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OK..... some weirdness going on here.

 

1995 Legacy 2.2 98K.

 

AC compressor cycles on and off and air gets nicely cold. However, randomly and frequently now, it decides to cycle off and then won't cycle back on again for some time. Car get's hot, windows go down and then it just starts to work OK again.:-\ All fans work fine.

 

The charge seems to be fine.

 

Any ideas?

 

Thanks in advance! Todd

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So you can actually tell that the compressor's clutch is not engaging after a while? The AC button is pressed, but the compressor's clutch does not kick in, correct? Have you hooked up a(set) of gauge(s) to see what your pressures are at? I would at minimum check the low side to see where it's at.

 

As a test, turn on Max AC and set your fan speed to 4 and let it run for a while and see if it turns off. The compressor should not cycle in this mode, and if it does, there is something wrong somewhere. This should rule out too much pressure building up in the system triggering the high pressure shut off switch.

 

If the compressor's clutch is not engaging after a while when it was before, try wiring the compressor right to the battery and see if it comes on, after it has stop working. If it does come on, then the compressor is most likely fine, it's probably a wiring/sensor/relay issue, not a mechanical issue. If the compressor does not come on, then most likely there is something up with the mechanicals of the system.

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the compressor gets the signal to turn on and off via pressure switch on top of the dryer most likely the pressure is low.. btw when you do the test with the A/C on and on high open the windows since on the evaporator core is a little temp sensing probe that also tells when to turn the compressor on or off.

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did you check pressures? sounds like it's off a little and just going out of limits at certain times. that it's "getting worse" may suggest it's a slow leak.

 

most common leaks are the two orings on the compressor fittings. 15 minutes and it's resealed. did an 00 forester and 03 OBW in the past year.

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did you check pressures? sounds like it's off a little and just going out of limits at certain times. that it's "getting worse" may suggest it's a slow leak.

 

most common leaks are the two orings on the compressor fittings. 15 minutes and a recharge and it's resealed. did an 00 forester and 03 OBW in the past year.

 

Fixed for you :)

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Why is the compressor cycling ON and OFF normal on a '95 Legacy?

My '93 Leg AC has been operating without being serviced for 18 years using R-12 freon. When the AC button on the '93 is ON the compressor never stops and unless some heat is blended in with the AC or its 100F I will need a coat. Blending heat with AC is the way to control cabin temp.

 

The compressor should only stop if there is a problem: high pressure-> cut off the compressor or there is moisture in the system and it freezes, pressure builds up then -> cut off the compressor.

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Why is the compressor cycling ON and OFF normal on a '95 Legacy?

My '93 Leg AC has been operating without being serviced for 18 years using R-12 freon. When the AC button on the '93 is ON the compressor never stops and unless some heat is blended in with the AC or its 100F I will need a coat. Blending heat with AC is the way to control cabin temp.

 

The compressor should only stop if there is a problem: high pressure-> cut off the compressor or there is moisture in the system and it freezes, pressure builds up then -> cut off the compressor.

 

Yes, cycling is normal behavior for some automotive ac systems. The compressor only needs to compress when it needs to cool, not all the time. That's what the txv is for (thermal expansion valve). Hence why whenever you add/fill refrigerant in an automotive AC system, you turn on max ac, and your fans on high. This is to make sure the compressor does not cycle.

 

If yours is on all the time, then there might be something wrong with it, since even my wife's 2005 kia spectra's ac cycles.

 

The last part of your statement I believe is just wrong all together...

Edited by eulogious
clarification
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See a good explanation on the txv. I would still say the compressor stays ON, then you blend heat or wear a coat. High AC means in a Legacy that no outside air is coming INTO the cabin, get a bigger coat.

 

Thermal expansion valve

 

From Wikipedia, the free encyclopedia

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300px-PHT.jpg magnify-clip.png

Thermostatic expansion valve

 

 

A thermal expansion valve (often abbreviated as TXV or TX valve) is a component in refrigeration and air conditioning systems that controls the amount of refrigerant flow into the evaporator thereby controlling the superheating at the outlet of the evaporator. Thermal expansion valves are often referred to generically as "metering devices".

Flow control, or metering, of the refrigerant is accomplished by use of a temperature sensing bulb filled with a similar gas as in the system that causes the valve to open against the spring pressure in the valve body as the temperature on the bulb increases. As temperatures in the evaporator decrease, so does the pressure in the bulb and therefore on the spring causing the valve to close. An air conditioning system with a TX valve is often more efficient than other designs that do not use one.[1]

A thermal expansion valve is a key element to a refrigeration cycle; the cycle that makes air conditioning, or air cooling, possible. A basic refrigeration cycle consists of four major elements, a compressor, a condenser, a metering device and an evaporator. As a refrigerant passes through a circuit containing these four elements, air conditioning occurs. The cycle starts when refrigerant enters the compressor in a low pressure, low temperature, gaseous form. The refrigerant is compressed by the compressor to a high pressure and temperature gaseous state. The high pressure and temperature gas then enters the condenser. The condenser precipitates the high pressure and temperature gas to a high temperature liquid by transferring heat to a lower temperature medium, usually ambient air. The high temperature liquid then enters the expansion valve where the TX valve allows a portion of the refrigerant to enter the evaporator. In order for the higher temperature fluid to cool, the flow must be limited into the evaporator to keep the pressure low and allow expansion back into the gas phase. [2]

