My 1971 280SL overheated badly with the AC on, even on a normal day, when stopped or moving slow. On a 100+ Georgia summer day, it overheated without the AC in these conditions.
I installed a 13" (I think--maybe 12") pusher fan in front of the radiator on two 1/8" X 1" metal strips about 12" long bolted to the front face of the cross piece below the radiator. I wired it to the AC on/off switch through a relay so it runs whenever the AC is turned on.
That helped at low speeds, but didn't completely solve the problem and it helped very little when stopped. I tested by putting a big industrial fan on the ground in front of the car---it made almost no difference. That convinced me that the overheating problem when stopped (ie, at idle) was attributable in part to poor coolant flow.
So I ran a 3/8" hose from the thermo switch outlet at the left rear of the cylinder head to the return hose between the thermostat housing and the top radiator tank. It worked better than I could imagine. Now it doesn't overheat, even in stop and go traffic on hot days.
It is hard to imagine that the small amount of coolant flowing from the rear of the head, through the 3/8" hose, and into the radiator could make that much difference, but it does. In fact, in winter if I do not disconned or block the 3/8" hose, the engine will never warm up (because the hose bypasses the thermostat).
This suggests that coolant flow in the rear of the head (where the temp guage sender is) is very poor.
For fittings, I got a metric/inch adapter kit for Autometer guages at the parts store for a few bucks to connect a hose barb to the cylinder head. I fabricated from old radiator inlet tubing a "tee" connection in the top radiator hose that a hose barb fits into.
Its not pretty, and one day I will get around to makeing it prettier, but it sure made my wife stop asking for a new car every summer.
Farmerford
Charles H. Ford, Jr.
