I have just received my 300D back from the shop with a newly rebuilt
engine. I also had the A/C checked and re-charged. However, now the
Secondary or Auxilliary fan is not working. I currently do not have
access to my shop manuals, and any assistance would be appreciated as
to what to repair on this--is it a relay, a switch, a temp-
controller? The fan will work when hot wired.
What color is the sensor on the drier? green or red? If you connect the two
wires at this sensor... not the sensor plugs... but the factory plug... will
the aux fan work?
If yes for above test and you have a red sensor, then you don't have enough
refrigerant to trigger the aux fan.
I think the first question is under what conditions is the fan
supposed to run? On my 116, the aux fan only comes on when either
the coolant temp is over a certain limit or the temp of the
refrigerant at the recv/dryer is over a certain limit. And those
limits are only reached on hot days, after heavy driving, then
stopping and idling, etc. Those two temp switches are run through a
relay, which then turns on the fan.
On older MB, they runs when AC is triggered... not at what temperature it
comes on. The thing is that if you did a conversion to R134a, alot of time,
people undercharged them and the fan never come on...
Same goes for staying R12 with green sensor, it will not come on if there is
not enough refrigerant.
Conversion to R134a shoulr use the red sensor.
Even newer two speed... low speed must be running at all time and high speed
is triggered when temperature is up.
What does older mean in the context of this discussion? My 116 is a
1980 300SD, which is in the same approx age range as the 123 series in
question. And my car is designed so the aux fan comes on only in
response to either high coolant temp or high refrigerant temp at the
Older system is basically before 1992. I know W126 is constant one speed
high speed. W123 should be the same. W124 up til 1992 should be one speed
and 1993 and up is two speed. All the newer cars after this... I think 1996
or newer are automatic triggered when needed.
That's clearly not true. As I stated above,on my 1980 300SD, the aux
fan only comes on when either the coolant temp is above a limit or the
refrigerant at the receiver/dryer is above a certain limit. I have
the wiring diagrams for the 116 series which ran from 1973 to 1980,
which includes many other popular MB, ie 450SE, 450SEL, 6.9, 280SE,
etc. And it clearly shows the fan wired as I've described.
When compressor start running and there is sufficient refrigerant in the
system, it will reach the temperature necessary to trigger the sensor on the
drier in no time... within 30 seconds. If you don't have enough refrigerant,
you will not reach that temperature needed.
The other sensor on the engine is for overheating safety... if for some
reason the aux fan did not come on and the engine temperatrure reaches above
105 degree C, then the aux fan will trigger... or that if AC is off and
engine is above 105, then it will trigger the aux fan.
It is not a simple one circuit but rather two multipaths and that makes it
difficult to diagnose why aux fan does not come on.
No, it does not work that way. And if it did reach the turn on temp
in 30 secs of AC use, obviously there would be no need for the temp
sensor on the receiver dryer at all an MB would never have put it
there. They could just wire it to come on when the AC goes on,
which is what you originally claimed.
I'm an original owner of that 80 300SD and I can tell you that:
1 - The aux fan is wired exactly as I described, which is that it's
turned on when either the engine coolant goes above a certain point or
the refrigerant temp at the receiver/dryer goes above a certain
2 - From 30 years of driving experience, I can tell you that the fan
rarely goes on. To get it to go on requires typically high ambient
temps, 85+, some highway driving, followed by idling, eg stuck in
traffic. Then it comes on. I can hear it come on during those
types of conditions when I come home and let the car idle in the
driveway for a few minutes. It most certainly never went on within 30
secs of the AC going on. Not when it was brand new, not now. On a
75 deg day just driving around town, it never comes on at all.
Which makes no sense, because the only 2 things that can turn the aux
fan on are the high coolant temp in the first place or the high AC
refrigerant temp. The engine coolant temp sensor is there to
provide additional cooling capacity under certain conditions, eg
highway driving, 85 ambient, followed by getting stuck in traffic and
It's really a very staightforward and logical implementation.
Additional cooling is needed if either the AC refrigerant gets too hot
or the engine coolant gets too hot. That's exactly how it's designed
It's actually very easy to diagnose if you understand how it is
actually wired and works.
I'll check that, but it sounds very plausible for what has been done
to this system. I (very reluctantly) had the system converted last
year. It has never worked properly, and still doesn't. This sounds
like a most promising theory. Will report back when I can check the
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.