once unplugged, fans should ideally come-on at full speed..if not, something is not right (resistor or relay)

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1993 300E, 2.8 M104 .....

Check your fan resistor. The connectors often get corroded - break off or wont condcut the fan current through the corrosion.

Fan resistor is underneath the ABS module. Caveat this is for the later M103 update version.

Pulled the two connectors with Key ON to ignition and fuel pumps energized....Nothing happened. I checked that resistor and I don't see any corrosion on the leads.

Is the relay one of the metal square boxes under the fuse box? I wonder if I can switch the one from my 88 300TE as I know that functions right as a test?

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

Slightly off-topic, but does anyone else feel that 105 is a bit too high for the aux fans to come on? My 300SD plants itself at 80 degrees and does not move a millimeter when in traffic, hot, cold, etc. When I got my 300E I was extremely concerned when it hit 100* in traffic and then went down to 60* on the highway. I have never seen a cars temp fluctuate so wildly. I take it 105* is "fine" then and will not hurt the engine? I get nervous when it hits the 100 mark.

Edit: yes, I do realize that diesels create less heat than gas engines, but in comparison to other gas cars, the temp gauge is wild.

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth

The relays should be interchangeable. If you pulled the 2-pole connector off the temp sensor (the farthest right one in the earlier photo) with key all the way on, engine off and the fan did not spin, there is a problem in the circuit somewhere. Have you tried bridging the pigtails coming off the drier pressure switch? That gives you low fan (KOEO), and would be a test of fan function. Other components to test: 2 fuses. I think they are 7 and D, one is for the coil side of the relay (labeled C in the attached photo) and one is for the load side. Then there's the relay itself, the ceramic step resistor in Ivanerrol's photo, and the wiring from there to the fan motor.

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't

You can just bypass the resistor.
Yes I agree. 105c is too hot. I'm eliminating the clutch fan on my 2.6 and triggering the aux fans at 90c with a 700 ohm parallel resistor in the coolant sensor plug. But it sounds like your thermostat has failed because the temperature shouldn't drop to 60c. I would replace it with a 79c thermostat.

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CENSORED due to not family friendly words

My 300E engine can never reach 80*C on the highway. It can in the city. With the engine not running the fan can still move. But when the engine is running it seems to run at the same speed. I notice that there is no electrical connection to the fan.

Is my engine running too cold? Is the fan clutch activated by electricity? If so then how can I test whether the clutch is still working and how to get its correct connection wire?

Not if he wants low fan speed.

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't

If the temp gauge sits just a hair below 80c on the highway, you probably have a 79c thermostat. I wouldn't worry about it. If its much lower than that, your thermostat probably failed.
I don't see the benefit to having a low fan speed. On my car low speed is only used by the AC pressure trigger and the resistor is another potential failure point. After 100k+ miles all our radiators are stuffed full of dirt, rocks, debris etc. Run a high pressure water jet through the back of the back of the radiator fins and you'll be amazed at the crap that comes out. The more air you can flow through the radiator at lower temp, the better.

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CENSORED due to not family friendly words

[QUOTE=tjts1;2465815]If the temp gauge sits just a hair below 80c on the highway, you probably have a 79c thermostat. I wouldn't worry about it. If its much lower than that, your thermostat probably failed.

Thanks, tjts1. To be exact the needle is roughly 2mm below 80C on the gauge on the highway. Another question: is there any control wire to the fan clutch? I don't see my fan have it.

The 300e should have a regular viscous clutch fan so no wire. When they fail, the clutch doesn't engage as the engine warms up. An easy test is to allow the engine to warm up (~90c on the gauge), pop the hood and try to stop the fan with a rolled up news paper. Be VERY careful with your fingers. If you can stop it or slow it down substantially with the newspaper, the clutch is probably toast. If you're temperature gauge never goes above 100c, the clutch fan is probably fine.

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CENSORED due to not family friendly words

When the engine doesn't run the fan is movable. When the engine does I can still keep the fan from moving by holding it but when I release it it seems to run with the same speed with the crankshaft meaning it always runs as if it is always engaged with the pulley of the crankshaft.

My car occasionally reaches 90C when running under the sun with the A/C on. Most of the time it is around 80C in the city and less on the highway. And the funny thing is my aux. fan starts when the temperature gauge reads slightly above 80C. Is there anything wrong here?

Running a too hot engine is definitely not good but what is the down side of a cold engine? I heard someone say normal operating temperature of Merc engine is 90C.

Which one is it?

When spinning it dead cold, it only spins about 1/4 of a turn. It definitely is not free spinning. Quite some resistance.

I bought a used working fan coupling either way(Seller showed video of it working and it is only 2 years old), but just curious as to the correct test procedure.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

Standard is an 87C thermostat. You should be running around 90C or so most of the time. Is your gas mileage poor? If so, then you may be running with the cold start injector going all of the time.

That doesn't sound right. Maybe the temp sender or the temp gauge on your dash is bad.