Computers - Inadvertent MIL ON Failure Explanation
Date: February 2001Order No.: P-SI-14.00/14
Supersedes:
Group: 14.00
SUBJECT:
MODELS WITH LH-SFI OR HFM-SFI, MODEL YEARS 1993 THROUGH 1996 VEHICLES INADVERTENTLY FAILING ON-BOARD DIAGNOSTIC INSPECTIONS IN STATE OPERATED TESTING LANES
Affected vehicles
This service information pertains to models with LH-SFI or HFM-SFI, model years 1993 through 1996. Please refer to the table shown for a detailed listing of the affected vehicles.
It has come to the attention of Mercedes-Benz USA, that the affected Mercedes-Benz vehicles have inadvertently failed On - Board Diagnostic (OBD) tests, in states conducting OBD checks (in their respective Inspection / Maintenance Lanes). These inadvertent failures are due to state guidelines, which were established without the knowledge of how specific Mercedes-Benz systems operate.
Testing guidelines read as follows for states checking the OBD system
Vehicles fail the entire OBD inspection if they fail either of the following criteria:
^ "Check engine" malfunction indicator lamp (MIL) is commanded on, with the vehicle in the key-on/engine-running condition.
^ "Check engine" MIL is illuminated after the vehicle is started, even if no fault codes are present, which could indicate a serial data link failure.
Please be advised, that certain Mercedes Benz vehicles will illuminate the "check engine" MIL during DTC read-out and while the tool is connected, regardless of whether or not any faults are present in the OBD system.
This feature was designed into Mercedes-Benz systems equipped with LH-SFI and HFM-SFI control modules during the early phase of OBD, so that a technician would know when a tool is properly communicating or "linked-up" with the control module. For this reason, these vehicles should not be failed in any state Inspection Maintenance lane. Furthermore, it is critical to note that this is only a "visual illumination". The MIL is not "commanded on" and bit (Mode $01, P1D $ 01, Data A, bit 7) is not set or activated in the OBD software.