A/C pressures: off vs motor running

[blb9t]

My A/C has been blowing hot air since it started to get warm around here. I figured it needed a re-charge, so I connected an a/c manifold gauge to see how low I was. With the car off I'm seeing readings of 50 psi on both the high and low port sides. With the car on and the a/c on max the low side is reading 30 psi, which is normal, however the high side is now reading 25 psi :confused Shouldn't the high side be in the 200-300 psi range with the car on?

[flyfishvt]

My manifold set has different scales for different types of refigerant. I see yours does too but I dont' see a scale for R134A on it.

Dont forget that you should let the a/c cycle a few minutes before you check the pressures.

[blb9t]

My manifold set has different scales for different types of refigerant. I see yours does too but I dont' see a scale for R134A on it.

Dont forget that you should let the a/c cycle a few minutes before you check the pressures.

the scales for each refrigerant are just the temps which I'm not too concerned about. It's the low pressure on the high side that I don't get. I'll let it cycle for a few minutes tomorrow and take another reading.

[benemorius]

It's never going to cycle off without any cooling going on, and no amount of cycling or crossed fingers is going to help a high side pressure like that. Are you quite certain that the compressor is engaging properly and fully? Your compressor seals or expansion valve would have to be very nearly nonexistent to cause pressures like that. Where do the needles go when the compressor stops, and more importantly how quickly?

EDIT:
Well after actually looking at your images, I see that the difference between running and not running is quite distinct indeed. More than anything it looks like a restriction in your refrigerant flow. I'd recommend a thorough servicing. Replace the expansion valve and drier, flush the entire system and in so doing locate and remove the restriction, and if you find a hint of metal shavings anywhere then flush extra well for good measure and replace the compressor too.

[328 Power 04]

Seems you are definitely low on refrigerant. If I remember correctly when the system was off in the Z3 (most recent AC work) I had 110psi on both sides. When compressor went on, low side went to the 30's and high side went higher to 250-300.

That or you could have a block... but it depends how the pressures respond when you turn on the system.

[blb9t]

It's never going to cycle off without any cooling going on, and no amount of cycling or crossed fingers is going to help a high side pressure like that. Are you quite certain that the compressor is engaging properly and fully? Your compressor seals or expansion valve would have to be very nearly nonexistent to cause pressures like that. Where do the needles go when the compressor stops, and more importantly how quickly?

EDIT:
Well after actually looking at your images, I see that the difference between running and not running is quite distinct indeed. More than anything it looks like a restriction in your refrigerant flow. I'd recommend a thorough servicing. Replace the expansion valve and drier, flush the entire system and in so doing locate and remove the restriction, and if you find a hint of metal shavings anywhere then flush extra well for good measure and replace the compressor too.

The a/c worked fine last year, so I'm thinking the seals couldn't have broken down completely over the winter like that. I replaced all of the o-rings in the system a couple summers ago as well as the dryer. I'll have to go back and see what else was replaced then; I'm pretty sure the compressor was not though. How can I tell if the compressor is engaging properly and fully?

Also, I thought a block in the flow would create a higher pressure than normal at the high side port?


Seems you are definitely low on refrigerant. If I remember correctly when the system was off in the Z3 (most recent AC work) I had 110psi on both sides. When compressor went on, low side went to the 30's and high side went higher to 250-300.

That or you could have a block... but it depends how the pressures respond when you turn on the system.

I was thinking I was low on refrigerant, however with the a/c on I'm seeing readings ~35 psi on the low side. Doesn't that indicate an adequate amount of refrigerant? The high side actually lost pressure when the system was turned on instead of increasing.

Took another reading yesterday. High and low side were equal at 65 psi at rest. With car on after running a/c for 5 minutes the pressures were: low = 35 psi, high = 30 psi. So, not too different from my original readings

[328 Power 04]

The a/c worked fine last year, so I'm thinking the seals couldn't have broken down completely over the winter like that. I replaced all of the o-rings in the system a couple summers ago as well as the dryer. I'll have to go back and see what else was replaced then; I'm pretty sure the compressor was not though. How can I tell if the compressor is engaging properly and fully?

