Control module HFM-SFI function - GF07.51-P-4103-02D

Motor 104 

General 

On models with the base version the idle speed control is integrated in the HFM-SFI control module (N3/4).

On models with

• cruise control/idle speed control

• electronic accelerator/cruise control/idle speed control
• 5-speed automatic transmission
• diagnostic module (California only) the HFM-SFI control module (N3/4) is linked through the CAN data bus to the control modules of the possible optional equipment versions.

Depending on the vehicle model, the HFM-SFI control module (N3/4) is supplied with voltage by the overvoltage protection relay (K1/2), base module (N16/1), relay module (K40) or the fuse and relay module (K40/4).

It detects the input signals and actuates the outputs in accordance with operating conditions.

Diagnosis/fault storage 

All inputs/outputs are checked for plausibility when operating. Faults recognized are stored in the fault memory. The HFM-SFI control module (N3/4) distinguishes the following faults when they are output:

• Faults which are current.
• Faults which were present over a particular period.
• Temporary open circuits (loose contact) which occurred more than 5 times during a trip.

A trip for the HFM-SFI control module (N3/4) is completed if

• vehicle speed >4 km/h
• engine speed >700 rpm
• ignition "OFF  " >30 seconds

The faults remain stored with ignition " OFF  " or after disconnecting the vehicle battery. Faults are automatically erased in the fault memory if the fault no longer occurs within 19 successive trips. If, for example, a fault reoccurs after 6 fault-free trips, a further 19 successive trips are necessary before the fault is automatically erased.

The faults stored can be read and erased at the data link connector (X11/4) contact 8 with:

• Hand-Held Tester (HHT)
• Pulse counter

Ignition system control 

When the engine is running, the HFM-SFI control module (N3/4) constantly monitors the primary voltage and primary current pattern of the ignition coils (T1/1, T1/2, T1/3) and is able to recognize on the basis of this monitoring whether faults exist over a number of about 50 ignitions in respect of:

• ignition coil (monitoring of primary current)
• secondary ignition circuit (monitoring of primary voltage)

If a fault is detected, the corresponding fuel injection valve(s) (Y62) is/ are no longer actuated in order to protect the catalytic converter from excessive thermal stresses.

If the fault no longer occurs over a number of about 255 ignitions, the injection valve(s) (Y62) is/are again actuated if:

• engine speed was >2500 rpm
• car in deceleration mode (inertia fuel shutoff)

Formation of substitute values in the event of signal failure for "Emergency running mode" 

If input signals fail, the HFM-SFI control module (N3/4) forms a substitute value on the basis of other signals in order to maintain system operability.

Example: 

If no signal is received from the HFM-SFI hot film mass air flow sensor (B2/5) a load signal is formed as a function of engine speed and throttle valve position and is used for determining the injection time.

Automatic transmission recognition of mechanical end stop of the closed throttle valve 

To ensure trouble-free operation of the idle speed control, it is necessary for the HFM-SFI control module (N3/4) to recognize the mechanical end stop of the closed throttle valve. Recognition is carried out with ignition " ON  " (90 seconds, selector lever in P/N). The throttle valve is moved to the end stop by the servomotor integrated in the idle speed control actuator (M16/6) which enables the HFM-SFI control module (N3/4) to detect the mechanical end stop and to store this position from the increase in current of the servomotor, in combination with the actual value potentiometer (throttle valve angle).

Self-adaptation of mixture formation  (TWC only)

If the HFM-SFI control module (N3/4) detects a variation from lambda=1 from the signal supplied by the O₂ sensor (G3/2) in the lambda closed-loop control mode, the mixture is constantly corrected by altering the injection time until lambda=1 is again achieved. Only a limited correction range is possible, however. The correction is stored by the HFM-SFI control module (N3/4). In the open-loop mode (lambda closed loop control not active), the mixture is adapted in accordance with the stored correction values.

This ensures that the mixture composition is not too "rich" or not too "lean" in the open-loop control mode.

Self-adaptation is carried out in the:

• idle speed range
• lower part throttle range
• upper part throttle range

The stored values of self-adaptation can be read and reset at the data link connector (X11/4) contact 8 with the Hand-Held Tester (HHT). It is necessary to reset the stored values after carrying out repair work on components which affect exhaust emissions or after replacing the HFM-SFI control module (N3/4).

The readout of the stored values of self-adaptation on the Hand-Held Tester (HHT) is:

• in kg/h when engine idling and means that this value is added to or subtracted from the air mass inducted by the engine for determining the injection time.

  For example:

  actual air mass flow: 12 kg/h

  readout: +0.8 kg/h

  mathematical air mass flow used for determining the injection time:

  12.8 kg/h

• a factor in the lower/upper part throttle range and means that this factor is multiplied by the air mass inducted by the engine for determining the injection time.

  For example:

  actual air mass flow: 12 kg/h

  readout: 1.11

  mathematical air mass flow used for determining the injection time:

  13.32 kg/h

Automatic recognition of vehicle version 

When the HFM-SFI control module (N3/4) is first operated, it recognizes from the assignment of the inputs the vehicle version in question, stores this and activates the corresponding maps for the vehicle version.

Possible vehicle versions: 

• Manual/automatic transmission
• Automatic 4-/5-speed transmission
• TWC/no TWC
• Federal/California version
• TWC thermocouple ( )

The "vehicle version recognition" can be reactivated at the data link connector (X11/4) with the Hand-Held Tester (HHT):

If this is done, the following actions occur simultaneously:

• fault memory erased,
• stored values of self-adaptation of mixture formation (TWC only ) reset,
• recognition of mechanical stop of closed throttle valve carried out.

If the control module is replaced as a check, it is then necessary to activate the "vehicle version recognition".