General Information

TO: OUR MERCEDES-BENZ PASSENGER CAR DEALERS

Date: March 1993

REF.NO. 07.5/15

SUBJECT:
Engine 104
With HFM-SFI Without Diagnostic Connector And TDC Sensor

Engines equipped with HFM-SFI no longer have the 9-pole diagnostic connector and TDC sensor for engine diagnosis.

Until suitable adapter equipment becomes available, the approved testers can only be used to check the ignition system scope patterns on the primary side of each ignition coil individually for each circuit.

The idle quality test and dynamic compression test which are possible with diagnostic equipment, but not with the test and adjustment equipment, cannot be performed without an adapter unit.

Connect the socket box to the HFM-SFI engine control module to test the ignition system. The signals from the socket box can then be read using the tester's universal connection cable.

The ignition spark burn time, burn time starting point, and decay characteristics for the secondary ignition circuit can be evaluated from the primary circuit.

The ignition voltage of the secondary circuit cannot be measured by means of the primary pattern, since the voltage induced by the secondary winding in the primary winding is limited by the output stage in the control module (Zener voltage). Defects in the secondary circuit, such as a defective spark plug connector, affect the burn time starting point and the spark duration, and can be identified by means of the primary oscilloscope pattern.

This can be done by carefully evaluating the primary oscilloscope pattern in the parade display mode and then in the individual pattern mode on a 5 or 10 millisecond scale.

A defect, for example, in one spark plug connector, will always affect an entire circuit. In other words, the fault can be seen on two cylinders simultaneously, since the two spark plugs are connected to the secondary winding in series.

For this reason, use a multimeter to check both ignition leads and spark plug connectors for excessive resistance with a multimeter in order to locate the defect.

WARNING!

^ Perform all work in the engine compartment and on the ignition system (including connecting testers) with the engine stopped and the ignition switch off.

^ Do not touch live parts of the ignition system with the ignition switched on or the engine running since dangerous voltages are present in both the secondary and primary circuit!

^ Persons with pacemakers should not work on this type of ignition system.

^ No exposed metal connectors or sending units may be installed in the ignition wires (e.g. ignition wire, cylinder 1).

Observe the warnings in the SMS Repair Instructions for Group 15.

The testers listed maybe used for testing. For "normal" and "defective" patterns, refer to the appendix.