All that happens when the AC is that the engine has to work harder, and the condensor adds heat to the air being pulled through the radiator. Maybe your cooling system is marginal and that's all it takes to push it over the edge.

A couple of things might be going on with the sensor. First, it may not be working and making the connection to ground. Second, the thermostat housing may not be a ground if it has a gasket and maybe the bolts are corroded. Easy to check. I dimly remember having seen ground wire running to the thermostat housing bolts from the frame or other ground points.

Another cause of boil-over is an air pocket in the coolant. You might try filling it by disconnecting the top radiator hose at the radiator end and adding coolant through the hose.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe