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Check Engine Light (MIL) Diagnosis
Mercedes S(140), SL(129), E(124, 210) and C(202) class have multiple systems which can turn on an Check Engine Light.
All related systems must be tested for codes and repaired before the light will extinguish.
Injection System Diagnostic Socket Pin and CS1000 System to Use
129 LH
(1992-95)
LH (pin 4 & 5), EA/CC/ISC (pin 7), BM (pin

, DI (pin 17 & 18) and DM (pin 19 check stored and
registered codes only)
140 LH
(1992-95)
LH (pin 4), EA/CC/ISC (pin 7), BM (pin

, DI (pin 17) and DM (pin 19 check stored and
registered codes only)
124 HFM
(1993-95)
HFM (pin

, EA/CC/ISC (pin 14), and DM (pin 3 check stored and registered codes only)
140 HFM
(1993-96)
HFM (pin 4), EA/CC/ISC (pin 7) and DM (pin 19 check stored and registered codes only)
202 HFM
(1995-96)
HFM (pin 4), EA/CC/ISC (pin 7) (except C220) and DM (pin 19 check stored and registered
codes only)
210 HFM
(1996-97)
HFM (pin 4), EA/CC/ISC (pin 7) and DM (pin 19 check stored and registered codes only)
Note: ME-SFI injected vehicles integrate Injection, Ignition, Electronic Accelerator,
Diagnostic Module and Base module into the ME controller module.
Mixture Adaptation:
The Lambda control system precisely determines fuel injection duration so that the fuel/air ratio is consistently kept at
Lambda equal 1 (Lambda=1 is 14.7 kg air per 1 kg fuel) under all operating conditions.
Should a malfunction occur in the form of:
u Intake air leaks
u Injector defects or carbon build-up,
u Air Flow Sensor defects
u Pressure regulator defects, such as a blown diaphragm.
u Fuel tank purge control valve defects or EVAP system leaks.]
u EGR defects
u Vacuum leaks of any kind.
u Mechanical engine wear, such as, chipped valves or leaking rings.
The engine control module automatically performs a mixture adjustment. The degree of correction is calculated constantly
and stored in KAM (Keep Alive Memory) RAM. The self-adaptation is performed at idle and under partial load. Maximum
correction towards rich or lean is 25%. After repair work is performed, the engine control module will automatically adapt
itself again after approx. 10 trips. After eliminating a malfunction or after trial installation of an engine control module from
another vehicle, the self-adaptation feature must be reset to its mean value.
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"Resetting and Reactivating for BOSCH Engine Control Module Memory"
For LH & HFM systems only.
To reset and reactivate the module :
1. Read and clear all fault codes
2. After display of 1 (No faults present) short the diagnostic plug (pin 8 for 16 pin diagnostic socket, pin
4 for 38 pin diagnostic socket) to ground for 6 to 8 seconds
3. Switch ignition off and wait at least 5 seconds
4. Turn ignition on, wait minimum of 10 seconds then restart engine.
Long Term Adaptation (Additive) - Engine at idle.
Short Term Adaptation (Multiplicative) - Engine at partial load
The correction towards rich or lean is + - 1.0 msec (Injection Duration) at idle and the factor of 0.68-1.32 at partial load.
After repair work is performed the engine control will automatically adapt itself again (ME injection) over the course of 10
TRIPS.
Codes Present in the Absent of Trouble Light
Not all control systems will trigger codes or turn on malfunction lights when codes are stored.
INJECTION/IGNITION system problems will trigger trouble codes, but may not turn on the MIL unless the fault results
in a change in the exhaust gases or can damage the engine in the short term. Also some mechanical problems in
earlier cars (pre 1988-96) can cause poor drivability without ever tripping a code or turning on the light. On later
models that is much less likely.
BRAKING/TRACTION systems will only turn on their check light when the system has become inactive. It does not
mean there isn�t a problem and codes haven�t been stored.
TRANSMISSION systems will turn on the �Check Engine� light if any shared system is detected as failing test.
AIRBAG and RESTRAINT SYSTEMS will turn on the light and record codes if incorrectly coded for the car. A good
example is the 1995 C280 and the 1995 S320. The same controller is used in both vehicles, however the S-class has
more features in the Airbag system such as side bags and baby seat detection. The coding of the controller for the
C280 masks the features not present so the controller doesn�t register a code. The controllers generally come off the
parts dept. shelf coded for maximum features.
In All Systems low or high battery voltages can trigger invalid or multiple codes without real failures of the indicated
components. Always check the battery condition before starting analysis.
Using the Data Stream to Diagnose/Confirm Faults
The Serial Data Information Stream of the ECU is a �window� into the operation of the system under test. By looking at
the values of the suspect components in operation and the computed values of the ECU, we can build a picture of the
operation of the system and what is causing the fault. The interpretation of the Data Stream is beyond the scope of
this book. Please refer to the Diagnostic Manuals from Mercedes Benz for complete discussion of the topic.
The nominal values for all Mercedes vehicles 1990-2000 can be found in the Mercedes Engine Diagnostic Manual
Volume 1 Section A.
A listing of these manuals can be found at the end of this document.
Multiple system and component faults can almost always be traced to faulty wiring
harnesses or water damage.