I have a 2010 ML550, vin #4JGBB7CB0AA581600

Wasn’t cooling, so I put on the gauges and both the high and low sides are reading the same ~100psi. I believe that means that the compressor is not operating as there should be a differential in the pressures if running.

Also checked and found no fault codes in the AC system. Ran some data points, and (if I’m reading it right) the data shows the compressor as on.

Back in the day, if the compressor wasn’t kicking in, we would drop in a little refrigerant, and see if the low pressure switch was being activated due to low refrigerant levels. I.e. bring up the pressure a little and if the compressor kicks in, then you know you are low on refrigerant.

However, at 100psi without the compressor running, I’m a little nervous about whether that is the right approach - I.e. is that low or not? I don’t want to overfill it.

A couple of questions before I do anything:

What should be the approximate equalized refrigerant pressure when off - I.e is 100psi high or low?
What should be the correctly filled refrigerant pressures on the high and low side - ambient air temp right now is about 76ish - but a little higher next to the car due to idling and little breeze at the time.

some pics of the gauges and code reader are below.

Thanks in advance for any help and insight that you can share.

 

It's hard to believe (not on your part) that there are no codes stored.

Hope this info helps.

 

I double checked - no codes. Cleared the anyway just in case. No change.

I did do a comparison test vs our 2009 ML350 that is cooling fine.

The pressure gauges on the ML350 show a differential between high and low before starting up of 55/115. After starting and running a minute, the ML350 settles in at about 25/125. On the code reader, you can track the high pressure and you can see it go up when the system is turned on. Cooling great. Interestingly, before startup, the pressures were not equalized - reading about 55/115 first, then 65/100 when off for a few minutes after running. It may be a little low on refrigerant, but it’s cooling good, so I’m not touching it.


The pressure gauges on the ML550 were pretty consistently 105/105 whether the car was off or on. On the code reader, you can see the pressure attempt to change slightly/periodically, so it’s either running - and not compressing right, or that is a phantom reading and it’s not running at all - will post a pic of the chart below. The 105 pressure is within range for the compressor to start (thanks for that info.)

Lack of differential pressure before startup was expected - just based on other previous experiences. The fact that I have differential pressure before startup on the ML350 (which is cooling great) tells me that this system is not supposed to fully equalize when off.

I have a little experience with cooling systems, but it’s dated and definitely not Mercedes. My impression is that either the compressor has failed or that some other component like the expansion valve has failed. Just wish there was a visible clutch so that I could confirm whether it was engaged or not. The car is new to us and I’m also wondering if it’s been overfilled. I may try to remove a little as that will be a cheap test vs the other alternatives.

Again, any insight on these further findings Is most appreciated.