If I understand you correctly, you are talking about the switch that is mounted on the venturi and tells when the throttle is off idle. On my '68 280 SE 111, the switch signal goes to the automatic transmission double acting solenoid. The part number for my car is # 000 545 33 04. Not sure what year your SL is, but very well could be the same number.
They can be cleaned by removing the three small slotted screws. The screws are peened on the back so they are a little hard to turn at first. Before you totally disassemble it, note how everything is oriented. Once apart the points are visible. Use contact cleaner to remove grease or dirt. A little filing may be in order to dress the points up. Once back together, you can test with an ohm meter. If the points are shot, you may have to find another. Salvage yards should have some but will probably need cleaning.
Bad news is that I bought a new one last year and now they are NLA from the classic center. Amazingly, that new switch failed after 6 months. There is a post on this site where I described how I cleaned the points and would work for a couple of weeks and then fail again. I repeated this many times with the same result. It turns out the point material was bad and was fouling itself out. I sent it to a electronics rebuilder, and they replaced the points and now it works again.
