We're talking about an '85 300D, California model. A few days ago, I noticed that the blower was working intermittently, and mostly not at all. So, I removed it, cleaned it, removed and resoldered its amp, and put it back together. Now it works well enough that I have heat and defrost and maybe it is working as it should be. I really never paid all that much attention to it before and the manual is not so detailed as the convoluted mind of Benz's engineers.

Now, the trouble is, I don't know whether what I am observing is correct operation or an anomoly. On defrost and high, the blower starts immediately, which is what I would expect. On low, automatic, and with the heat on any setting other than defrost, there is a delay between my pushing the button and the fan coming on. This delay, which seems quite regular, really confused my diagnostics.

Does anyone among you know the logic behind the blower motor controls?

On my '85 300CD (Cali model too) I notice the same thing. Defrost blows immediately and the other settings delay for a second. It seems like the delay is while the vac controlled flaps are moving.

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Chad
2013 Jeep Unlimited Rubicon
OBK#44
"Pleasure in the job puts perfection in the work." - Aristotle (384-322 B.C.)

SOLD
1985 300TD - Red Dragon
1986 300SDL - Coda
1991 - 300TE
1995 - E320
1985 300CD - Gladys
2006 Nissan Pathfinder LE
1998 Acura 3.0 CL

As the blower gets old, it also slows down. Might need internal greasing and/or new brushes for the lower current to get it started immediately.
On mine, I need to start it out on high speed for it even to start, and then select low or auto.

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The blower seems to come up to speed as it should. When the low setting is selected, I hear a distinct click of the relay just before the blower starts to spin. I don't hear the relay when high is selected, however, probably because the blower starts to spin immediately.

Something else: I read somewhere, perhaps in the owner's manual, that there are several more speeds than two. Presumably, in the "Auto" setting, the various speeds are utilized. That is, if there are various speeds. So I ask: how many speeds does the amp have to choose among, and what does "Auto" really mean?

I believe there are 6 speeds. And the delay is normal.
If everything is working right the blower shuold not come on until the engine temp is higher than the cabin temp, unless it is defrost, or high.

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83 300SD Dark Silver Dark brown inside

Blackestate is right. On "automatic" (the middle speeds) the CCU thinks for awhile before starting the fan. High, Low, and Defrost (which defaults to the highest speed) stary immediately. A blower that will start on high speed but not on the lower speeds, or that requires a stiff kick, will usually be found to have worn brushes and/or commutator.

Jeremy

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"Buster" in the '95

Our all-Diesel family
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I'll try to explain how it works on my 84 500SEL which i believe is the same. The fan does different things depending on settings, temperature of engine, temperature of air in side and out side and temperature setting on dial. I'll explain what happens if the system is in Auto mode with temp wheel set at 72 deg F.
If the engine is cold and heat is called for, the blower will delay until the engine gets to aprox 150degF then bring up the fan speed as needed. If the engine is cold and cooling is called for the blower will delay for a few seconds until the evaporator has a chance to cool off then bring the fan speed up an needed. Both of these are to keep uncomfortable air from blasting out as soon an the car is started.
If the engine is already warmed up the fan will start after a short delay and start the fan at low and bring the speed up as needed.
Any time defrost button is depressed the system goes to full heat and high blower regardless of any other inputs. Economy mode works just like Auto except the compressor is turned off. The system still tries its best to reach the set temp. if it can.
I set my controls to 72 degF and auto fan and the center mode button. In the winter I will change the mode to Econ or Defrost only if needed.
If the temp is set to max heat or max cool then most of what I said above no longer applies.

Paul

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84 500 SEL (307,xxx miles)