Also, I thought a block in the flow would create a higher pressure than normal at the high side port?



I was thinking I was low on refrigerant, however with the a/c on I'm seeing readings ~35 psi on the low side. Doesn't that indicate an adequate amount of refrigerant? The high side actually lost pressure when the system was turned on instead of increasing.

Took another reading yesterday. High and low side were equal at 65 psi at rest. With car on after running a/c for 5 minutes the pressures were: low = 35 psi, high = 30 psi. So, not too different from my original readings

Well the compressor actually has to compress to get to the high point of 200+psi. So either you're really low, or your compressor may be going out. The high side is what needs to get compressed, and it is the difference between the high and the low that will give you cold air. I said you may be really low because your inactive pressures are too low.

[blb9t]

Well the compressor actually has to compress to get to the high point of 200+psi. So either you're really low, or your compressor may be going out. The high side is what needs to get compressed, and it is the difference between the high and the low that will give you cold air. I said you may be really low because your inactive pressures are too low.

ah, ok that makes a lot more sense now. I really hope it's not my compressor. I'll try adding a little refrigerant to see if the high side pressure remains the same or goes up. I just worry about over-charging it as the low side port is showing a normal pressure of 35 psi.

[jsmn4vu]

I'm thinking the seals couldn't have broken down completely over the winter like that.
It's important to run the A/C every week or so during the winter to keep the seals sealing. It's possible that they have not been damaged, and you just need a recharge.

[heifetz17]

It's important to run the A/C every week or so during the winter to keep the seals sealing. It's possible that they have not been damaged, and you just need a recharge.

+1. Once you get it sorted out, run the a/c on a regular basis. Leaving it off for months on end doens't allow the oil in the freon to circulate, and the seals dry out and leak. #1 cause of a/c problems!

[benemorius]

You can observe the compressor as the clutch engages to see that nothing looks or sounds out of place. Things like noisy engagement and slipping belts would be out of place. I'm not suggesting you're likely to find the problem here, but it certainly deserves a quick look so you don't end up kicking yourself later for skipping something simple.

Restrictions will cause varying things as the locations of potential restrictions and of the service ports are variable. It's the drop in high side pressure that particularly gets my attention in your case. The endpoints of the familiar high and low sides are formed by the compressor and expansion valve. Adding a restriction effectively adds another side, so the two service ports can't show the whole picture. As I recall, the e36 service ports are located on the firewall near the evaporator. Adding a restriction anywhere between the compressor and high side service port would reduce the pressure on the gauge. That includes almost the entire high side including the condenser and drier. A restriction would cause a drop in refrigerant temperature at the point where it occurs, so you can look for one by feeling along the lines anywhere your hands can reach. And speaking of which, I should have asked about that already. How do the lines feel going into and out of the evaporator and compressor? That alone will tell you a lot.

The low side pressure is not an acceptable indication of the amount of refrigerant in the system. There is no good indication for that. You can only know what's in there by drawing it out, and that's something most folks don't have the equipment for. Considering that, it's generally safe to try adding a few ounces to see what happens. And also considering that it worked well recently, I imagine that's exactly what I would do. It's quick and easy, and you don't stand to lose much. Just don't overdo it. If you don't start to see signs of normal operation, don't continue. Being engulfed by an icy cloud of refrigerant will ruin your day, and your clothes will never stop smelling like refrigerant oil. Not to mention - it can kill you.

[blb9t]

+1. Once you get it sorted out, run the a/c on a regular basis. Leaving it off for months on end doens't allow the oil in the freon to circulate, and the seals dry out and leak. #1 cause of a/c problems!

wow, I have never heard that but it makes sense.


You can observe the compressor as the clutch engages to see that nothing looks or sounds out of place. Things like noisy engagement and slipping belts would be out of place. I'm not suggesting you're likely to find the problem here, but it certainly deserves a quick look so you don't end up kicking yourself later for skipping something simple.

