heat in passenger compartment (450sl--1973)
 

While driving the other day I noticed alot of hot air coming into the passenger compartment.I moved all the a/c levers up ,down,closed but I still can't stop the hot air from coming in. please advise.

On early SLs the heater valve was shut with vacuum controlled by a switchover valve (vacuum relay so to speek), that was controlled by a microswitch at each heater lever. either side asking for heat opened the water valve.

Problems can be the valve, the vacuum, the switchover valve and the micro switches. Make the heater valve go closed or pinch off the flow to evaluate.