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.

------------------

dbg

David, I had the same problem on a 74 450SL. My problem was the ignition timing. It had slipped to 30 degrees retarded. It still ran but overheated. Also, The radiator core could be the wrong type. Other things to check. Wrong impeller clearance on aftermarket water pump use only OEM, Fan clutch, faulty or no aux fan, clogged AC condenser coil. I have also heard of a desert package that includes a bigger fan blade for hot weather driving. If your car has a tranny cooler in the radiator check the tranny to make sure it is not overheating.
Thats all I can think of now.

BTW the 107 is prone to overheating just for the fun of it.

Bill