I just discovered my 1997 E420 has the following A/C problems when I set the temperature at "Low":
(1) When outside temperature is 65-70F, both driver side and passenger side have cold air.
(2) When outside temperature is 72-85F, the driver side air is getting warmer, but the passenger side is still receiving cold air.
(3) When outside temperature is 92-106F, the driver side only gets hot air, but the passenger side is still receiving cold air.

Any professional and technical recommendations to resolve this problem is greatly appreciated. Thanks,

Benze420

First, check the temperature at the vents with the temperatures both set to "LO" but "EC" illuminated. They should both blow warm, but the same temperature. If so, go on. If the temperatures differ, you need to troubleshoot the heater.

Next, ensure that you have the correct refrigerant charge. Being a bit low on refrigerant can cause this. However, if the refrigerant level is correct and you still have warm left-side vents, there is likely a restriction in the system.

You can approximate the refrigerant level check by connecting gauges and reading the pressure charts for the AC system, but that really is very approximate. However, if your high side pressure is way below specs when you get the warm air out of the left side, the problem is almost certainly a low refrigerant charge.

The only way to be sure of the refrigerant charge is to reclaim what is in there and recharge by weight.

Thanks for your suggestion. Matt.

I do also have a problem with my W210 A/C, which I suspected to be due to low refrigerant: blown air is not very cold. Went to a workshop, they tested the pressure in the lines with a hand gauge, stuck an electronic thermometer in the dash vents, read some codes and claimed it works fine. However, although the blower motor blows as it should, air is just moderately cold while in other Mercs, namely the W204, it nearly freezes inside. If there is refrigerant, pressure OK, blower motor OK, what should it be? Are the cabin filters to be changed?

~Nautilus

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1990 260E Sportline (that's 300E 2.6 for our American friends) -> sold
2001 E320 4Matic Elegance -> my Dad's daily drive
2005 Seat Leon FR 1.8T

What was the vent temp reading? I find that "freezing cold A/C" is at best an anecdotal experience. You need to know the actual temp of the air in the center vents to determine if it's not blowing cold enough.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

Thanks for all the kind responses to my AC issue. I have checked the center vents for both driver and passenger sides and discovered that the drive side only has hot air and the passenger side still has cold air when outside temperature is around 85-106F. Also, I noticed that the air flow has slight less volume compared to the passenger side in the center vent. Is that possible that the Freon level is OK but the internal damper of the duct for the driver side is not taking the temp. control signal from the temp. control panel on the dash board? Your kind professional suggestions are really appreciated.

It's too hard to tell, since a warm driver's side on a W210 almost always indicates low charge. You really should hook up a pair of gauges and report back with the pressures. If you don't own a set, AutoZone rents them (at least the one by me does). They'll be grungy and oily, but they'll work and will give you a general idea of the state of charge of your system.

It's also possible that your duovalve is stuck in the "on" position for the driver's side, but again I would rule out the possibility of low charge first.

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

I would venture to guess you've had your evaporator replaced. They probably reversed the two Air Box temp sensors somehow during retrofit or some other maintenance - left is right, right is left

If both temperature settings are on LO, there is no possibility that the heater temperature sensors are the problem.

A warm driver-side vent indicates insufficient refrigerant entering the evaporator. It is often accompanied by a hissing noise at the center vent.

This can be caused by a low refrigerant charge. However, if the refrigerant charge is not low, there is likely a restriction in the system. Restrictions can be found with a thermometer.

My car had a restriction in the dryer. I recently replaced it, and let me tell you that "freezing cold A/C" is more than anecdotal. I measured 38F at my left-side center vent at idle, 90F ambient, fan on position 4 (just greater than half-way) and recirculate off. Under these conditions, the high-side pressure reads 14 at the pushbutton unit, and the auxiliary fan comes on at low speed.

Yup, mine is freezing cold too, 38 degrees under the same conditions.

MattL, what I meant by "anecdotal" is that many people say "freezing cold" or "warm" or "not very cold," etc. when they really should post a vent temp. It's hard to quantify otherwise.

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

Taken in that vein, I do agree. $5 isn't too much to invest in a thermometer.

Thanks for all of your kind suggestions, Gentlemen. I got AC technician checked the Freon level for me and found out it is low. After the recharge, AC is functioining normally now. Your kind professional help is greatly appreciated. One more question is regarding ATF (Auto Transmission Fluid) change. I am the 2nd owner of 97 E420. The car has 103,000 miles. Three MB dealers suggested me to machine flush + drain and change filter and pan gasket. One MB dealer told me not to touch it because the old fluid already broken down and difficult to be drained off. If I machine flush it and refill with new fluid, it would cause more break down and gear shifting problem including the potential leaking issue. Your kind technical suggestions to clarify my confusion are really appareciated.

Personally I would go with just a drain, filter change and fill procedure but that is just me. On a side note I drained the tranny fluid out of a 1990 190E the other day that the transmission had never been serviced in 219k miles. The fluid looked like something that would have come out of the oil pan after 5k miles but it also had a bunch of metal particles in it. Needless to say the tranny is shot.

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Jim

Thanks for the suggestion. Jim

My problem is a little different. My A/C works fine, but I just can't turn it off. Temp control works as it should, but when I turn the EC light on to shut off the A/C it just keeps running. Any ideas? If I posted the question incorrectly, please correct me. I'm a newbie here.