Hey guys, every so often as I'm driving my a/c starts to blow warm air randomly. When this happens I usually rev the engine once; a foul odor comes from the a/c vents and then starts to blow cold again. Any ideas?

 

does it do it while driving , or when u parked?

 

it occurs while driving and parked.

 

This happened to me the other day in my '16 E350. Was very brief, almost like it just switches on and switches off in 3 seconds or so, no odor though.

 

with my condition it will continually blow warm until I rev the motor. I'll take it to the dealer and see if they can discover the issue.

 

When this happens, put your hand up to the center air vents. If it's only affecting one side, it's a stepper motor in the ductwork.

If it happens to both vents, it's the compressor kicking out and you've probably got a refrigerant leak somewhere. The "foul odor" you mention makes me think this is likely it, with the odor being mildew on the AC evap coil.

 

It's been a while but I recall playing with the AC on/off button and recall a noticable difference right away and a slight change in smell to the air coming out. No change in fan speed though.

You should pop the hood and check the compressor to see if it is running. Set the air on low so it should stay on.

 

Both center vents blow warm - all of the vents do actually. This condition has been happening for 2 years now, I just never bothered to look into it. If it was a refrigerant leak don't you think I'd be out of refrigerant by now? considering this has been occurring for years.

I popped the hood to see if the compressor is engaging during the problem, and yes, it is spinning.

 

Yes, the compressor is engaging. No matter whether I turn the AC on and off it won't make the air blow cold. The only remedy I've figured out is revving the motor.

 

Take it to the dealer, they can try re-programing climate system control unit first, something similar happened to my 11' and reprograming climate ecu helped, just a thought and worth a try if car under warranty,

 

Yeah, if your refrigerant was low it would blow warm (at first or after a minute) with each use, not just once in a while. You could check the refrigerant pressure using gauges from a home recharge kit (or one you can borrow from auto parts store) but you've got to be sure the compressor is running for that check. At least you'll know if you have a pressure issue which is the first thing they always rule out since it is easy to check.

Since it blows cold most of the time and it seems to stop working randomly, my guess would be an issue with the AC relay sticking ($) or a failing compressor clutch ($$$). The pulley is spinning all the time from the drive belt but the clutch (center of the pulley) doesn't move until current is sent to its coil to jump start the compressor shaft spinning. You can see it start spinning and hear the compressor engage if you have someone in the car working the AC for you. If the clutch is going bad, then the compressor may not be turning on when the car tells it to. Since it cycles on/off while the engine is running, it may start off fine but sometime while you're driving it just doesn't engage again when it should. Running the climate control without the compressor on leads to warm air and warm coils which tend to smell when cooling starts again.

But why does it start working when you rev the engine? I'll venture a guess for fun. When you floor the gas pedal, the alternator kicks in it's higher output mode (pushing out 14.3v instead of the fuel saving 12.6v), and that could provide the juice needed for the electromagnetic coil in the clutch to engage. Once it is turning it will keep running until it cycles off. Also, if these cars have a WOT/throttle/load switch that cycles the compressor when you're revving that could be forcing the compressor to cycle.

If you can locate and swap out the AC relay that would be a cheap and easy thing to try and wait and see if the issue persists. If the problem continues, then you're probably looking at a compressor replacement.

 

I've been having this problem too! Sometimes I'll turn it off then back in and that remedies the problem with the odor as the AC kicks back in. I found that when this doesn't work, if I take out the J relay in the fuse box and plug it back in (all with the car off) it'll work instantly. This has been happening for a while with me too.

Anybody know what that J relay controls?

 

Would also like to say a shop told me it was the compressor was failing....nearly 2 years ago...I live in south Texas and have my AC running all the time. If it was the compressor shouldn't it have totally failed by now?

 

Did that shop guy actually look at the car? A good mechanic would check the relay first just in case.

No idea what the J relay does. You may want to PM konigstiger for more info. Why not replace it and see if your intermittent issue goes away.

 

im assuming that J relay is under the hood? drivers side?
get another one , or if you have friends that have another benz just swap it out see if it changes anything....

 

They did look at the car...it's a trusted mechanic I use but he was very busy that day.

I've tried swapping them around and I think the those times just taking out the J relay was a coincidence. It's still happening...but the only remedy I've found this far is taking relays out and plugging them back in.

 

J = terminal 15 relay (n10/1kJ) is switched by front SAM pulling relay in effect simply resets system. Pull codes using MB SDS (diagnostics) many variables involved see attachment.

 

I found same problem now.

Did you resolve it?

 

On my W212, it was caused by bad air mixing valve motors. MB replaced the driver's side, but refused to replace the passenger's side one which continued to malfunction. Their excuse: They never noticed it in the shop, therefore it didn't happen. Got rid of the W212 -- I liked the car, but MBUSA and the Tampa MB dealership were terrible, to the point that it wasn't worth keeping the car.

 

Thanks, Beach Bunny.
I think your case is not same, though.

 

The issue I was having with mine was that the car would randomly go to FULL heat, usually only one side at a time. It'd stay in full heat for a minute or two, then go back to what it was supposed to. Usually only happened after the car had been on for some time (>30 minutes) and the cabin was near the set point.

 

The issue in this thread is probably about a compressor or electrical system.


Anyway,
All vents are same every time.
It really seemed AC button lost the function.

I never see this trouble every time.
When this trouble occurs, I can resolve it soon.

It’s resolved by Rev the engine to 2500 rpm. Otherwise, kill the engine once then restart.
I don’t know the J relay could affect anything about this problem.
Actually I swapped it because there are same relay parts in my fuse box, and resolved, However, it might be resolved even if I didn’t swap them.

I hope to know this reason of this problem actually.

I would like to check compressor-oil quantity and gas pressures next time.
Have you guys checked them?

 

Something else to check -- the cooling fan. I've seen VW ACs kick out if the engine's cooling fan wasn't working properly.

 

Thanks, BeachBunny.
Next time when I see same issue, I'd like to open hood and check the cooling fan for the condenser.

Do you think that if the gas or oil are not enough, does the compressor occur such problem?

 

Something else to consider... W212s equipped with ECO Stop/Start do experience a decrease in airflow during instances when ECO Stop operates. If it possible that some of these intermittent issues are related to that system? And can be considered normal?