The electric, or auxiliary fan on my 300e does not come on, even when the temperature gauge gets all the way up to 115 celsius. I hooked the fan directly up to the battery, and it works. I did an extensive search on this subject, but I am still a bit confused as to the location of the temp. sensor that engages this fan. There is one green sensor right next to the air cleaner, a little past the middle of the head, with male pin ends sticking out of it. The two pins that are right next to each other on the sensor measure -1 on an ohmmeter when the temp. is at 110 C, and 1 when cold. When I pull the two corresponding wires off this sensor and connect them, I here clicking coming from the relays behind the fuse box. I switched two relays with identical part #'s, but the fan did not come on.

The other possible temp. sensor is located towards the back of the head. It is also green, but has two female pin ends, with the male on the wires that connect to it. This temp. sensor measures .3 when engine is at 110 C, and,of course, 1 when cold. I connected the two male ends that come off these wires with another piece of wire, but alas, the fan still did not come on.

My questions are:

1. Which sensor is for the fan?
2. Shouldn't the sensor closer to the back of the head measure .00 ohms when the engine is at 110 C?
3. Considering the fact that bypassing both these sensors still did not make the fan come on, where should I look next? (although the sensors might very well be faulty, too)
4. My A/C does not work because of a lack of freon. I know that this stops the slower A/C controlled fan speed from engaging, but is it possible that it could stop the higher, engine cooling, fan speed as well? Or are they completely separate circuits?

Thank you very much for any and all replies.

p.s. #7 fuse is good.

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headrivenoise... listen.

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Fuse 7 only feeds power to the coils of the aux fan relays... that is why you hear the relays clicking in. BUT the fan LOAD fuse on your chassis is fuse 'D" -16A..check that one.
If that's OK, take a peek at the connection at the dropping resistor behind the drivers side headlamp......

You can also check the low [ a/c] fan by jumpimg the high pressure sw at the drier. The a/c need not be fiully charged for this test.

dropping resistor? I unplugged the fan to test it at the plastic pin connector behind the driver side headlight. this connector slides into a clip that is attached to the car. Where is the dropping resistor in relation to this? Or is it closer to something else?

So, since I hear the relays clicking in when I bypass the sensor near the air cleaner, does it mean that this is the one that controls the fan?

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headrivenoise... listen.

Dropping resitor is around there somewhere .
This is what you are looking for :
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1A70KFNW8&year=1987&make=MB&model=300-E-002&category=G&part=Aux.+Fan+Resistor
It's main funtion is to reduce the fan speed for the /ac circuit. They series it into the fan circuit with the a/c aux fan relay to get the low speed.
But the reason I want you to look there [ after checking the proper fuse] is cuz they also use it as a connection terminal for the main line to the fan. It is quite common to find these connections burnt to the point of no longer conducting power.
If the relay is clicking and the fan is still not turning, I doubt you have a sensor problem. You have a fan motor power problem.
The sensor test for that circuit is to ground the bn/gy wire at the 3 way sensor. But this only clicks the relay in. The load side of the relay is where you want to be checking components [ ie .-fuse D and dropping resistor connection]

ahh, that thing! I know exactly where that is. For some reason I pictured a resistor looking quite different. Will check today. Much obliged, Arthur.

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headrivenoise... listen.

About your fans...on my E320 I thought they might not be working as the temp reached 110C on a hot day with the a/c on...but they were just running at the slow speed. I think they should be running maximum by 115. I know the 300 is a different motor, but on my car fan speed is first tripped by water temperature. There is a single wire lead off the Water Pump, this is for the dash guage...then there is a four wire clip which goes into the harness. It might be brittle...be careful...but with the engine running just pull off the clip and the fans should operate at maximum. Maybe it is different on your car, or maybe this is the problem.
As a last resort to combat overheating, you could always wire the fans directly to 12V with a dash switch inside the car (even in one of the extra slots in the row of toggle switches) and just flip the switch when the temp goes up.

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2002 Mercedes-Benz E320 4Matic
1995 Mercedes-Benz E320 Wagon
2005 Land Rover LR3 V8 SE
1999 Audi A8 4.2L quattro

Thanks Arthur, the fan works!!! I switched out fuse D (which actually looked O.K. anyway and is now being used as the fuse for the window switches). I also took out the aux. fan resistor, which was noticably brittle with a very rusted inner coil. I am just a bit confused because the fan works right now with or without connecting the two wires that ran to each side of that resistor. Is it possible that the fan is wired in such a way that the electricity can bypass the resistor if it is not present, but the resistor will "trap" the current if it is hooked up, but not conductive?

Electronics boggles me, but I do enjoy it.

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headrivenoise... listen.

The reason your fan will run without connecting up the resistor is cuz the resistor is only in the circuit for LOW [a/c] fan. It is bypassed for the high
[ coolant temp] fan.
So, you now have high fan, but no low..hook it back up.
The same fuse [ D] feeds both high and low fan .
Sometimes the fuses just have to be rotated in there socket to remake a good connection...

yeah, I thought maybe the fuse just needed to be jiggled a bit. problem solved cheap and easy. Thanks Arthur.

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headrivenoise... listen.

Cool..
Actually, there is a point to bring up here for any others reading this post..
When checking fuses for aux fan circuits, one wants to note that there are TWO fuses
One is the fuse for the sensor/coil side of the relay[ low current] and the other is for the load/contact side of the relay [ high current]
and...on top of that ...some have individual load fuses for each speed [low and high, as there are 2 relays]
soooo... if you check for a fuse problem, be aware of this before ripping stuff apart and buying sensors.

It is usually the load side fuse that blows/fatigues, but not always