A/C What would cause the low pressure side to over pressure and high side to be ~0

[Tomcat BMW]

Ps- my A/C is not blowing cold like it used to?

[m6bigdog]

Ps- my A/C is not blowing cold like it used to?

I'm not a where of anything other than the A/C manifold gauge is being used incorrectly, high side valve is open or the connection is missing the pin to open the high side schrader valve.
If the high pressure side is 0 then the low pressure side must be 0 as compressor will not run unless the high side pressure is above the Freon ambient gas pressure.
If the expansion valve is plugged the high side will be over pressure and the low side will be under vacuum.

[Tomcat BMW]

If the expansion valve is plugged the high side will be over pressure and the low side will be under vacuum.

I have just the opposite condition. When I hook up my manifold set with the a/c off the low side is high and the high side is low pressure (unfortunately I did not record the readings but they were on the wrong side of normal). As soon as I engage the a/c, the low shots higher (off the scale high) and the high is evacuated to 0 psi.

[Tomcat BMW]

Overnight/system off Low 90 psi, high 90 psi
Start car and compressor Low bumps to 95 psi then back to...High goes to 0 psi

[sleepy81]

Wasn't there a post about one year ago where someone has trouble identifying the high & low valve even though the pressure gauge will only fit the high pressure valve? Lost of talk about pipe size etc.

[Tomcat BMW]

Yes there was...but that had to do with tube size vs valve size. I have had this car for 15 years...and although I do make about every mistake, in this case I have the right pressure ports identified.
Thanks,
Tom

[m6bigdog]

Overnight/system off Low 90 psi, high 90 psi
Start car and compressor Low bumps to 95 psi then back to...High goes to 0 psi

A 1991, E31 would have originally been charged with R-12 and the Schrader port connection will both be the same flare threaded size.
Equal pressures (Freon gas pressure) in the A/C lines is normal when the compressor is not running and the system has had a few minutes to equalize.
Freon gas pressure is determined by the temperature of the liquid Freon that is generating the pressure; you will want to refer to a Freon temp/pressure table.

Has your 8 been converted to R134? with the R134 port adapter installed?
With R134 ports the High side is the larger port and the smaller port is the low side.

Was the system working one day then fail to cool efficiently over time or did it just suddenly stop working one day?
95 psi is not the compressor (typical high side is 180-280 psi) running but an effect of warming the liquid Freon in the system but I can't explain the high side pressure going to 0.

No matter what;
The Red gauge (higher pressure) and hose is the high-side and Blue gauge (lower pressure) and hose is the low-side measurement, Yellow is the center manifold port and the Freon charge line. When you hook up the hoses run the Schrader connection back to the A/C line where it TEE's off to make sure the connections are correct. Then open each manifold valve separately for a few seconds to flush air out of the hoses through the yellow charge line.
The large refrigerant line/tube (with insulation over some of the line) is the low-pressure side. This line runs from the cooling coils in the passenger compartment fan enclosure through the fire wall to the compressor.
The smaller refrigerant line/tube (has no insulation on the line) is the high-pressure side. This line runs from the compressor to the condenser coil in front of the radiator, from the condenser coil to the receiver dryer, from the receiver dryer through the fire wall to the expansion valve.

After that a normal system function as follows:
The compressor is the only component that generates pressure in the A/C system and the only reason there is pressure on the low-side is from the Freon gas being returned through the expansion valve.

Pressure flow:
The small line from the compressor delivers hot high-pressure Freon gas to the condensing coils in front of the radiator which turns the high-pressure Freon gas into a cooler liquid Freon for delivery through the receiver dryer to the expansion valve.
The expansion valve sprays and expands the high-pressure liquid Freon into a cold low-pressure Freon gas in the cooling coils which are inside the passenger compartment fan enclosure.
The large line out of the cooling coils returns the cold low-pressure Freon gas back to the compressor low-side to complete the cycle back to a high-pressure Freon Gas.

So could the compressor reverse the pressures so the pressure on the low-side exceeds that of the high-side?
A functioning compressor could not and there in nothing in the system that would provide that as a failure condition.

The compressor has check valves that insure the Freon gas only flows one direction and prevents the high pressure gas from blowing back into the low pressure side when the compressor stops.
I anticipate even if you reversed the compressor direction it could not develop pressure on the low-side due to the check valve.
If the compressor or check valve was damaged could it generate pressure on the low-side and vacuum on the high-side? I doubt it; but then, I never say never!!

all the best

[Timm]

I thought the ports were the other way around - big valve = low pressure, small valve = high pressure?

[m6bigdog]

I thought the ports were the other way around - big valve = low pressure, small valve = high pressure?

Tim,
My description is accurate: large port (RED) is high-pressure and small port (BLUE) is low-pressure.
Also see attached images below:

[unity]

Yeah, it seems weird at face value. The smaller high pressure line has a larger port. The larger line, which is low pressure, had the bigger port. I think the idea is that since the high pressure side is potentially more dangerous, there is no way at all the lower pressure connector will fit. If it was reversed one could somehow force or hold it on top the port. Not sure.