Restrictions will cause varying things as the locations of potential restrictions and of the service ports are variable. It's the drop in high side pressure that particularly gets my attention in your case. The endpoints of the familiar high and low sides are formed by the compressor and expansion valve. Adding a restriction effectively adds another side, so the two service ports can't show the whole picture. As I recall, the e36 service ports are located on the firewall near the evaporator. Adding a restriction anywhere between the compressor and high side service port would reduce the pressure on the gauge. That includes almost the entire high side including the condenser and drier. A restriction would cause a drop in refrigerant temperature at the point where it occurs, so you can look for one by feeling along the lines anywhere your hands can reach. And speaking of which, I should have asked about that already. How do the lines feel going into and out of the evaporator and compressor? That alone will tell you a lot.

The low side pressure is not an acceptable indication of the amount of refrigerant in the system. There is no good indication for that. You can only know what's in there by drawing it out, and that's something most folks don't have the equipment for. Considering that, it's generally safe to try adding a few ounces to see what happens. And also considering that it worked well recently, I imagine that's exactly what I would do. It's quick and easy, and you don't stand to lose much. Just don't overdo it. If you don't start to see signs of normal operation, don't continue. Being engulfed by an icy cloud of refrigerant will ruin your day, and your clothes will never stop smelling like refrigerant oil. Not to mention - it can kill you.

I finally got around to adding some refrigerant and the a/c is now blowing cold air :cool (Thank God!), however the high side pressure is still really low. I don't know why but as long as it's blowing cold I'm happy. Perhaps my gauge is just broken. Thanks for the replies.

[328 Power 04]

What does your really low consist of?

http://forums.bimmerforums.com/forum/showthread.php?t=1474309

I put this thread up for information. It has some printouts from BMW TIS, the first one is about pressures that should be normal according to TIS. I hope it's of use to you.

[flyfishvt]

I read over this entire thread again. My manifold guage has quick disconnects that go onto the high and low ports. Im sure yours does too but mine also has an on/off valve on each connector like on a garden hose faucet. I was checking my daughterss pressure readings the other day and I forgot to open the high side valve.

Any chance that is what is going on? My pressure readings are 35 low side and 175 high side. How low are yours now that you added freon?

[328 Power 04]

I read over this entire thread again. My manifold guage has quick disconnects that go onto the high and low ports. Im sure yours does too but mine also has an on/off valve on each connector like on a garden hose faucet. I was checking my daughterss pressure readings the other day and I forgot to open the high side valve.

Any chance that is what is going on? My pressure readings are 35 low side and 175 high side. How low are yours now that you added freon?

Your pressures are right on the dot according to bmw's normal values. You have 2.3 bar low and 12bar high, those are pretty much in between spec.

[flyfishvt]

My A/C works great. Everyone keeps saying the high side is supposed to be well over 250. As long as mine works i'm happy. I check the pressures a few times a year. As long as they don't change and everything works Im not worried about it.

[328 Power 04]

Normal for the low is 15psi to about 35psi,
and high 175psi to 320psi according to bmw.

All these numbers are relative to ambient temperatures around and of the condenser etc.

[blb9t]

I read over this entire thread again. My manifold guage has quick disconnects that go onto the high and low ports. Im sure yours does too but mine also has an on/off valve on each connector like on a garden hose faucet. I was checking my daughterss pressure readings the other day and I forgot to open the high side valve.

Any chance that is what is going on? My pressure readings are 35 low side and 175 high side. How low are yours now that you added freon?

I made sure to check that both sides were open. After adding the refrigerant I'm seeing a high closer to ~100 psi, which still seems low, but a/c is blowing ice cold.

[Radioman]

Actually, having a slightly UNDERcharged system will cause lower evaporator temps. The downside is the amount of heat it can pump is reduced. So, in your case, running with normal low side and abnormally low high side actually generates very cold air, but on very hot days it wont be able to keep up.

Low high side pressure can also be caused by a blockage in the line from the compressor or compressor failure in general. If there is indeed a blockage, the pressure behind it would RAPIDLY exceed 350psi and you would most likely blow a line right away.

I realize that the sight glass is meant for R-12, but it can be used for R134a as well. Because of the different densities of refrigerant, in an R-12 system you would not want to see any bubbles going past the sight glass; but in R-134a, you should see only a few bubbles, much more liquid than gas. If my system was that low id be taking a look around and seeing where it went. My A/C on a diff vehicle lost a charge like that over winter and it was from a pinhole in the condenser that was aggravated by ice buildup.

