Let me start off with It has been record setting hot in Texas lately. One Friday we took our 21 GLS450 to go visit family for Father's Day weekend. About 45 minutes from home we noticed that we were getting very little air from the center 4 vents in the dash. The outer vents by the door were still blowing cold air. All of the rear vents were blowing cold air as well. As we continued on the center vents stopped blowing altogether. I noticed if I would change blower speeds, change from auto to manual, or vice versa that we would get some air for about 10 seconds before it stopped again. We could hear the blower motor running full speed the entire time. Coming home on Sunday we experience reduction in flow from the center vents, but it did not stop completely. At first I was thinking the evaporator core was freezing up and blocking flow (this has happened on another car in the past), but all of the other vents were blowing cold air just fine. We haven't experienced this much on shorter trips around town.

I took it in to the dealer today for them to perform Service B and look into this. I got a call late today that they couldn't duplicate it on their short drive. They requested permission to do a longer drive tomorrow which I granted. They explained what they think is happening is because of the logic/priority built into the AC. Apparently the outer vents in the dash are to always be on. The center vents share with all rear vents and will lose flow (possibly completely) to properly cool the rear of the vehicle to the requested temperature. I could see how this could cause the situation. I was wondering here if anyone else has experienced this or is familiar with the logic/priority of the AC system?

The reason I am curious is that I have asked them to verify the freon charge is appropriate and have not been able to get confirmation that this has been done. Secondly, the service writer who wrote it up when I dropped it off told me that sometimes the AC "case" goes bad and needs replacing. This contains all diverters, actuators, etc. He is not my usual service writer and handed off to my usual writer when he was back this afternoon. My service writer hasn't mentioned this as a possibility.

I just want to be confident in the resolution before leaving on a long road trip with the family this summer.

 

I've noticed the AC is very weak in general and the solar radiation is significant, especially with the roof screen open (glass closed).

 

The AC on my 2007 GL was about 4 times more powerful than my 2020 GLS.

 

Ceramic tint to block a majority of the infrared heat might be something you should look into. Do the windshield in clear too. You won't need to blast the AC as much.

 

That's exactly what I have lined up. Ceramic 35% on the front doors and 86% on the windshield. I have been having trouble getting my schedule to coordinate with my tint guy since he is sought out by many in my area.

 

I have the acoustic package which blocks infrared but I'm thinking about doing ceramic on my panoramic sunroof, plan to wait a year to see if the heat gain thru it bothers me enough.

 

I have pretty much the same issue with my 2020 GLS 450. When I drive it for about 45 minutes, the air flow thru all vents go down to almost to a stop regardless of the operational mode (Auto or not; fan speed at 7; recirculation or not). Dealer checked and says no faulty codes and pressure is fine. Anyone experiencing a similar issue? What was the reason. Appreciate your help. This summer has been very uncomfortable! Thanks.

 

When you are stopped at a light, does the refrigerant stop working and you only get circulating air? For most other brands, the auto start/stop also stops the A/C from working (fan may circulate).
The GLS has a secondary battery that should keep the A/C running! Perhaps there's a refrigerant leak?

 

A bit confused. If the fan is at maximum speed, air must come out of the vents; otherwise, the air is going elsewhere. If air coming out at the windshield vents? Footwells? Can you hear the blower when the speed is at MAX?

 

I have Eco off so the engine is running all the time. All vents appear to be doing the same (dash, floor, windshield). I hear the fan running loud, but almost nothing is coming out. What comes out feels cold, but flow is near non-existent.
BTW, if I switch off the AC and run only the fan few minutes and switch AC back on, cold air starts coming out like there is no tomorrow for few minutes. Then the cycle repeats!

 

Great. There is a problem with the AC blending doors. It seems they are closing at the same time instead of blending to adjust to the specified temperature.

The blending doors may need to be "normalized", or there is some sensor triggering a wrong signal.

Do you have a bi-directional MB-aware scanner? If you do, go to the AC module, and look for the "normalization/initialization" of the blend doors. Fingers crossed it is just that.

 

Thanks!!
No I do not have the mentioned scanner. I am due for service and will ask the dealer to check this out....may be they will learn something from this??

 

My vocabulary may not be perfect for these components.

I think MB calls the blending doors -> AC actuators.
The "normalization/initialization" -> AC relearn

In older models, there used to be a procedure to reset the system by a combination of the buttons in the AC control panel.

If the actuators were stuck (which I doubt in your case), some owners have tried setting the AC at the lowest temperature first, then at the maximum temperature (hot) so the actuators reach their limits. However, I do not know if that is similar to the relearn/normalization process.

Keep us posted on your progress

 

The dash fan provides flow for both the front and 2nd row vents. After picking up our GLS the fan ran full speed the whole way home without cooling down the car. After some tinkering I discovered that the front vents were set to 70 and the 2nd row was at 80. In our case it was trying to heat the 2nd row seats and wasn't doing much cooling for the front seats. Syncing the temps fixed it. Another possibility is that your front temp was set higher than the rear temp. If it calculated that the front temp was satisfied but the 2nd row was not it likely sent all the airflow to the 2nd row. The mechanical fan speed button seems to average the fan speed based on the demands of the front and 2nd row. EX: 2nd row set to fan speed 5 and front row set to fan speed 1 will result in high speed fan but not a lot of air from the front. The HVAC fan speed logic is weird IMO. I find myself regularly having to adjust the individual zones to get the fan speed to do what I want.

 

Thanks!
I have sync button turned on. Don't front and rear become the same temp setting when that happens? I am using the controls on the screen to adjust all that. Hope I am not missing anything.

 

I had this issue with my 2020 GLE350. The AC was blowing cold, but not from the center vents. There were a number of software updates applied which finally fixed it - but there was nothing more annoying than hearing the fan blow and not getting cool air from most of the dash vents.

 

Took the car to the dealer. They replaced the temp sensor for the evaporator coil (apparently it was freezing). Drove the car for about an hour on Sunday and seem to have fixed the issue. Hope that was it!