it worked fine after that and there was no refrigerant problem in that car? Didn't own it long for much work or a check - was for me almost free and didn't care for that one of zillions of cars of my own. Was last full size GM body for the models a 1976 Buick LeSabre with an A6 I think?

                          Nothing wrong all around, I just didn't like the car so forgot all about that thing done that way on that model then - of that I'm not mistaken. Guess it's now very old days used to keep 4-5 cars at a time for myself and loaners so that was a loaner the person bought needing anything that ran at the time.

                          Good driveline (real Buick Engine, 350) but distributors in the front always got soaked in rain a constant problem otherwise nice smooth total pigs on fuel,
                          Last edited by Tom Greenleaf; 05-29-2017, 12:44 PM. Reason: Add more info
                          Tom
                          MetroWest, Boston

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                          • #28
                            Yeah, it had three terminals, often it was right on top or near the compressor, but could be far away. The three terminals were 1) power in from the A/C or frost switch 2) power out to the clutch 3) wire to the temp probe in the compressor, When the probe heated up above a set temp, it closed to ground, which burned a section of Ni-Chrome wire in the "limiter" breaking the connection to the clutch. You could test by jumping the 1st two terminals.
                            The down side is if the temp probe wire fell off the compressor you had no protection. Something that did happen as it was a "pin" type terminal I don't know what the set point would be, but it would have to be well above underhood temps.

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                            • #29
                              Back to last post by Huntindog" ">Todays computers changed everything in the automobile world. Many systems will not allow the vehicle to hurt itself... Past systems that allowed a compressor to run as low as 7 PSI can hardly be called a protection system.

                              Is that the case in this instance? Don't know. I do know that nobody yet claims to know how it works for this car. And nobody has any specicfics on just what the suspect switch does...It cannot even be found.

                              I also know that 110/20 pressures are too low. Running the compressor longer will not improve that... It could wreck the system worse than it already is... Perhaps the computer is stopping it from hurting itself.

                              At any rate, I would find a shop that can evacuate and measure what is reclaimed.... It will most assuradly be less than it should be. If it holds a vacuum, then put the proper charge in it. If the system then works, chasing all these electrical gremlins is not necessary.

                              If it was low, then a slow leak must be found. The most likely places are the schrader valves, or the front compressor seal.<"

                              The fail safe features of this is now a 2010 are most likely sensing pressures and computer disabled. Many as said there just recover a known amount and replace to known amount you know how low if was or if low at all. I just can't say individual vehicles if will auto reset or you reset it with a specifically enabled code reader/scanner able to do so, some can be dealer only as they cost way too much.

                              Leaks still first for a reason for A/C problem but 100s of other reasons. Need to know if Schraders, ball valves and if two ports or one. I think Subaru has kept two ports still can't be sure by the year what changes might be?

                              On the comments if a vehicle holds vacuum you think and probably right that it doesn't leak.

                              Know some things: You are only under vacuum holding OUT 14.7 PSI of air not holding IN perhaps 300+ PSI if only an area or momentarily. Mess with your head more some "O" rings will seal under vacuum and audibly hiss with pressure not liking changing direction of pressure.

                              Side note: My own now newest vehicle is over 20 - super low miles. Another almost 30 can't be left out in colder than may 10F or figured out shaft seal of compressor the seal and shaft do not expand and contract at the same rate when that (a few now) left in especially below zeroF temps as they are found low in Spring and don't leak?? Leave it in garage it goes years and doesn't leak for the car now antique by chance still all OE and original R-12 just 3 boosts by me known since car was new and history of it!

                              The snag is you find it low and not leaks nor oily evidence?? It wasn't running at all when it leaked can be the only answer. I believe shaft is steel and seal is a porcelain and do not keep the seal when that cold and just FYI lowest cold last Winter where I am was watched by me on memory thermo was -14.5F which is within norms for where I am just a little colder than some most years.

                              Point - stuff happens you may not find active or evidence. Would a code reading recall this info even the best code readers? NO. After "X" number of combo of miles and cycles of cold to operating temps codes that do not reoccur are lost/forgotten,
                              Tom
                              MetroWest, Boston

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                              • #30
                                Originally posted by Cornbinder89 View Post
                                My point is the pressures are well within operating range, and evacuating and attempting to charge before solving the electrical issue is putting the cart before the horse. I.
                                Think about what I said, the pressures both static and dynamic are well within the operating range of a properly charged system, So even if PCM controlled, it should at least start. The low side can dip below the operating range on start-up and any PCM would need to account for that. Could be done with a time delay or a trip pressure below say 10-15 PSI.
                                Tom I know what you mean about holding under pressure but not vacuum, A quick-connect (found on some HD trucks driers) will leak under vacuum but hold pressure, can send you chasing your tail.
                                When I made an APU (aux power unit) I had quick-connects where it tied into the system, and they would leak under vacuum if you tried to charge the system with the APU disconnected from the A/C system

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