[Tomcat BMW]

Thanks for all the information. The car has been converted to 134a and has had good a/c for the past 10 years.
Some component recently went bad/failed.
QUESTION: What component/problem is on the high pressure line between the compressor and the high pressure port that evacuates the residual pressure and doesn't allow the pressure to register at the high pressure port (after the compressor starts spinning)

What I (think I) know is this is normal (although a little high):
Overnight/system off Low 90 psi, high 90 psi
Something is making pressure (compressor spins and system has 90+ residual in it and pressure rises when I'm trouble shooting with compressor on).
What component is between the compressor and the high pressure port that evacuates the and doesn't allow the pressure to register at the high pressure port (after the compressor starts spinning)
Start car and compressor Low bumps to 95 psi then back to...High goes to 0 psi

[m6bigdog]

Thanks for all the information. The car has been converted to 134a and has had good a/c for the past 10 years.
Some component recently went bad/failed.
QUESTION: What component/problem is on the high pressure line between the compressor and the high pressure port that evacuates the residual pressure and doesn't allow the pressure to register at the high pressure port (after the compressor starts spinning)

What I (think I) know is this is normal (although a little high):
Overnight/system off Low 90 psi, high 90 psi
Something is making pressure (compressor spins and system has 90+ residual in it and pressure rises when I'm trouble shooting with compressor on).
What component is between the compressor and the high pressure port that evacuates the and doesn't allow the pressure to register at the high pressure port (after the compressor starts spinning)
Start car and compressor Low bumps to 95 psi then back to...High goes to 0 psi

The 90 psi is the correct pressure for the system when the compressor is not running with R134 liquid @ 82.5 degF. Of course the pressure changes approximate 10 psi for every 5 degF temp change. I anticipate the 95 psi is just an artifact that the A/C lines are heating up from the engine running.

Again, the system pressure you are experiencing are not possible for any A/C system failure I can conceive of; other than some kind of unknown compressor failure.
The worst failure I had was a plugged receiver dryer that ruptured and allowed the dryer granules to migrate all the way to the expansion valve particulate screen...what a mess!!
I would suggest you post some images of your gauge test setup with A/C lines, just to help with the analysis and perform some of the test I suggested below.

First, I would recommend you test your A/C manifold gauge set.
Test to make sure the schrader valves are opening when the gauge test ports are connected (before the compressor is running); by opening (normally shut) corresponding Red high-side valve and observe venting through the center Yellow hose and then repeat this test by opening the Blue low-side valve and again observe venting through the Yellow center hose.
Test another car with your gauge set.
Also, I suggest you get another gauge set and use it on your car to confirm the readings (I suggest you get another gauge set to eliminate all test equipment failures and false reading).

Second, If the compressor runs; (clutch engaged - clutch hub visually rotating)! that indicates the system high-side pressure is not 0 psi but is within the A/C high/low-pressure compressor clutch cutout switch limits (switch S98 @ receiver dryer: Low-pressure contacts should open at 22 psi and high-pressure switch opens at 441 psi).
Is it possible that this circuit was hacked to eliminate the switch pressure limits and allow the compressor to operate with 0 psi on the high-pressure side is something for you to investigate, if you can confirm the actual pressure on the high-side is 0 psi (I believe the receiver dryer and switch is locates under the passenger side head light assembly).

Third, if the clutch disengages after a short run, I recommend a simple charge check by connecting a can of R134a to the Yellow manifold hose and opening the Blue low-side valve. This should put enough pressure in the system to close the low-pressure switch and re-engage the compressor.

Answer to: [What component/problem is on the high pressure line between the compressor and the high pressure port that evacuates the residual pressure and doesn't allow the pressure to register at the high pressure port (after the compressor starts spinning)]

The components on high-side in sequence are: flexible-line with various hard-lines, Condenser coils, receiver dryer, Test port, particulate screen and expansion valve.
I assume the flexible-line, various hard-lines, condenser coil or receiver dryer could get completely plugged up preventing the compressor generated pressure from reaching the high-side test port, but then the the low and high side test ports would read a very low pressure after a short compressor run time if not a vacuum.
Therefore, I am completely perplexed as to how the high-side test port which is on one side of the expansion valve could have 0 psi and 95 psi on the low-side test port which is on the other side of the expansion valve and on the intake side of the compressor.

Also, I am confuses as to what this statement means? (Start car and compressor Low bumps to 95 psi then back to...High goes to 0 psi)
That is: Start car and compressor Low bumps (low side does not pump it is the intake for the compressor so it "sucks") to 95 psi then back to (what is this"..." mean, back to what)...High goes to 0 psWith that: The low side of the A/C compressor will pull a vacuum if the Freon flow is restricted from entering the low-side line and/or the compressor intake!!
Like wise if the high-side line is plugged somewhere before the high side test port preventing the flow of Freon, the low-side the pressure will drop below the normal running pressure depending on how complete the restriction is. Hence a plug in the high side before the high side test port would most likely make the high-side and low-side pressures decrease together with a very similar pressures reading.

good luck

[Tomcat BMW]

Thank you for all of the above...I am digesting!
The pressure gauge shows a low side "surge" of about 5 psi for a few seconds and then the pressure drops back to about 90 psi and the high side quickly goes to 0 (zero).

[Tomcat BMW]

It turned out to be a plugged expansion valve...~$600 with flush and refill R134a

[ORL]

Equal pressures (Freon gas pressure) in the A/C lines is normal when the compressor is not running and the system has had a few minutes to equalize.

Sorry to hijack but this is the perfect thread to add this question to: Can one *diagnose* anything from it equalizing really quickly? I put manifold gauge set on a recently-repaired (1995, R134a) 840; the equalization from running (high/low) pressures to resting (equal) pressure took more than 10 minutes. On a failing (1995, R134a) 840, the low side went from running pressure to equal pressure in only a minute after being turned off. Can you determine anything from this, like some "crossover" or leaky seal inside the compressor, or an open expansion orifice or some such thing? (Yes, I also have a slow leak somewhere, else I wouldn't have put the gauges on the good car to compare its pressures (because adding refrigerant to the bad car by pressure @ same ambient, not by weight). But the quick equalization makes me wonder if there's something more failing than the slow leak.)