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.

I also just changed my crankshaft position sensor on my 99 C280. It cured 2 things:
1. Engine died when it gets hot. (really dangerous. I died on me while I was on the Highway)
2. Erratic Fuel Gauge. 3 weeks before I changed it, my fuel gauge were far from accurate. Especially after I filled up the tank. Sometimes it even read empty.

It also costs me $200.

i am happy for you

We all would need a certain test procedure of this sensor.

My car is talling or fuel starving (impression) so I have to try this solution as well.

__________________
Keep us posted especially if your problem is solved
Present cars:
My car: E-class 420CDI, 2008, W211, V8, 180 000km

Wife's one: C-class 220CDI Sport Coupé, Euro, 2002, W203. 345 000km

Son's one: GLK class 220CDI, 2009, W204

Sold E class 260E, W124, 1988 beloved car sold after 489 000 kilometres of reliable services (engine M103, clutch and 5 speed manual gear box all original).

E-class, W210 320CDI, 2000[/B], 225 000km, Sold

Just for completeness of this thread, I should mention that there is a recommended test (AllData) of the Crankshaft Position Sensor (CPS). You disconnect the CPS at the EZL/AKR Ignition Control Module (N1/3) and measure the resistance to ground. It is supposed to be 680-1200 ohms.

However, my CPS passed that test--and later the CPS was still found to be bad. So, there is a test--but it's not reliable. (I just checked the resistance on the new CPS. Resistance is infinite--not a twitch from the ohm-meter.)

My car is performing much better at all speeds since the replacement of the CPS. And the dreaded "Check Engine" light is out for the first time since I've owned the car.

I am happy for you.
Indeed we need to get rid of the "black box philosophy" and try to find objective testings of all these devices.

__________________
Keep us posted especially if your problem is solved
Present cars:
My car: E-class 420CDI, 2008, W211, V8, 180 000km

Wife's one: C-class 220CDI Sport Coupé, Euro, 2002, W203. 345 000km

Son's one: GLK class 220CDI, 2009, W204

Sold E class 260E, W124, 1988 beloved car sold after 489 000 kilometres of reliable services (engine M103, clutch and 5 speed manual gear box all original).

E-class, W210 320CDI, 2000[/B], 225 000km, Sold

My mechanic said that you have to do that test the moment the symptoms happen. In my case, when my car stalled out. If you test it after 10-15 min it will say that it is okay.

When I decided to change mine, it was plainly based on the information from the forum members. (After reading numerous postings)

I just came across this and find it very interesting.

I know it's fixed *but* the CPS as you call it, is just a passive device (magnetic 'ampflier') that consists of wire around the magnet. As the flywheel magnet passes by a 'blip' of current is induced the coil which results in a small blip of voltage. As you know the car won't run without it.

If you measure resistance of the CPS, between the limits you mentioned, the device is GOOD!

Now it could be intermittant: that the coil of wire could short out (similar to the "yoke" on our old tube tvs - OR - the connection to the connector end is also intermittant. Then the car will give the symptoms you had.

The CPS MUST have a DC path through the coil, so if it was 'flexed' and 'pulled', it could break that path, thus killing the function of the CPS.

But are you SURE, that the new one measures INFINITE resistance??? Sorry, it can't! Maybe you would re-measure it, to satisfy me (thank you), and use a higher resistance range on the MM. It should measure in the range you mentioned; 680 - 1200 ohms, the center being 940 ohms. Most of the ones I've measured come out about 860 ohms.

__________________
Regards . . . . JimF
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'94 S500 Cpe

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