Diagnostic Connectors X11/4 and X11/21
The standard diagnostic test point for all W140 autos is the X11/4, 38 pin connector located in the F23 module box in the right rear portion of the engine compartment. The module box is closed and air cooled by a blower motor (phased out at end of '95 model year). The cap which allows access to X11/4 contains a rubber gasket to prevent warm engine air and water entry. Note the auxiliary Diagnostic X11/21 switch with LED for local DTC readout of the DIAGNOSTIC MODULE (N59) only.

The five (5) ECUs in the F23 box are (California):
N16/1 = Base Module
N59 = Diagnostic Module (California)
N4/1 = Electronic Accelerator/Cruise Control/Idle Speed Control Module
N30/1 = ABS/ASR [AntiLock Brake System/Anti Slip Control] Module
N3/1 = LH-SFI (Sequential Fuel Injection) Module

The picture below shows all possible combinations of ECUs for both California and Federal version vehicles.

X11/4 Pin Map

ECU Connection Pins of Diag Connector X11/4

Controller Area Network
The controller area network (CAN) connects all ECUs together electrically so information can be transmitted from one control module to the others. This exchange network results in more information for the control modules, less wiring and lower vehicle weight.

Purpose
The Diagnostic module (N59) monitors the engine control systems and their ECU's to assist the technician during the check/ troubleshooting process. If the module detects a system/component malfunction, it illuminates the 'CHECK ENGINE' light in the instrument panel.

The diagram below shows the interconnection of the five (5) major ECUs (black boxes) and controlling/system monitoring produced by each module. The CAN bus ties all of the ECUs together. All faults are stored in the DIAG module for convenience (California cars only.)

DTC Fault Levels
Some ECUs have multiple levels of fault storage. These are called 'Active', 'Registered' and 'Stored'. Here's what they mean.
Active Faults - These faults are detected while the car is running at idle or speed. They represent a component that may be or is currently failing. These codes cannot be erased. They are only meaningful with the ignition on and the engine running so DTCs found in this system with the "key-on-engine-off" usually have no meaning. Sometimes it shows a components not present on the car as failing so these codes do more to confuse than to serve as a diagnostic tool. But if the code persists, and the CEL lights, then it indicates a bad component but not necessarily the DTC that's stated!!! Thus the confustion.

Registered Faults - These faults are recorded in the temporary memory of the of the car's ECUs. This temporary memory records the the fault indicating that it has occurred and is continuing to occur but it hasn't exceed the preset number of times to make it a Stored fault. When that certain number of failures has occurred, the fault is now moved to permanent (Stored) memory and the CEL lights. Some ECUs that have internal fault registers, the CEL may stay on after the Stored or Permanent fault has been erased if another occurrence of the fault has happened since the 'permanent' fault was stored. To extinguish the CEL, you should always erase both Stored and Registered faults after fixing the faults indicated.

Stored Faults - These DTCs are sometimes called 'permanent' codes. When that preset number of failures has occurred, the fault is now moved to permanent (Stored) memory and the CEL lights. These faults are recorded in the ECU's permanent memory and are the main cause of CEL light. They have been generated because the fault exceeded the preset number of times set up for for that function. These DTCs can (and should) be erased after the fault is fixed.

Erasing DTC Memory
After eliminating a fault, the respective DTC readout must be cleared as follows: Press the PB for 3 to 5 seconds and read out the DTC. Then within 10 secs, press the PB for 8 to 9 seconds. Then turn the ignition off for 15 secs. Note the EACH DTC must be cleared INDIVIDUALLY! If the fault has been eliminated and its respective readout erased, the DTC will no longer be displayed when performing DTC readout.

AST Scan Tools
Assenmacher Specialty Tools (AST), whose been in business since 1977, distributes automotive specialty tools around the world. All of their tools are designed and manufactured in the U.S.A. Their first priority is quality, backed up with the best technical support possible. They are fast becoming known for their expanding line of diagnostic tools. Once you own their diagnostic tool(s), you can contact them for application assistance and future updates to stay on top of the ever-changing software technology found in the latest cars. The AST line includes tools for Audi, BMW, Ford (U.S. models), Mercedes-Benz, Saab, Subaru, Toyota, Volvo and VW. For more info check out the AST web site.

POINTER Scan Tool
The Pointer Scan Tool is an economical alternate designed to recall and reset fault codes from the following European autos: BMW, Mercedes-Benz, VW/Audi/Skoda/Seat/Ford, Volvo, SAAB SRS system. Fault Code Cartridges: Read and reset fault codes for the same autos. The Pointer scanner plus other MB scanners can be purchased at Technic Tool Supply.
Update: This scanner has been discontinued. A good replacement is the PALM Scanner.


DTC Readout
To read out DTCs, locate the X11/4 connector near the right rear side of the engine compartment. Remove the protective cover exposing a 38 pin connector. Plug the mushroom adapter to give access to the 38 pins based on the ECU being tested. Plug in the three wire leads (Red, Black, Yellow) to the pins referenced for each DTC chart in the Main Menu above. This powers up the Pointer Scan tool and prepares you to perform the DTC readout.

Now turn the IGNITION to ON making sure that all display lamps are illuminated. Then push the Enter key to cycle the program. A display of 'No Faults' means the system is clean! Additional faults are displayed by pressing the UP key. All faults are displayed in plain English on the LCD screen rather than the actual DTC itself. Repeat this step until the first DTC is displayed. Note all faults by writing them down. After repair of each fault, repeat the above procedure but this time select 'RESET FAULTS' from the menu. Completing this action clears all DTC codes from the selected module's memory.

Home Brew DTC Tool
You can build a very handy DTC reader from parts available at Radio Shack or surplus store. The simple tool allows you to read DTCs from the car without having to spending a large sum of money for a scanner. This tool can be built for under $15. Radio Shack part numbers have been included in BLUE for your convenience. Of course you can use a multiplicity of other parts.

The only drawback: You still need a reference document to see what the LED flashes mean. But that can be done, so here's the schematic of the Code Reader. Note that this unit works for all MBs with ANALOG systems, ie most cars up to and including 1993. Also some 1994 systems were mixed, both analog and digital.

The nicely drawn schematic is courtesy of Tony, N4HSR.

After you build the scan tool, you'll need a DTC Manual to show what the 'blinks' mean! Click here for an answer to your problem!