I recently replaced the AUX fan in my '85 300D so the A/C would work properly. Now the fan blows constantly as soon as I turn the ignition to on. This happens with the A/C off completely and the coolant temp "normal" (below 80 C). I've replaced the fan relay to no avail. Any ideas?

sounds like the temp switch on the receiver/dryer has failed closed. on your '85 300D, the AUX fan has nothing to do with coolant temps, only refrigerant temps. The switch on the dryer is supposed to close at 144*F, energizing the aux fan relay, and turning it on.

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cbjukraine
'84 300D

I agree that the Temperature switch on the Receiver/Drier is most likely the culprit.......

SB

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Diesels:
'85 300D, "Max, Blue Benz", 155K, 27.0 MPG
'84 190D 2.2, "Eva, Brown Benz", 142K, 40.2 MPG
'77 240D (parts car)
'67 Eicher ES 202 Tractor "Otto" (2cyl, Air Cooled, 30HP)
Gassers:
'94 Ford F-150, "Henry", 170K (300 Six) 17.5 MPG
'85 190E 2.3, 148K....Parts Car
'58 Dodge W300M Powerwagon (Flat Fenders) Less than 10 MPG

Not being familar with AC systems can you just swap this part out because I have the same problem. Will it cause the system to discharge?

Yes! The probe is measuring the temp of the refrigerant directly and when this part is removed, your refrigerant will escape. I disagree that the AUX fan has nothing to do with Coolant temps. Running it will help keep your engine cooler in stop and go driving and my fan will cycle on when my AC is not running when my engine heat reaches 100C! My 300d is a 1983 and has two sensors for activation. I don't know if all have this feature, but it sure is nice! A second sensor is something I would consider adding if you live in an area with hot summers!

Thanks for the info; I've ordered a new switch as the fan cut-off when I unplugged the electrical connector to the temp switch. I thought the fan automatically came on with the A/C; not the case? Now I'm wondering if I should just leave it "broken" since I live in Alabama and its about to be hot...

Is this also true for an '82 300SD?

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags

The aux fan is certainly related to overall engine temps, but what I meant is that there is no direct coolant temperature sensor that turns on the aux fan, only a refrigerant temp sensor on the dryer that closes at 144*F. This is all according to the wiring diagram in the FSM for your model year 83 300D (same for 84 and 85). I'd be interested to know where the extra temp sensor is on your vehicle and how it was added to the circuit. I've though about adding a manual switch in the cabin to turn on the aux fan, but don't know if it's really necessary.

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cbjukraine
'84 300D

Some temp switches are installed in a threaded port which is not drilled into the interior of the receiver/drier. If your switch looks like this one, you can replace it without discharging the system:
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=22816GTJL22816H93V&year=1982&make=MB&model=300-DT-001&category=R&part=A%2FC+Temp.+Switch

I'd be interested in knowing where the second sensor is too! I really don't know. I just know that when my AC or climate control system is off and the motor temp reaches 100C, my electric fan comes on. I never looked at the wiring diagram. I just assumed this was a factory setup. I've heard it start up in stop and go driving and when I can get up to 30 MPH or more it shuts off by itself! I've never looked for the second sensor, but I will now when I get the chance!

when your engine runs that hot, the entire coolant volume should be moving through the radiator. the radiator is probably easily heating up the freon in the condensor to 144*F, which closes the switch on the dryer, and voila, your aux fan comes on. no extra sensor needed.

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cbjukraine
'84 300D

This is incorrect. {sorry Knightrider} Tango has the correct explanation, and it even comes with a link to a picture......
The pressure switch is the only one that taps into the freon "circuit".
I also don't know this for sure, but I believe the explanation by cbjukraine of the aux fan coming on when engine hot and AC not on is correct. At least it makes sense. {ie: heat from radiator to condenser lines to temp switch} My '80 wagon and my '85 both did this.

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Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John

Well I had to change my temp switch on my receiver/drier and I thought the same thing too, that it would be in a port that would not allow the refrigerant to escape. Boy, was I wrong! PSSHHHT and get the hell outa the way! well I replaced it and now my system is working fine and no problems. My fan would not come on at all and when I jumped around this switch, it would start. Maybe I have a aftermarket drier in there. As far as the fan starting at 100C without the AC, I'm all for that!

Some replacment rec/driers have two ports for the temp switch. (This is in addition to the port for the low pressure cut-off switch.) One is external, the other allows direct contact with the refrigerant and requires a different temp switch than originally installed.

I didn't replace my receiver/drier, but I did put the switch where the other one came out. I guess I'm lucky then because my new switch was totally sealed up and I put it back with some threadlock softset and have had no problems or leaks. I'll leave it alone until it needs something else! I know my engine gets up to 100C around here in summer so it wouldn't suprise me if the temp sensor was turning the fan on by itself. Phoenix can get to 120 degrees in the shade during the summer months