This issue is very most likely a semi open ( jammed open ) TXV of the front AC system, the EVAPORATOR.
Return pipe from EVAP is bigger diameter ( for R134a in gas form ), than the input pipe which is smaller and handle r134a in liquid form.

 

I ran his VIN and he does not have a rear A/C (i.e., no Code 582). So, he should have exactly (or close to) 970 grams.

If a rear A/C is present, it was added afterwards and would require 1220 grams.

 

Ohhh, dang its not a W212, its SUV W164 .... thanks jetta,

 

I made the same mistake, but he did identify his car in his first post. I just assumed it was an E-Class W212.

 

ALL the ML of that generation (W164) use permanently engaged clutch, or clutchless compressors: 7SEU17C.


For this particular, gasoline V6, the compressor should look like this:


Therefore, the only way to deactivate the compressor is via the ECV (proportioning valve). If the valve is always active, the compressor will never stop cooling.

Q: is the ECV always active?

Can we see the XEntry AAC live data when the AC is running?, and if possible rent the AC pressure gauges from AutoZone (or similar), or buy an inexpensive set so we can monitor both low and high pressure sides.

Disconnecting the ECV should deactivate the compressor, and defrost the line unless the compressor is internally stuck in active mode (unlikely)

 

Can the proportioning valve be jammed such that AAC read low duty cycle but valve forces compressor actuation plate into high rates?

Sealed circuit is gonna need to serviced to replace ECV... is it fair to say XTV diaphragm got forced opened by overpressure... damaged?

 

What if the cabin temp sensor reads too high and the poor A/C system just tries to do its job freezing the evaporator over?

There is mention that the line is frozen. It is frozen only in the surface. What freezes and is a problem is the surface of the evaporator fins that leads to the plugging of the air flow thru the evaporator coil. If this happens the OP should feel air flow drop down though the fan is running.

Very efficient A/C system to do that and I would really look into cabin temp reading trying to find why it does it.

 

Good thing is no need Xentry for HVAC scanning. a good scanner can read all HVAC temperature data and compressor valve actuation in AMP. Max is 1.0 AMP but usually 0.95 amp.
When system is already cold usually 0.45 to 0.55 amps only for the compressor.

Pressure gauge at the HVAC LP and HP r134a port is a must have too, to troubleshoot.

 

 

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Theoretically, the solenoid (ECV) could get stuck in the active position regardless of the requested "duty cycle". In that case, disconnecting the harness from the compressor, and running the AC system should NOT cool, but if it does, oops, the ECV is stuck in partial/max cooling.

Interesting XEntry reports a defective compressor that can freeze a line. Perhaps it should say (assuming requested low/NO duty cycle) : AC Compressor does not respond to requested duty cycle or similar.

 

Here's what you need to know to diagnose your pressure readings.

https://www.acprocold.com/help-and-a...-instructions/

 

Here is the chart by Nissens (AC compressor manufacturer) which highlights the ECV behavior explicitly. Not many of the existing charts have that section since many are based on the older fixed displacement compressors


or a video explaining why diagnosing these compressors can be tricky if not familiar with them

 

Many thanks guys for all the information and recommendations!
I will definitely follow them and share more information once we test the system.

 

This is my experience for w212, you can apply.
A/c pipes freeze after the evaporator freezes. That is why the voltage supplied to the air conditioner is always 0.95A instead of 0.4-0.6A when operating stably. The main causes include:
- The temperature sensor in the car reports incorrectly.
- Lack of gas in the air conditioning system.
- Expansion valve failure.You have checked and ruled out these 3 causes. So only need to check evaporator sensor. In w212 facelift model, is only one sensor for evaporator temp, not 2 sensor as W211. When the wind blows through the condenser, it may be because the evaporator is dirty, causing the air flow to not reach the correct temperature of the evaporator. This leads to the evaporator gradually freezing and the sensor giving increasingly false alarms due to insufficient airflow through the evaporator. Check by inserting another sensor to compare at the same time with the sensor's reported temperature. In my case, when I put another sensor into the evaporator, the temperature on the left side was colder than the temperature on the right side - where the car's original temperature sensor was mounted. So I moved the sensor's position to the left side with just a 1 meter of wire and a little trick after that everything works perfectly, no more freezing of the evaporator.
Good luck.
Sorry for my bad English.

Goodluck
w

 

I really wonder how can A/C with low refrigerant freeze return line ​​​​​​... how can this be?

 

https://www.mastertechplumbing.com/b...h%20the%20coil.

 

JR!, you know that's only true with an old fixed opening systems.
Our cars and many homes A/C use a TXV to prevent the evap. from freezing.

This is done by automatically regulating opening based on evap. temperature.

A frozen coil will mechanically shut off circulation. Here we likely have uncontrolled pressure buildup + stuck TXV that got overheated.

 

Well, I'm an old fixed system.

 

Yes, there's some truth to that

In 2001 my Acura TL already had a TXV and a CAN-Bus with ESP, ECU, TCU, SAM, DCU's... keyfob worked and battery never drained - I did not fully appreciate how good this all was.

 

I don't know where this writing comes from but I highly doubt it has much truth to it.

R134a freezing point at normal pressure is at around -153F. At 34 psi that the low side pressure port in my car reads the temperature is just below 40F. Pressure in the evaporator is higher than at the pressure port as it is further away from the compressor so the temperature also is higher. I don't believe the R134a can freeze inside the piping, no way.

What freezes is the outside of the evaporator coil and ice blocks the fins leading air flow to stop. This, of course, means air condition stops until the coil melts again and allows air to flow.

 

The link to the image is above the image.

 

It does have some truth to it but is misleading because it is not applicable for our TXV system.

When you read the article it makes matters more complicated than it is to fish business.

The loud applicable take-home is:
When in doubt... CHANGE NEW FILTER!

 

Hi, were you able to find a solution? I’m experiencing the same issue with my W212. Thank you.

 

The ECV was tested by changing it with new one, still the same issue, the compressor is under constant pressure.
What we did with my mechanic was to disconnect the electricity on the compressor (sorry I am not good in terminology), the socket that is connected with the compressor, still it is under constant pressure.
So all leads to a compressor fault.
Will keep you updated once we change it.
Thanks!

 

You mean that when disconnect ECV and compressor still running? That mean it running all time and no under control of ECV. So change your Compressor.

And remember that Xentry only can read ECV voltage and Amps. So you can see amps to see how compressor work. When evaporator around 2 degree C, Amps will drop to 0.4A. If evaporator around 4 degree C, it will running. And as my post before, Sensor of evaporator can't down to 2 degree C, Compressor still running nearly max at 0.8-1A, it will frozen evaporator and low line back to compressor.