For the benefit of others who are experiencing the displeasure and danger of "Limp-In Mode", I would like to report on the final resolution of a problem which has endured over many months and has been somewhat difficult to diagnose.
Symptoms
My 1992 400SE would intermittently go into limp-in mode. It happened at very inconvenient times (e.g. middle of an intersection). It sometimes occured as frequently as twice in a day and sometimes as infrequently as once every few weeks. The failure occurred at all speeds (e.g. 10MPH and also at 70MPH). The "Check Engine" light was "On".
The Diagnostic Module was displaying a
code 6 (Idle Speed Control). The CC/ISC Module was displaying
code 10 (Engine speed signal (TNA) from base module (N16/1)).
Clearly, the CC/ISC Module didn't like the TNA signal it was getting from the Base Module. Tracing backwards, the following units were involved in generating the TNA signal:
Crankshaft Position Sensor (L5) ---> EZL/AKR Ignition Control Unit (N1/3)
--> LH Control Unit (N3/1) --> Base Module (N16/1) --> Cruise Control/Idle Speed Control Unit (N4/3).
Another symptom was that the tachometer and the oil pressure guage had the "yips" (i.e. they would periodically and simultaneously fall to zero and then back up to normal again). The wiring diagrams revealed that the same TNA signal which went to the CC/ISC unit also went to the tachometer.
What to Fix?
Hasty troubleshooting could resulting in needless replacement of some very expensive components (e.g. Air Mass Sensor, EZL/AKR Ignition Control Unit, LH Control Unit, Base Module, Cruise Control/Idle Speed Control Unit, or Instrument Cluster).
With the codes mentioned, the recommended fixes (AllData) were: Base Module (N16/1) or wiring.
Further incrimination of the Base Module was the fact that its input signal appeared to be good. The Base Module and the LH Control Unit both test the input signal (TN) which the Base Module needs to create the TNA signal. So, Base Module replacement was indicated--but maybe not!
At one point, I even suspected the Instrument Cluster--reasoning that the Instrument Cluster might be loading the TNA signal. (I temporarily removed the TNA signal from the instrument cluster to disprove that theory.)
Final Resolution
Pure and simple--a bad Crankshaft Position Sensor (L5) caused this problem. I've just replaced the CPS and all the symptoms are GONE!
Happily, the CPS is one of the cheaper solutions (It's about $200 at the MB dealer. Still quite a lot to pay for a wire!).
Installing the new CPS is NOT a do-it-yourself job. The lower-end of the CPS connects to a relatively inaccessible area (i.e. flywheel portion of crankcase) of the engine. So, pay someone to put it up on a lift and get the job done quickly and safely. It's a 15 minute job when a lift is available.
Many thanks to the many ShopForum contributors that have reported closely related situations that helped me arrive at a successful conclusion to this problem. I hope this report is helpful to others in the future.