Sorry, Paul, but that's not true. Low speed aux fans only come on when refrigerant pressure is above the set point. On my M119, aux fans generally do not come on when ambient temps are below 80 degrees F even with AC on.

I'm with buckwheat - I would wonder about how "good" the water pump is. They are known to have impeller problems, so they can still pump coolant but not as efficiently as they should. As I recall, this is a particular problem with aftermarket pumps, but even OE ones can eventually wear.

The fact that coolant temps are getting to the point that they are really seems to point to inadequate coolant circulation.

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Steve
'93 400E

I disagree. With the a/c on, the aux fan should be on nearly all the time.

I think we'll have to leave it to Enrique at Mr. MB Motors. He's had more than enough time to figure this one out.

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".

Here's what JimF says:

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The fans are supposed to come on at (around) 100C w/ the A/C "off". 107C is a bit high but I've seen a few cars that kick-in at the temp.

If it's a HOT day (90F) and you start the car w/ A/C "ON", the aux fans will come on when the pressure reaches around 14 bar or 206psi. For example, my fans come on at 15bar, apx 240psi. Check MENU#18 about in the middle of the section; the table shows the pressure and temperature kick points for an S car. Yours is similar except you don't have the number of stages (3) that the "S" has.

If the weather is cool, the fans will not come on even w/ A/C on if the pressure is not exceeded. Here in So Cal, my fans don't come at all even on an 80F day for normal driving until the PSI or TEMP is exceeded.
If you want to turn them on at a lower temp, check out my page and the "Cool Harness".
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Regards . . . . JimF
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Not wanting to argue - just think that going after the aux fans is barking up the wrong tree in this case.

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Steve
'93 400E

I think what Jim is referring to is the high speed fans.

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".

Jim refers to the "aux fans". What the difference between the aux fans and the high speed fans?

Jim's writeup doesn't go into detail about the different speeds of the "aux fans." On the W124 chassis, at least, the aux fans are designed to come on at "low" speed in response to high AC refrigerant pressure (to prevent compressor damage). They are designed to come on at "high" speed in response to a coolant temperature switch reading 107 degrees C.

He makes a reference to their being a third stage on some S-class chassis vehicles. I know nothing about this...

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Steve
'93 400E

I hate to keep beating this topic, but my car is STILL overheating. Enrique did install a 'cool harness' to make the aux. fans come on earlier and installed a new temp sender. The new temp reading was 80 to 100. After getting the car back, I noticed my mpg had dropped from about 16 to 11. I've since removed the 'harness' and my mpg has gone up to 13-14. I don't know why.

Now the aux. fans are on almost all the time, temp is between 100 and near 120. Stopping at a traffic signal, the temp will climb to near 120, but as soon as I move, it'll drop to 100.

Two nights ago I let the car idle and got the engine temp to near 120. The aux. fans were at full speed, sucking air into the radiator. I popped the hood and put my hand behind the fan, and there was hardly any air moving toward the engine block. I revved the engine and still no air flow. I think I probably could have kept a candle flame lit. I shut the engine off and watched as the fan kept spinning and slowly came to a stop.

If I understand the way a fan clutch is supposed to work, my fan should have been engaged and stopped immediately with the engine. I think I have a defective clutch. As I said in my first post, it's a Vemo brand, made in Germany. My gut feeling is to return it for a genuine MB part.

Any ideas? Thanks.

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1994 E420, Pearl Black/Black. 2.82 rear diff., AMG front spoiler, painted lower half. SOLD

1972 & 1974 BMW 2002tii's.

You are correct for the W124 chassis and your writeup is 'right-on'!

For a W140, MENU#18 shows a table in "BLUE" in the middle of the page. That one table says it all for Aux Fans (A/F) operation for both A/C and no-A/C, just engine temperature.

My car has a three (3) speed system; low, mid and high. For A/Fs to come on LOW, the coolant temp has to be around 100C ---- or ---- if the A/C is on, the pressure has to be greater than 14bar (206psi). So if EITHER of these conditions are exceeded, the A/Fs will come on.

And the corrollary is true, if neither the compressor pressure or engine temperature is exceeded, then the A/Fs will NOT come on.

When driving on a cool day w/o A/C, the fans should NEVER come on except where you exit into stop/go traffic; then they MIGHT if the conditions described above are exceeded.

For Dcraig's car, the temperatures described are not reasonable; should never be that high.

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Regards . . . . JimF
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'94 S500 Cpe

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