The electronic throttle assembly may suffer an electrical failure. It is often cheaper to send the valve to a service center for repair that to purchase a new part.Google+
Car Problem Reports
Mercedes-Benz SL600 Throttle Valves May Experience Electrical Failure
Mercedes-Benz SL600 Problem
Average Mileage: 110,000 mi (55,000 mi - 165,000 mi)
When the engine wiring harness failed, my mechanic also found degraded insulation in both throttle body wiring sets. The insulation was cracked throughout and falling off, exposing the bare wire. This repair required replacement using high-temperature insulation. I understand that the computer system then has to be re-balanced to compensate in changes in the throttle body wiring (or simply because they were removed and re-installed, not sure).