I have a W212 which I have been using since the past 3 years and have been very happy about the performance. It's a 2012 model. Lately it has been having a problem where the Transmission does not shift normally while using the AC. With the AC turned off, the transmission shifts flawlessly and smooth at 1500RPM from the 1st gear upwards and feels just about right as I have always been using it. But lately with the AC turned On, it feels like the car has to force itself to change from the 1st to the 2nd and 2nd to the 3rd gear and so on till the RPM reaches around 2200. On a normal traffic day, it gets very annoying as the car is mostly stuck up at the 2nd gear most of the time and I really have to speed up quite a bit to force itself to change the gear up. On turning off the AC, the problem immediately goes away and the transmission is smooth and flawless. Have even tried doing a transmission reset by turning the key to position 2 and pressing the gas pedal all the way down for like 30 secs and then the rest of the procedure but to no avail. Any suggestions would be very helpful.

 

Have you tried resetting your adaptive transmission on your vehicle?




oops my mistake...seems you've exhausted that route. The only thing left would be to hook up to the OBD and diagnose the transmission module.

 

Yes I have mentioned that in my post. It doesn't help with anything once I drive with the AC switched on. The problem remains. As soon as the AC is turned off, the transmission is perfectly fine. Even while using the paddle shift, no improvement whatsoever. The AC button once activated , changes the way the transmission behaves.

 

AC compressor is less efficient at lower RPM and I bet a strategy is to keep the RPM higher due to AC load.

Personally I’ve never noticed it on either our E350 or E550.

The again, having the car hang at 2k rpm or higher wouldn’t bother me either. I prefer that over the norm in E/C modes that keep the RPM near lugging… especially around town or back roads. Highway is fine when cruising to keep the RPM lower.

 

Drove the E350 into the office…

85* outside, bit humid, AC on, car warmed up already (driven 1 hr earlier)

35-45 mph, casual drive shifting around 2-2.5k, maintains 1250-1500 RPM on flat road. Will stay up at 2k on hilly or when accelerating a bit more.

 

Curious this comes up, I too am having the same issue with the RPM, but it only happens when it's too hot outside. I wonder if the a/c system is running out of gas, and it has to keep the revs up to push the A/C system compressor.

I am going to have the system checked, and report back.

 

That’s another thought I had… if the system is underperforming due to charge, dirty condenser, sticky RCV, etc…

 

how old is your battery... may be its showing its age driving around town under a heavy load with minimal rpm
-or the engine block cooling needs the water pump to spin faster.

The way the tranny shifting goes right back to normal without A/C... means it is out of question.

 

Mine is less than 1/2 year old.

 

I’d like to verify the transmission “reset” procedure with key/pedals via Xentry… it gets brought up every so often.

Does it work on W212s, seems like mixed results. Having Xentry, I haven’t tried the other method.

 

I think that the increased RPM in the shift points with the AC on is normal. My does the same thing.

 

It sounds high rpm to me for gear shift unless you are accelerating faster than normal when it does oit but you say it is different that what it has been while you have driven the czar for a couple of years. It is your car so you know when it acts differently.

If you are in a very hot climate the refrigerant pressure level in the A/C system goes up. This means the compressor is harder to turn against higher pressure.

I can tell from my own car (2010 E550) that without any prior work in the A/C system the system stropped giving cooling. The fan in the front of the car started running very fast, screaming I would say. My first thought was the system had a leak and it was low in charge so I got a refrigerant bottle with a gauge. The gauge showed slightly in green range over 60 psi. I added refrigerant to half way in green range in the gauge but no help. I then let refrigerant out so the gauge went to the low edge of the green range and the A/C system started working again. The A/C system in my car was over charged and this is a bit of a mystery as the system was as it left the factory.

I don't remember any higher engine revs at shift points when this happened. Perhaps the V8 would not need to use higher revs for the extra load from the A/C compressor but the smaller engines may need to as shift points happen based on the engine speed AND engine load.

