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Old 06-03-2006, 08:12 PM
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Join Date: Jun 2006
Posts: 1
Fan on 1978 450 SL

Hello, First I want to say I have owned this vehicle for 2 years and wish I would have found this sight then! Anyhow, being and amature I have a few simple questions I have always been curious about.

First, I was wondering how the fan works on the 450 SL. I drive school bus, and I know on those the fan is not operational untill the engine reaches about 210 deg, then the air clutch engages and shuts off when the temp drops to around 190 again. Obviously I know a Mercedes does not have an air clutch, but does it have any kind of clutch system?? If I listen carefully it sounds like I can hear the fan slow or stop when I reach a certian RMP. (steady speed) and then it is running again if I go back to idle and start again. This has just never been clear to me exactly how the fan system is supposed to operate on these older cars. Any info would be great.

Also, what is ideal running temperature at travel speed?? (55 and over)
mine seems to stay right around 175-180 (unless idling for a period) I always thought 200 was normal temp (for example 200 is normal temp for my pickup truck) it must vary per vehicle. Thanks!

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Old 06-03-2006, 08:52 PM
Tomguy's Avatar
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Join Date: Aug 2002
Location: near Scranton, PA
Posts: 5,217
The 4.5 was designed to run at 175-180 all day except it can get warmer in traffic.

As for the fan clutch, it's hydraulic. With age they can get weak. If you start running above the (212?) mark above 175 in traffic, you MIGHT need a new one. There are many other things that can cause hot running too (scaled radiator at this age is very, very possible)

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