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Thanks for proving my point. The txv is a metering device, meaning that it controls the flow. So when the compressor compresses enough, and the temp is correct, guess what happens? It shuts off because the txv won't let anymore refrigerant through, causing the pressure to build up on the high side, causing the compressor to stop compressing due to the pressure. Then when the temp rises, the txv opens, allowing refrigerant to flow and also causing a pressure loss at the same time, and naturally kicking the compressor on to raise the pressure. And this is called cycling. So thanks for linking to that, I really didn't want to do that earlier :)

 

Argue all you want, but my kia, my folks' mdx, my legacy, my old loyale, and my mr2 all cycle so ya. I could go on and on with examples, but I will take what actually happens over what someone "thinks" happens :rolleyes:

Edited by eulogious
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Just came to me. When the '93-9X Legacy AC compressor is ON it remains ON until you drive up a hill with the pedal to the metal. This AC cut off switch is intended to give max horsepower to the engine, hence AC goes OFF until you back off the accelerator pedal

 

The fans may run ON/OFF to keep the condenser in balance, but the compressor is "ALLWAYS ON". The windshield defrost works similarly compressor would allways be on and may need to blend in some heat at times.

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check your ac thermoswitch, everyone jumps to , ohh your ac system is screwed, when in actuality its the thermoswitch shutting the system down cause its going bad.

 

the ac on my 91' legacy is original r-12, system is full, no leaks, etc. of course i only have 98k on it (original)

the ac system shuts off and on at random, and at random speeds. my compressor will kick on and off sometimes in 1/2 second intervals makin the car shudder.. other times it will run fine for a nice long drive..

 

i've had to bypass the thermoswitch to have my ac work consistently. if the pressure switches are working fine, there will be no cause for alarm with the thermoswitch unhooked..

 

just my 2 cents..

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WOW...so much AC knowledge!! Thanks!

 

I will again check pressures. Does any one know the correct #s? I do know it's not the compressor clutch, as I can jump it to operate with the battery. It's worse when it's "not so hot", such as trying to defog in cooler wet weather. Also seems to cut out for l o n g e r periods when it's really hot. Works best on "medium" days. (high 80s -low 90s)

 

Thermoswitch....is that at the evaporator?

 

Lot's to look at now!

 

Thank you guys so much. I will report back.

 

Todd

Edited by SUBARU3
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High AC means in a Legacy that no outside air is coming INTO the cabin, get a bigger coat.

 

Just want to correct this statement as well. You are correct when you say that max ac means no outside air coming into the cabin, but what you don't understand is what else that does with the ac system, especially because of the txv.

 

The reason that you turn on max ac is so that you can get the air flowing the fastest over the evap. The reason you want this is because you need to keep the evap at a temp that will keep the txv open. If you don't keep the txv open, the compressor will cycle, which is not what you want when you are adding refrigerant to the system or checking the system, or when you want the ac to put out all that it can, like when you press the max button. When air flows over the evap, the cool air is "lifted" from the evap and combined with the moving air creating cold air which cools the cabin, and at the same time causing the evap temp to rise and therefore causing the txv to reamain open to allow more refrigerant through to try cool the evap. If you are not using the highest available air flow setting, you risk cooling the evap enough to close the txv and cause the compressor to cycle off. This is why you run the ac on max, and the fan on high. This has nothing to do with the temperature inside the cabin of the car, and therefore there is no need for any type of coat at all, since you are standing out side the car working on the ac system. I don't understand why this coat keeps being brought into the picture and why you keep mentioning blending air. Seriously has nothing to do with the OP's question.

 

Just need to make sure that there is no mis-information being thrown around here.

 

I will again check pressures. Does any one know the correct #s?

 

I believe the low side should be between 30-40 psi, give or take a little. The high side should be about 2.2-2.5x the ambient temperature, so on a 80 degree day it should be between 176-200 psi, give or take a little.

 

I do know it's not the compressor clutch, as I can jump it to operate with the battery. It's worse when it's "not so hot", such as trying to defog in cooler wet weather. Also seems to cut out for l o n g e r periods when it's really hot. Works best on "medium" days. (high 80s -low 90s)

 

Good to know it's not the compressor. The rest of this statement tells me right off the bat that you are low on refrigerant, and here's why. One of the cool things (pun totally intended :)) about ac is that the warmer the air outside, the higher the pressure is, naturally. Even without your compressor pumping, the pressure in your system will be higher when it's hotter outside. This works great because it allows for great cooling in hotter weather, all done naturally. This is also why I mentioned that the high side pressure is related to the ambient temperature. So when it's colder out there is not as much pressure, so the system turns off. But when it warms up, there is just enough pressure to turn the system on.

