Every Summer I have the same cooling problem with my 1975 450SLC. The car runs extremely hot at sustained highway speeds (70-75MPH) with the AC on. With the AC off, the temp is fine at highway speeds. In cooler weather, I can run at highway speeds with the AC with out any problems, but in the Summer Texas heat (100 degrees or more ambient temp.), the car wants to over heat. I have recently replaced the water pump, all hoses and thermostat (75 degree). Approximately two and a half years ago I recored the radiator. I am wondering if this is a trait of this vintage SL-SLC, and if the radiator capacity isn't large enough. I am also wondering if the capacity can be enlarged by adding an additional core or two to the radiator and still have fan clearance. I don't want to re due the radiator again and still have the same problem. Any suggestions would be greatly appreciated.

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dbg

Some of these cars did not have an aux fan. Does you car have one and does it work? This is a key. Also, be sure the fan clutch is new or close to new.

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Benzmac:
Donnie Drummonds
1991 GMC Syclone
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

Benzmac made a good suggestion. The fan clutch is often overlooked when there are cooling system problems. Check and make sure that yours is not "freewheeling" when the engine is warm.

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I hate to say it, but you might just have to learn to live with it. A common trait of these early SLs was hot running. Since that big engine is crammed into a fairly small compartment, they tend not to dissipate heat very efficiently. My '77 450SL will begin to get hot when sitting in traffic and that is also with a new radiator, new t-stat, new water pump, etc...I've seen many a 450SL/SLC with this symptom. But follow Benzmac's suggestion and see what happens.

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Rgds,
Aaron Greenberg
MB technician
Precision Motorcars, Cincinnati, Ohio
'67 250SE Cabriolet
'77 450SL
'80 300SD
'85 380SE
'86 420SEL
'89 420SEL
'93 300E 2.8
'74 Jensen Interceptor Mk.III
'81 DeLorean DMC12
'84 BMW 745i Turbo

Yep, mine runs hot also on the higher temp days of summer or when stopped in traffic for long periods.

I have a 1975 450SL and it also gets very hot under the hood. I too have a new water pump, thermostat and hoses. One more thing to keep in mind, at least with my vehicle, the Catalyctic Converters are situated just under the manifolds on each side of the engine also causing a lot of heat.

I’d say ditch the fan clutch and get a good electric fan.

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1993 e300
1995 e320
1994 e320
2006 s500 4matic
2004 Jeep wj overland
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2004 Honda shadow aero