Your gonna love me!
If your ASR light stays on but you are not reduced to " Limp Mode" then your problem is the cheap part - The Idle Speed Actuator. It costs about $41.00 (maybe cheaper here on the fast lane or Benzbin.) It is in a weird place. You remove the driver's side rug and it is in the area around the gas pedal. It sort of looks like that pig-tailed thingamabob that ItalianBenz is selling for premature aux fan control engage. Peter Vanderveer replaced that part first, cleaned the TA, cleared the codes, reset the Throttle Stop and it ran GR8 - for about a mile
Your problem can not be the TA yet as you would immediately feel it in the gas pedal. You would have to press it about 3/4 down before you would get any throttle. You may wanna also check your harness between the TA and the ASR. The Kraut engineers just layed it over the intake manifold and the back part of the valve cover on it's way to the ASR computer. The factory supposedly now instructs the tech to install rubber holders to keep the harness elevated above the hot engine parts in it's path back to the ASR.
psfred! You are just about 100% correct. Non Dealer techs can not repair these MB or BMW post '89 vehicles any more without shelling out $4000.00 for a HHT with limited capacity to read DTCs.
Fortunately used parts in the post '89 vehicles are in GR8 abundance in the bone yards for just the reasons you stated. Most problem parts are bolt off-bolt-on, plug in- clear codes and go if you have the electro-mechanical skills and/or willingness to learn and use.