 

It might also be cutting out on really hot days because the compressor compresses to much on the high side because of the pressure. The only way to create pressure is by moving liquid, and if there is not enough liquid to compress on the suction side, and the txv is closed, then all the fluid gets stuck in the high side, then obviously it will turn off because you will start creating negative pressure on the suction side because there is no fluid to compress since it's all stuck in the high side lines, causing the compressor to shut down as well.

 

So my money is on low refrigerant at this point. But I am not a betting man, so take that for what it's worth :lol:

 

Thermoswitch....is that at the evaporator?

 

I believe he is referring to a temp switch/sensor in the engine bay ( or somewhere...), not the txv. The txv is located right next to the evap. The temp switch will tell the ac compressor to not kickin/kick in depending on the ambient air temp. This is because the ac is used on the defrost settings, and during the winter, if the compressor turns on and starts cooling things down, things might start freezing and bad things could happen. So there is another temp switch that reads the ambient air temp to tell the compressor when to turn on based off the temp out side. But I do not know where that sensor is located...

 

HTH!

Edited by eulogious
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It's worse when it's "not so hot", such as trying to defog in cooler wet weather. Also seems to cut out for l o n g e r periods when it's really hot. Works best on "medium" days. (high 80s -low 90s)

 

the predictability of it suggests pressures, which we've already talked about rechecking, are going to tell you something. good luck, hope it's simple.

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Ok...I played with the AC today. (105 degrees!!!! :slobber: Car at idle, shop fan in front of radiator, windows closed).

 

It seems the high pressure cutoff is cutting the AC and then it has to wait for the pressure to come down to cycle back on. The cutoff seems to be 400.

 

After removing some charge, the high side climbs to about 300ish when revved to 2000 rpms......and the low side stay about 30ish. The air is cold in the car at 43 out the vent. It seems to cycle appropriately now, but I will drive it tomorrow in the heat of the day and see.

 

Oh, If I add any charge at all, the high end goes too high.

 

Do I have a marginally failing expansion valve? Shouldn't the low end be a bit higher and the high end be a bit lower.....or is this hell we call "TX...it's a dry heat" causing it?

 

Thanks!! Todd

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Hmmm... Too much charge, wasn't really expecting that!

 

Here's a link to a good run down, troubleshooting guide I found that explains the AC stuff pretty well. Best explanation I could find!

 

http://www.aircondition.com/tech/questions/82/

 

So after reading that, it does look like your expansion valve might be stuck open, causing your AC to act weird :( I am not an expert, but that does sounds like the symptoms you are having, not exactly, but a little bit like it. It seems like your compressor is fine, since it will compress up to 400 psi just fine, so that really only leaves one other moving part of the system, and that's the txv.

 

Luckily the txv is only like $20, and it's super easy to replace it. The downside is that you have to evacuate the system, and then refill it. Unless you have access to this stuff, then it's no problem. Took me about an hour to a hour and a half of actual "work" to get it all done, including let it pull a vacuum for a good while. So it's easy to do, if you have access to all the AC stuff to do it correctly...

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Well, today it didn't break 100 degrees in Dallas! Yipee!

 

Anyway, pressures running 30ish and 250ish at idle. However, when you "1st" turn on the AC after the car has been sitting, or the 1st start of the day and the AC will not turn on for a few minutes. It seems that the engine heat may need to increase the low pressure enough to allow it to cycle on. Maybe that is why it won't turn on in cooler weather. The low pressure is triggered. If I add freon, the high side cutoff starts!

 

So all that combined with a higher than normal pressure that climbs quickly with RPM increase, seems to be pointing strongly to the exapansion valve. You agree?

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I don't think I would rush to change the txv, I'm guessing you may have an ambient sensor keeping it from coming on initially. (I haven't looked at a schematic for your vehicle so I don't know for sure what it has) If you jumpered 12volts to your compressor clutch to engage it and the system acted normal it would point to a control / safety in the circuit. Also a question I would have is if you have non-condensables in your system causing the high side pressure to be excessive. It could be easy to get some air and moisture into the system during charging or checking pressures and that would influence your high pressure safety tripping off.

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There is something up with your system, but what it is I really don't know. Its not really following the txv troubleshooting stuff. I would start looking at the sensors and seeing if they are all working properly...

 

When was the last time any work was done on the AC system of the car? Maybe a good evac, suck down, and recharge would help a bit. This really isn't easy if you don't have access to the stuff though, so that's kinda a last resort. BUT if you do have access to AC, might be an option.

 

Capn_r is the one who helped me with all my AC work, so he's the man when it comes to AC, IMHO :)

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Actually, I could draw it down. I have a vac pump. Just have someone recover.

 

It's never been serviced....original system.

 

If I draw it down, is it worth it to replace the compressor with a rebuild? Or just keep the original. It seems to be working OK. I have a rebuilt on the shelf....I got cheap on eBay!

 

Todd

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