81 300SD.

Last year I was running a bit hot in traffic and I'm trying to be pro-active this year. The question concerns the aux fan control.

I can get the fan to come on if I jump the dryer plug, so I know the relay/fan is OK, and last year the fan would come on when the ac was on and the pressure was high enough. But I never heard the fan come on without the ac on even at 110C.

I removed the wire from the sensor on the t-stat housing and grounded it - I even grounded it to battery neg just to be sure. Fan never came on. I suspect the sensor, but I tried the same thing with my wife's 300SD (Practically Perfect in Every Way) and the fan would not come on either. I find it strange that both cars sould have a bad sensor, but maybe.

Is there more in the circuit for the t-stat sensor? Does it use the same circuit as the AC sensor?

Am I on the right track?

Thanks

__________________
1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer

"Is there more in the circuit for the t-stat sensor? "
BINGO !!!!
I hate electricity... If I were you , given what you said about last summer, I would wire up that fan where you could turn it on and leave it on when you are in the car. They tried to be too fancy on some of these things ( they may have worked when the car was new.. but trying to keep up complicated things may not be practical )... and it would not hurt to have that fan on all the time.

Just for the record... you need to check, clean and straighten the fins on your AC condensor and your radiator...and put new proper strength MB antifreeze here in the spring.. which is specifically when MB suggests you install it...

Except that the $100+ fan will die far sooner as the brushes wear out.....

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

I really did not think they had brushes in that type fan...
OK.. get a Spal High Performance fan for that location....

Yes that fan has brushes.....I bought a new one for my W116 last year as it was DOA....and got lucky as I got it for $80 from Bigbabbo the guy with the reputation here.....haven't seen another for under $126 since then....

Tore my old one apart but the bearings were shot in it....so I tossed it.

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

On my 84 300D the thermo switch on the housing grounds at 50 C to turn on the climate control so cool air does not blow before you get heat. When you ground it listen inside the car to see if you hear a click.

__________________
-disav

99 E300 TURBODIESEL Astral Silver 282K - AMG brakes, suspension, monoblocks, speedo & interior - Full Load Maxed on IP by custom Speed Tuning USA Chip - T3/T4 Garrett - EGR/MAF delete
98 E300 TURBODIESEL Alexandrite Green on black leather 289K
95 E300 DIESEL Green Queen 267K SOLD
84 300D 216k SOLD
87 300D 299K #22 head - intercooler - full load adj. - 8sec 0-60mph - SOLD
76 300D 214K SOLD

Ah ha! A clue!

My climate control turns heat on before there is heat. Usually about 1 minute after starting the car. There is no heat available until about 5 minutes though. Not sure why the 1 min delay, but definately doesn't wait to come on until it's warm.

Last year I straightened the fins and cleaned the radiator, replaced t-stat, and water pump. Radiator is only 5 years old.

Still wonder about the circuit for the aux fan. Why won't grounding the sensor turn on the fan? Is there a 2nd relay just for the t-stat sensor? Doesn't make sense.

Anyone know for sure?

__________________
1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer

No. Not according to the electrical diagram for the 1982 300SD.
Per the diagram, the aux cooling fan operation is controlled by the receiver/dryer temp switch and nothing else. (Power being supplied through the aux fan relay.) Meaning that the engine coolant temp is unrelated to aux fan operation.

Interestingly, the 380SL diagram shows a coolant temp switch in parallel with the receiver/dryer temp switch. Meaning that the aux fan could be actuated by either high refrigerant temp or high coolant temp.

I recall that others have insisted that their fan was controlled by coolant temp. But the MB diagrams for the 1982 300D/SD do not support that.

Hey, do we need that fan?
I havn't had one for 2 years...
but I have no AC either.....isn't that what it is for?
Is that a stupid question?

__________________
Think Alternative Energy!
300CD '80 (now gone but not forgotten...)

DOH!!! Don't throw the fans away! They RARELY wear the brushes out, but they very commonly destroy the front bearing (some bonehead -not you- didn't spec out a sealed bearing, so 99% of them are rusted and falling apart.)

I've done a fix on them with a $12 bearing and drilling out and re-riveting in the new one (along with lubing the lower bearing with synthetic grease.) This "rebuild" pretty much cures the bad-fan syndrome and the fan should last twice as long as original with a QUALITY and SEALED bearing in the front.

Look up my post on rebuilding the pancake fan motor.

Again, most fans have perfectly good brushes, but a bad front bearing. Replace that and it's a nice, smooth, long-lasting motor.

__________________
1984 300D Turbo - 4-speed manual conversion, mid-level resto

1983 300D - parts car

1979 300TD Auto - Parts car.

1985 300D Auto - Wrecked/Parts.


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"If you don't know where you are going, any road will get you there". Lewis Carrol

ruling out a sensor because another nearly identical car does the same thing is an excellent way of isolating a problem - unless you are talking about the darn temp sensors. My climate control comes on too early and the sensor to control the aux fan is screwy too. It does whatever it d*** well pleases. All I'm saying is, your diagnosis procedure makes sense except these sensors seem to be fairly problematic when they get old. Mine are.

__________________
'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

A clear picture is imerging. This AM it was pretty warm and the climate control came on right away. Makes me think that the sensor (a) works since there was a change over cold temps and (b) the sensor may be only for the climate control (as suggested by tangofox).

Sounds like only the AC can turn on the aux fan.

When it gets hot, I may try to rig a switch that will allow me to turn on the fan at will.

Thanks for your help.

__________________
1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer

yes, that's it. I wasn't really clear on my last post - the sensor on the t-stat housing is the one that cuts out the heater blower until the coolant temp rises. The aux fan comes on with A/C, and with temp sensor on the receiver/dryer I think.

__________________
'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

This can get confusing, because the 603 is what you were originally thinking. It can turn the fan on 1/2 speed from the pressure switch on the dryer and it also has a 105°F. coolant switch that turns the fan at full speed when tripped.

No. The aux fan is actuated by the temp switch on the receiver/dryer. The pressure switch allows the a/c compressor to run. Or more specifically, it prevents the compressor clutch from engaging if there is inadequate refrigerant pressure in the system.