Hi - New to MB World. Car has 110,000 miles now. Second owner and purchased it with 57,000. Have done the scheduled services.

Looking for help to diagnose my AC that is not making much cold air. If the system is on "recirculate" it makes a little bit of cold air if driving, but nowhere near where it was even a week ago. No cold air if the AC is on but not on "recirculate". Temperatures here in Chicago area have been 88-92 for the last ~2 weeks.

I searched the forums and tried the easy thing to add R134a. When I connected the R134a can and hose to the low pressure side, the gauge read 85 pounds. It was not clear that any R134 was pulled into the AC system.

About 4 weeks ago I noticed a few small drops of dark oil-like fluid under the car which I'd not seen before. Not completely sure they were from the GLS. Did not find any leaks. When I looked under the hood today carefully, it does seem some fluid was in the pan under the engine, and it could have come out of the compressor (cannot tell it that's the case or not without removing the undertray).

I am not experienced with AC repair, but I know basic operation. I don't have a diagnostic or code reader for this car. Also, I don't have AC gauges currently. The car has the "basic" Climate control package with a single system that serves the 1st and second row (no control buttons in the second row). No "Zone" button on the dash, and there are NO on vents in the 3rd row at all.

Question is whether this is a compressor problem or a sensor problem or a control valve problem and how to determine that. I'd like to fix it myself versus going to a local independent shop, since I probably have the tools, and maybe just need the guidance.

I have a pretty good set of mechanics wrenches, breaker bars, etc. (from working on my BMW 5-series and other cars) and I have jack stands and hydraulic jacks.

I did read other forums before I posted here. These were two that were informative, but not a direct fit for my vehicle and situation ...

AC only working intermittently - MBWorld.org Forums
https://mbworld.org/forums/gls-class-x166/894223-ac-only-hot-air-2017-gls.html

Thanks in advance! Thoughts?

 

When you tried to add refrigerant, did you have the compressor running? 85 psi low side is too high if the system is running. You want to be closer to 35.
You'll need a set of gauges and a vacuum pump. You may be able to replace parts, but to evacuate and refill the system you do need some specialized equipment.

 

When I tried to add refrigerant, the compressor was running throughout. The compressor could not provide cold air to the cabin unless it was on "recirculate", and then the air was only cooler than outside, and not cold enough to provide adequate cooling. This was true while parked and while driving.

The fact that the low side pressure was so high makes me think the compressor performance may have dropped significantly and perhaps that is the primary problem with the system.

Thanks!

 

If it was 85 psi before you started adding, then either it isn't compressing, or the system is way overcharged. What is the pressure with the compressor (or the whole car) off?

With 85 psi low side pressure you'd be hard pressed to get the evaporator temp much below 80 degrees F.

 

If it was 85 psi before you started adding, then either it isn't compressing, or the system is way overcharged. What is the pressure with the compressor (or the whole car) off?
With 85 psi low side pressure you'd be hard pressed to get the evaporator temp much below 80 degrees F.
__________________
John

Thanks! John -

The car was home and not driven for about 90 minutes. Ambient temp was 81 deg F. I ran the following test. Using the only "gauge" I have which is basically the same as an AC Pro and 80 psi is the max reading. All pressures were measured at the "low" side.

1. Without starting the car, I connected the gauge, and it went off the scale to the "peg". I disconnected the gauge.
   
2. I started the car and kept the AC off. I connected the gauge, and it went off the scale to the peg. I left the car running and the gauge connected.
   
3. I turned the AC on, and the gauge dropped below 60 psi then slowly climbed to 65-66 psi. I watched it for several minutes and the reading remained the same. During this time the cooling fan behind the radiator turned on and continued to stay on. I left the car running and the gauge connected.
   
4. I turned the AC off, and the gauge climbed up to and past 80 psi over the course of maybe 2 minutes, maybe less.

Then I turned off the car.

 

It could be overcharged or the compressor could be weak.

I'm not up on the subtle specifics of this system, but the fact that the fan came on could indicate the compressor is working fine.

I think the next step I'd take is to get a set of AC gauges so you can read the high side pressure.

The big question is, how could it have gotten overcharged? If you want to take a brute force desperation move, with the system running, bleed the low side down to about 40 psi. This is easy to do with a gauge set, but dangerous (don't try it) with the gauge from the refrigerant tank. If reducing the low side pressure improves the performance, great. If not, you have other problems.

 

Thanks. The AC system has not been serviced since I have owned the car, and the AC performance has been completely normal until just this last week. And we've had some pretty hot days prior to that but always felt that the air was cool as usual.

Regarding the cooling fan turning on, it's possible that the fan turned on because the car was stationary and the engine was still somewhat warm from the most recent drive and then me testing the pressure for about 20 minutes (I repeated the steps twice to be sure I had the details right). If I recall the car had been parked ~1 hour?

I have to travel so will have to wait to get gauges to test further.

 

Then the problem is not because it suddenly became overcharged.

 

I returned from my trip and got gauges from Autozone. I connected them to low and high side, started the car and the air conditioner with AC on and interior temp set to lowest possible setting. No cooling/heating zones were set - all one zone. Ran the car ~5 min before testing pressures.

Temperature outside was 70 deg F. Inside temp set to lowest possible setting
Low side pressure 58-60 PSI and High side pressure 150-155 PSI
Air from vents inside was below 70 but not near 40 deg F
Cooling fan behind condenser turned on shortly after starting the car
I let a small amount of R134a out of the yellow line off the manifold. Was surprised it had green dye in it.

Previous test of only the low side pressure at ambient of 81 deg F (a few days ago) the low side pressure was ~65 PSI (I did not have manifold/gauges to test high side on that day).

 

I don't know for sure, but my guess would be a bad compressor. Get a second opinion, though.

 

Thanks - John. I will post in the other thread for a second opinion. (sorry I made 2 threads)