When cruising on the highway, I see 45F max on the left-side center vent. When idling, I see about 41F. This is lately, with 95F+ ambient temperatures, and the fan set to three bars.

However, whenever I leave idle and accelerate onto the highway, the left vent rises in temperature and humidity; water condenses on my thermometer. I don't have a thermometer in the right vent, but it feels much colder than the left when this occurs. After about a minute, the condensation evaporates and the thermometer starts lowering. After a few minutes, it settles in to a nice chill.

Has anyone seen these symptoms?

I suspect that I'm a bit low on refridgerant, since the system only gets to about 16 bar on 100F days, and the auxiliary fan is starting (there's a very short delay after the 14 bar criterium is met before the fan starts), which promptly lowers the pressure to below 14. Often below 11, shutting down the fan again (unless the ECT exceeds 95C, of course).

Yes, I drive while the climate-control computer is in current-sensor readout mode, watching number 7. Pretty much all the time.

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Wish I was that low on juice. 41 degree vents temps at idle. You ain't low. I envy you.

Some cars will disengage the compressor momentarily while accelerating. Can't sat this is your case as I'm not familiar with your system.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

Matt L,
On the two zone system, a very common reason for cold/warm difference on each side IS low refrigerant. I think you are right.
Read Steve Brotherton's take: A/C system blowing hot and cold in W210

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

What surprises me is that it happens only during transients from idling to high-speed operation. I don't know if reving the engine in place will do the same thing; I should check. It is worse at lower ambient temperatures, or so it seems.

I put too much in it before the last time the temperature hit these kind of highs, and starting the compressor immediately put the high-side into the upper twenties. I removed about eight ounces at that time, but have since put about two back in. Perhaps another ounce or two will fix it up.

With only one kg of refridgerant in the entire system, I should expect some severe sensitivity to a few ounces high or low.

I would evacuate and recharge by weight to be sure.

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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

That would be ideal, but it's not going to happen until something actually needs to be replaced.

Check your duo-valve. It could be faulty. It works with your compressor to obtain an optimum evaporator temperature. Your A/C is strong enough to overcome a passing valve but when the compressor briefly kicks off on full-throttle acceleration, the extra heating on that side caused by the sticky half of the duo-valve could possibly make its presence more obvious.

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'96 C280 (gone)
'97 C36
'05 C230k

The heater core can't increase the humidity in the air, thus it doesn't seem like the duovalve could be at fault. Whenever this happens, the air from the left side is quite humid. I can smell it, and quite a bit of water condenses on my thermometer.

This effect happens at any acceleration. I rarely put the pedal to the floor. The high-side pressure always plummets though, probably due to the greatly increased air at the condensor. However, I do not have any indication of whether the compressor is running. Perhaps I should hook up a lamp.

Yes it can. Warm air holds much more moisture and with the water in that area that has been removed from the air by the A/C (most drains, a lot remains on the evaporator and by default, in the area of the heater core, as the heater core is usd to keep the evaporator from freezing), any increase in temperature, the air will pick that moisture up instantly, condensing on your nice cool thermometer.

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'96 C280 (gone)
'97 C36
'05 C230k

Does the evaporator sensor read the air temperature before it hits the heater core? This reading also goes way up under the fault conditions, leading me to suspect the air-conditioning system rather than the heating system.

WIS mentions a fault in early receivers, restricting flow and causing high left-side center-vent temperatures. Perhaps I should have mine replaced, if I decide I can't live with the climate-control as it is.

It's certainly not hard to live with, as the temperature will rise for only a few minutes, then settle back down to about 41-45F.

I added a few ounces of refridgerant to the system, and now the fan comes on pretty much whenever I stop, holding the pressure to 13 or so until I start moving (still with high-90's ambient). I'll have to wait and see what it does when it's over 100F ambient, to ensure that I don't have too much. But the air-conditioning acts exactly as it did before.