1984 300D Blower starting earlier

Hi all and happy holidays. My car runs great and is easy on the eyes too, but something has changed recently. When I left my climate control unit set on auto and the temperature wheel on heat (in the red) the blower would not switch on until the engine and coolant were warm, thus blowing warm air. Presently for some reason the blower fan comes on shortly (10 seconds) after start-up and consequently blows cold air until the engine and coolant warm. CCU? Water temperature sender? If so which one? Any takers on this one.
Mike G.

Forums

Diesel Engine Specific Aid

Chassis Model Number

W123

Haven't fixed mine...but let's compare symptoms

Mine is similar....I have read that the foam tube from the temperature sensor to the CCU goes bad and you get two operational states: works on minimum (auto) about 10 seconds after startup and blows cold air until the engine warms up (then never steps up to medium & high on auto) and works on manual high & defrost. Mine worked fully very recently by a fluke so I think the symptoms may match (it stepped up to high by itself one day).

According to the description of the CCU from the factory manuals, it is a simple analog computer that balances the input from the temperature sensor, a resistor, to a reference value with input from the coolant temperature sensor and the temperature dial resistor. It forms a semi-Whetstone bridge to then balance the inputs by varying the outputs of fan control and vent positions (the temperature control circuit is very simple but is complicated by the failsafes such as defrost and manual operation).

I'd like to get mine fixed and will order that foam tube, provided I can find it, and report back. As I am an electronics tinkerer and engineer, these problems excite me and I want to delve into the mysteries of the pre 81 vacuum/electric based climate control as well....surely the $600-$2400 servo can be easily rebuilt! Just a circuit board in the engine bay attached to vac lines and coolant taps with the aux pump. The later models just moved the circuit board and vac lines to inside the passenger compartment.

In the same boat. Same interests with tinkering

I know this is a little late after the post, but I wanted to add my thoughts also.
From what I understand based on reading about the "intended" function of the CCU. The delayed startup is based on the temperature controlled vacuum switch at the front of the engine near the EGR valve. This switch is what delays the CCU startup until the engine is warm (according to my understanding).
I would first check the vacuum connections there (high heat breaks down that hose fast) and then maybe check to see if the switch itself is bad. My guess is a loose or bad vacuum connection. I have a picture of mine if you have trouble locating it.

Slightly unrelated. I think my temperature dial is bad. I have no temperature control at all (full heat, or full cold). I am wondering if someone can do an electrical inspection of theirs that is working. I plan on posting this as a separate question later.

late post

Hi again. Maybe Kent will read this and step in, because he is like E.F. Hutton. When he talks we listen. But anyway I don't dispute there is a sensor that signals when the water is hot enough to make heat but I don't think it is the one you mention. That thermo-vacuum switch near the thermostat housing and the EGR valve allows vacuum to make it to the EGR valve and let EGR function. The switch with the black wire next to the vacuum switch is for the auxillary fan. Kent is this right ...could you confirm?

Missed it by that much...

Yes, yet again I am wrong. Below that vacuum switch should be a single wire electrical connector. It looks just like the temperature sender on the driver side of the engine. I believe this is the electrical switch that controls the blower delay until the coolant is at the minimum temperature. That cable could have come unplugged, but I doubt it, it is protected pretty well.

Another note (according to the service manual) is that if you switch functions from either "defrost" or "top & bottom" to one of the other functions while the car is running, then the blower delay doesn't apply anymore. Did you happen to change the function selector after you started the car?
I am assuming you have the automatic climate control like mine.