Erratic Hi/Lo Pressures

[Rogermija]

8-11-25 Pressures Annotated.PNG (74.61 KiB) Viewed 664 times

Winnebago Navion on 2016 Sprinter 3500 Chassis. The AC r134 pressures seem to fluctuate excessively and are not in the expected range for an outside temp/humidity of 80F/60%. Low pressure in Black and high pressure is Red. The graph shows the OFF condition with the engine not running. The On condition is with the engine at idle, max fan, 60F set point. The engine rpm was increased to 1800 for two minutes, then to idle, then to 1800, then to idle, then to off. The pressure seems to have a lot of variation at each stage. The pressures took a long time to equalize. What symptoms does this indicate? Thank you.

[JohnHere]

Winnebago Navion on 2016 Sprinter 3500 Chassis. The AC r134 pressures seem to fluctuate excessively and are not in the expected range for an outside temp/humidity of 80F/60%.

Never having owned one, I don't know much about RV's. Nonetheless, you didn't post the pressures using a Manifold Gauge Set (MGS). What is your expected range of pressures at that ambient and humidity? And also, does your RV have a variable-displacement compressor (VDC)?

Low pressure in Black and high pressure is Red. The graph shows the OFF condition with the engine not running. The On condition is with the engine at idle, max fan, 60F set point. The engine rpm was increased to 1800 for two minutes, then to idle, then to 1800, then to idle, then to off. The pressure seems to have a lot of variation at each stage. The pressures took a long time to equalize. What symptoms does this indicate?

Pressures will vary with engine RPM, unlike home HVAC systems that run at a constant speed. That's normal behavior for both fixed-displacement compressors and VDC's.

Pressures at idle speed don't mean much because the compressor isn't moving much refrigerant, even a VDC at full or nearly full displacement. The standard method is to monitor pressures using a MGS at ~1,800 RPM under maximum heat load.

Diagnosing the system can't be meaningful unless the system is fully charged according to the manufacturer's specifications, and the operating pressures are indicated. The trace is confusing because the pressures aren't shown (but maybe they are?), and the X-axis / Y-axis labels are illegible.

What center-dash vent temperatures are you seeing?

[Rogermija]

Thank you for your response. The pressures were taken with Testo 549i. The low pressure black scale, starting at the bottom, is 10 psi, 22.5 psi, and 45 psi. The high pressure red scale, starting at the bottom, is 0 psi, 75 psi, 150 psi, and 225 psi. My question is about possible cause(s) for the pressure fluctuation. Is the fluctuation caused by the TXV, low pressure sensor cycling, or something else? I was expecting smoother pressure profiles.
Static pressures are about 80 psi indicating reasonable charge.

Thank you.

[Cusser]

Static pressures are about 80 psi indicating reasonable charge.

Not, so fast, grasshopper !

Diagnosing the system can't be meaningful unless the system is fully charged according to the manufacturer's specifications

Rogermija - Static pressures are just one data point.

[Rogermija]

Understood and accepted. The system has not been breached since new in 2016. If anything, some r134 has escaped through Schraders, shaft seals, etc. My focus is on oscillations and the possible causes, so troubleshooting can have direction. Does the TXV remain partially closed after the AC is turned off, thus contributing to slow equalization? Do the oscillations relate to freezing/thawing somewhere internal such as a blockage or at the TXV? Why would the compressor cycle at the indicated rate? Inquiring minds want to know.
Thank you.

[tbirdtbird]

As has been suggested, the testo graph is not all that helpful.
Please run engine at 1800 RPM , doors open, AC set to max fan and max fan, install a standard MSG onto the schraders, and get a suitable digital thermometer into the center vent, and report back when the compressor clutch is activated. ie doing work. Also record ambient.
That is how the pros do things. And as been pointed out, automotive pressures vary quite a bit unlike residential systems.
Please post and we can be of assistance
A properly running system has NO freezing/thawing going on. Any water that could possibly freeze/thaw should have been evacuated when a vacuum pump was used to first install the refrigerant.