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Old 04-22-2003, 07:45 PM
Registered User
Join Date: Apr 2002
Location: Portland Oregon, USA
Posts: 65
Aux Fans broke? / 92 300e 2.6

Hello All,

I have an interesting story to share about the Aux fans on my 300e:

So i bought the car approx 6 months ago, and haven't once heard or seen the aux fans (my car has the dual fan setup) turn on..ever.. regardless of temperature or AC setting.
I went to inspect the fans themselves, and discovered that the fan on the passenger-side would NOT turn by hand- it appeared to be stuck!.. Closer inspection revealed that a mechanic's wrench had long ago fell from the engine compartment area and had lodged up between the blades of the fan (!)..preventing movement of the passenger side fan unit.I removed the wrench and also discovered that the wrench broke off one of the blades of the fan.
I keep wondering if, because of this, the fans have "burned out" (because the passenger side fan could not physically turn)? Oddly, the drivers side fan DOES turn by hand without resistance, but I have never seen it rotate by itself, under power. Is there a way to force the fans to power on (for testing purposes)..? Can I replace the whole assembly? (FastLane has an assembly listed).. or is there some sort of relay/fuse for me to test??

ANY help is (as always) *greatly* appreciated..Thanks!!

92 300e 2.6 Silver

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Old 04-22-2003, 08:26 PM
Registered User
Join Date: Jun 2001
Location: Omaha, NE
Posts: 962
Have you checked the fuses for the aux fans? If one of them was jammed, you probably blew the fuse.

The aux fans only run if 1) the engine temp is too high; or 2) the AC pressure exceeds a certain value. They do NOT run whenever the AC is on, as they do on some vehicles.

If you haven't already done so, do a search and you'll see lots of info on diagnosing aux fan problems. But start with replacing the fuses ...
'93 400E
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Old 04-22-2003, 09:02 PM
haasman's Avatar
Registered User
Join Date: Jun 2001
Location: San Francisco, CA
Posts: 3,097

Be there, done that! Only in my case it was a nest! Some varmint had built up quite a wad in between the fan blades over th winter. What, to me, is most interesting, is this car was used daily .....

Yes you can test them ... find their connector, get a 12v source and ground and see if they will turn.

I make it a regular item to check now, doing just what you did and checking to see if they will turn.

Keep us posted,

'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)
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Old 04-23-2003, 12:44 AM
Registered User
Join Date: Oct 2002
Posts: 188
I believe your car has the M104 engine.

You can test the operation of the fan by un-pluging the ECT sensor harness. Some say the aux fans should turn automatically. For my car, W140, this test dose not activate the fans. But I have read many W124 turns this way.

If it dosen't turn, than you can use a resistor and connect it between the harness. Basically the fans come on at different speed depending on the Ohm reading from the ECT sensor. By puting a resistor between the harness it will trick the computer thinking the car is hot enough to turn on the fans.

How many ohms??? I don't have the figures with me but if I remeber correctly, 300 ohm should do the trick. If not, I am sure anything lower like 2 hundred something will definitely do the trick. Let me know if you need exact figure. This test also help you to test different stage as the fans at different speed depending on ohm readings
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Old 04-23-2003, 09:05 PM
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Join Date: Apr 2002
Location: Portland Oregon, USA
Posts: 65
Hmm.. My car definetly has the M103 engine (2.6)- the smaller version of the 3.0. Would this be the same procedure? Hopefully there is some way of switching them on, at least temporarily, to verify if the motors still work.. BTW i did check the fuse (#7).. it was intact...

hmmm.. the mystery continues!...


92 300e 2.6 Silver
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Old 04-23-2003, 11:19 PM
Registered User
Join Date: Oct 2002
Posts: 188
OK... your aux fan should come on at low speed when your car reaches 100 degree. The ECT sensor should feed 310 Ohm to the harness and therefore, the car knows its 100 degree thus turning on the fan.

Here is the test, unplug the harness from the ECT sensor. Than get a 310 Ohm resistor, or something lower to be sure like 200 Ohm, and cross the connector. This should turn the Fans on. If a 310 Ohm won't turn it on, a 200 Ohm must turn it on as it should register something way beyond 100 degree on the car computer.

Test your ECT sensor too. Just get a Multi-meter and check for resistance across the 2 pole on the ECT sensor. At 100 degree it should register 310 Ohm.

Once you confirm the fans as well as the sensor are working, the next thing to test is the different stages. The car have either 2 stage or 3 stage fans. each of these stages kicks in depending on temp thus Ohm from ECT sensor. If I am correct, its 100 degree for 1st stage and 2nd stage at higher degrees. Again I don't have detailed spec with me but I am sure other members know. Find out what Ohm will trigger each stage and start testing by crossing with the approporate resistor.

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