Hello Dick,
When cold the thermostat closes and prevents the passage of coolant to the radiator and at the same time it opens up a by-pass, circulating the coolant while by-passing the radiator. There are actually two functioning gates (round discs) built into the themostat. Notice the short by-pass hose between the themostat housing bottom and the waterpump. As the engine warms up the second gate in the thermostat closes off the bypass and opens the other disc so the coolant can be routed to the radiator.
If the themostat is left out the bypass will be wide open and some coolant will bypass the radiator.
By turning on the heater, you are increasing the capacity of your cooling sysytem, or enlarging your radiator in a sense. This could indicate that your radiator is a bit weak. If your car continues to get warmer as your car travels down the highway at speed, this is a sure sign that the radiator is not up to snuff.
If the engine cools down at highway speeds then there can be many other issues, radiator, themostat, waterpump, viscous coupling, fanblade, clogged cooling system, engine problem.
I took care of some late W113 cars in the early 70s when they were only a couple of years old. The late W113 USA cars stuck in traffic jambs on very hot days would run hot and eventually overheat. One of these belonged to the late Allen Funkhouser, National MBCA President at the time. His 280SL was show condition and the maintenance was always on time and by the book. He bought his 1971 280SL new. If caught in traffic on a very hot day, eventually, he would have to switch off the AC in order to keep the car from overheating.
Joe Alexander
Blacklick, Ohio