Hope that helps

[vlakci]

If there is indeed a blockage, the pressure behind it would RAPIDLY exceed 350psi and you would most likely blow a line right away.

That would happen only if the highpress switch on the drier would be bridget. With an original non adapted A/C the compresor will shut down with to low press (leak) and with to high press (blocked pipe). That's way the amount of gas has to be exact if not the system will not engage in both ways (to low and to high). The third safety is the inside core sensor that monitors the evaporator temp. If under 5-7°C it will shut don the compressor so ice doesn't form around the evaporator and causes air to be abstructed. On a E30 that sensor can be addapted with and extra resistor so the temp. of the cut-off is reducet to ~3°C. With the E36 could be done the same, but then the automatic airconditiong wouldn't blow as should do in auto mode

[328 Power 04]

I had a blockage in the expansion valve in the Z3, and the High port stood the same as off, and low port dropped deep into the negatives when the compressor turned on. So you can't really just say that it will happen that way, it depends where the blockage is.

[blb9t]

That would happen only if the highpress switch on the drier would be bridget. With an original non adapted A/C the compresor will shut down with to low press (leak) and with to high press (blocked pipe). That's way the amount of gas has to be exact if not the system will not engage in both ways (to low and to high). The third safety is the inside core sensor that monitors the evaporator temp. If under 5-7°C it will shut don the compressor so ice doesn't form around the evaporator and causes air to be abstructed. On a E30 that sensor can be addapted with and extra resistor so the temp. of the cut-off is reducet to ~3°C. With the E36 could be done the same, but then the automatic airconditiong wouldn't blow as should do in auto mode


Actually, having a slightly UNDERcharged system will cause lower evaporator temps. The downside is the amount of heat it can pump is reduced. So, in your case, running with normal low side and abnormally low high side actually generates very cold air, but on very hot days it wont be able to keep up.

Low high side pressure can also be caused by a blockage in the line from the compressor or compressor failure in general. If there is indeed a blockage, the pressure behind it would RAPIDLY exceed 350psi and you would most likely blow a line right away.

I realize that the sight glass is meant for R-12, but it can be used for R134a as well. Because of the different densities of refrigerant, in an R-12 system you would not want to see any bubbles going past the sight glass; but in R-134a, you should see only a few bubbles, much more liquid than gas. If my system was that low id be taking a look around and seeing where it went. My A/C on a diff vehicle lost a charge like that over winter and it was from a pinhole in the condenser that was aggravated by ice buildup.

Hope that helps

It's been in the 90s here (almost 100) and the a/c temp is still ice cold. So, it seems to be keeping up so far. If there was a blockage in the system wouldn't the air be warm, or at least semi-warm? I'm having to turn the temp up its so cold.


I had a blockage in the expansion valve in the Z3, and the High port stood the same as off, and low port dropped deep into the negatives when the compressor turned on. So you can't really just say that it will happen that way, it depends where the blockage is.

How did you find your blockage?

[328 Power 04]

How did you find your blockage?


I swapped out the compressor and dryer, no change, I already had a new condenser on there, so it couldn't be that. Next I thought of the expansion valve, (it's easy to remove it on the Z3) so when I took it out, there was much trash inside. Put in a new one, and the AC works perfectly, and it's colder than ever.

When it started going hot, it did it in waves (hot, cold, hot, cold) faster and faster until it was only hot. Then I hooked up gauge to it and started diagnosing.

[blb9t]

I swapped out the compressor and dryer, no change, I already had a new condenser on there, so it couldn't be that. Next I thought of the expansion valve, (it's easy to remove it on the Z3) so when I took it out, there was much trash inside. Put in a new one, and the AC works perfectly, and it's colder than ever.

When it started going hot, it did it in waves (hot, cold, hot, cold) faster and faster until it was only hot. Then I hooked up gauge to it and started diagnosing.

So, basically I should wait for it to stop blowing cold air before doing anything :stickoutt