In my 83 300TD, when I push the economy button on the climate control after turning on the car, the blower fan won't start spinning unless I press the normal or defrost modes first. Once the fan is spun up, I can switch it over to economy mode and everything works as expected. I have an extra climate control button module laying around and the same thing happens when I swapped that in, so it makes me thing there must be something like a weak relay causing fault. Has anyone else run into this issue or have some suggestions as to what could be the cause? The blower motor is a fairly new URO replacement and seems to work fine.

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Garage:
1983 300TD
1985 300D
1993 190E 2.6

Social:
Metric Restoration on Instagram

Yes I had this exact problem.

On my car it had to do with a disconnected coolant temp switch. MB didn’t want to give you a gush of cold air when you start the car on a cold day so the blower will not turn on when the water is cold.

You can override it by pressing the buttons the way you describe. This was my experience.

I dumbheadedly replaced my button controller and the brain box behind the glove box and it didn’t fix it.

Turned out my coolant switch (located at the front of the engine near the thermostat and upper radiator hose) had become disconnected and the frayed end of the wire was intermittently grounding on the chassis of the car.

Hope that helps.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

Brilliant! Thanks so much!! The connector for that sensor is in fact broken and the sensor appears to have been replaced at some point with an incorrect part. You just saved me from replacing the control module behind the dash. Cheers!

__________________
Garage:
1983 300TD
1985 300D
1993 190E 2.6

Social:
Metric Restoration on Instagram

Hah! I wish I could have saved myself from buying that box.

You can test it by just disconnecting the wire or touching it to the block. Those are the two states. The sensor is just a thermo switch.

ETA-here is the thread covering it. At least my climate control works well.

Blower not turning on with EC button CCU/Programa works with DEF SOLVED

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

Yes, M-B couldn't risk blowing cold air on rich Germans who already fret over "eine drafte". The electric "aux water pump" is similar idea and unneeded. I always push the solid white climate button. The economy button disables AC as I recall. The others with cryptic up and down arrows are just confusion. If your climate system is working correctly (rare), you can just keep the fan on "auto" and the system will adjust blower speed (6-speed I recall). That is managed by the black box above the glovebox. I recall it is just a bunch of relays with a separate input wire for each speed, controlled by the "Temperatur Regular" silver box next to it (or maybe the push-botton box, check wiring schematic).

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Your pain has paid it forward and it's much appreciated.

It looks like the previous owner replaced the sensor that sits in the thermostat housing with the coolant temp sensor that should be in the side of the head at some point. D'oh! I'm having a hard time finding the part number of the sensor that should be in that spot. Does anyone know what sensor it should be?

__________________
Garage:
1983 300TD
1985 300D
1993 190E 2.6

Social:
Metric Restoration on Instagram

Forget the sensor. Just attach the lead wire to ground and save yourself a lot of trouble.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Well, I spoke too soon and have not escaped climate control hell so easily.

Shorting the sensor wire to ground does not solve the problem of economy mode not turning the blower on.

I've noticed that only defrost or the button with the upper/lower vents will turn on the blower if the climate control is off.
Pressing the button with the lower vents or economy mode will not get the blower running until I've pressed one of the first two buttons.
When I have the climate control running in economy + heat I will get nice hot air for a random amount of time before it goes cold on me.

I did replace the broken vent tube that connects the temperature sensor in the middle of the dash to the module behind the glove box. No change.

__________________
Garage:
1983 300TD
1985 300D
1993 190E 2.6

Social:
Metric Restoration on Instagram

Following up on my own post in case any one else is having the same issue. I was able to fix it and had two problems. First was that the temperature sensor that sits on the thermostat housing had been replaced at some point with the wrong part so the climate control module was not getting correct info to turn the blower on. Second was that I had a weak relay inside the climate control unit with the push buttons. I swapped it out with a spare I had hanging around and now everything seems to work as expected.

__________________
Garage:
1983 300TD
1985 300D
1993 190E 2.6

Social:
Metric Restoration on Instagram