I would not be worried about it and wait until weather cools down and see if this helps the issue, but there is an easy way to try: Get a bottle of refrigerant and a hose with gauge. Run the car with A/C on max cool and connect the hose to check the low side pressure. Then disconnect the hose and let some refrigerant out. Check with gauge and try to let out like 5-10 psi. Go drive and see if the system still cools the car and if it changed the shift points. If small improvement and cooling work let more refrigerant out. If you let out too much your system is low and cooling stops and you need to re-charge the system. With this method you could find the charge level that returns your shift points like they used to be yet cooling also works. If nothing positive results from this then use the hose and add refrigerant back to the same pressure it was at before you let anything out and it will be back as it was before any changes.

A can of 134a at Walmart is about $5. The hose with gauge from car parts store is probably around $7-$8.

You may also want to add some oil in the system as with escaping refrigerant you also let oil out so a half can of oil with the new refrigerant fill would be good too. Can of oil is about $8.

 

No it's not. It was never this way. The way the car is supposed to be is exactly the way I drive with my AC off now. With the AC on, the driver can easily feel the engine being burdened and the transmission struggling to shift up. For eg, if its driving on the 3rd gear and the RPM drops even a little, it gets back to the 2nd gear and stays there till I get past 2200 RPM, which is unreal on a normal traffic route. With No AC, and at the slight press of the gas pedal, the car smoothly shifts up from the 1st to the 2nd to the 3rd and so on in No time just like it has to. Since this problem started , it also sucks up a lot of Diesel than it has to and I end up refilling the tank far earlier than i need to. Just like driving a Manual transmission car on a particular gear for longer than required where u can feel the pressure in the engine as well from the sound of it.

 

As you say it definitely does it with A/C ON while it OFF the car behaves normally.

You have an issue with the compressor or there is over pressure that the compressor must pump against. Over pressure could be from over charge of the A/C system. In a severe case you could be turning the gas into liquid in the compressor, which would be very bad situation.

Like I said in my earlier post letting some refrigerant out would show if this is the case. If it does not help just replace the 134a you let out with some oil too.

Another reason for over pressure could be partially plugged expansion valve or dryer in the system.

 

Have to correct my pressure reading I mentioned in my post. I remembered it wrong. The 60 psi was the higher value of the green range in the gauge. The lower range was 30 psi.

I just checked my cars A/C system pressures with car on idle. High side 180 psi, low side 32 psi. Air temperature out the center vent 44F, outside temperature 92F.

Air temperature at center vent goes down to about 40F when I drive the car.

 

I've performed it on my S212 and it works.

 

I have reset the transmission adaptations numerous times on my E550 before I bought the Sprint Booster that eliminates the need for the reset. Reset works fine but it last only a week or so and you are back to the slow pedal.

I did the reset also on my S550 and the exact same procedure works on that too. I need to get a SB for that too but the twin turbo is so strong the pedal lag does not feel nearly as bad as on the E550, but I will get SB for it..

 

Drove my E3504matic today with AC on. Does the same thing. Low throttle shift points are raised to about 2200-2300. Normally below 2000 RPM like 1800ish. It does feel like it is hanging onto the gear more than necessary but this is what it does.

Edit: I should add that sometimes my AC doesn't come on when the AC switch is pressed on. That was the case yesterday when I drove it. The shift points were still high in that case. Moments later it started cooling well, same shift points. I have observed this in the past as well. Point being I believe that the shift points are a function of the AC switch position not the actual AC load.

 

You're right. Trans shift is function of the AC. So you can change it with xentry in A/C control. That "switch point delayed for increase cool" turn it of. And your shift will no longer function to A/C on or off.

 

Pardon my ignorance but what's a Xentry ? And how do u turn off the switch point as u mentioned ?

 

XEntry is the Mercedes shop computer system, a subset if it is available to independent mechanics for an annual subscription. AFAIK, only dealers have the module to do coding and 'flashware' upgrades and such. having looked over the shoulder of my indy, its a really complicated system to use, which isn't surprising, these are really complicated cars with a ton of different computer systems and data busses onboard.

 

...

 

here that I talking about.
you can try with xentry and code it NO

 

Just had the car checked up with a Sevice engineer. He says there is not enough Torque speed upon Diagnosis and that the AC is loading the Engine where there may be a possible leak in the fuel injector. Needs more sophisticated equipment, only available with the company to have that checked. Xentry does not suffice.

 

This this check actual valve of ME.

Do you have xentry or can ask some one with xentry and access to A/C box, Author data and see what I've posted.

Torque output low with make some code on ME. If no code, don't touch to your fuel injector.