Here's what I experience:

1. AC works fine.

2. Heat works fine.

3. With moderate outside temperature (say, above 40F), everything seems to work fine: I can set a temperature, set everything to auto, and I will get variation in fan speed, variation in air temp from vents...things seems to settle at the temp I want...although I have noticed I have to set things pretty close to a fully cold position.

4. HOWEVER, when it's cold outside (this morning 25F), when the system is on it will always blow hot even after it warms up in the cabin. The only way to get cooler air is to click the wheel to the AC position.

What would/might account for this? Ordinarily with a problem like this I would think it's that temp sensor that goes to the CCU. But, as I say, everything seems to work fine above a certain ambient temp.

Thanks.

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14 E250 Bluetec "Sinclair", Palladium Silver on Black, 153k miles
06 E320 CDI "Rutherford", Black on Tan, 171k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

In-car temp sensor or aspiration of that sensor.

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MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

Check the aspirator tube. Also, hold a tiny square of tissue up to the aspirator intake and see if it stays sucked in place. If not, there is a problem. If it stays in place, the sensor could be bad.

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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

In case you don't know where that little intake is...

You'll see a little square grill just to the left of the dome light. Place a piece of light tissue paper over the grill. If the fan is working, it will hold the little square of tissue paper over the grill.

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2012 E350
2006 Callaway SC560

Tiny piece of tissue paper stays in place.

If the problem is the sensor, then why don't I have the problem when the outide temp is above a certain point? That's the thing that confuses me.

__________________
14 E250 Bluetec "Sinclair", Palladium Silver on Black, 153k miles
06 E320 CDI "Rutherford", Black on Tan, 171k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Next time the car is acting up see if the tissue paper stays in place - the aspirator motor in my W124 was intermittent.