PEC control module, location/design/function

P07.52-0003-06

1 Crankshaft position sensor (L5) connection

2 Electrical connection

3 Intake manifold connection

4 Reference resistor coupling PEC (R16/7) connection

N3/6 PEC [PMS] control unit

Task

 Controls the systems according to the evaluation of the input signals:
- Injection system
- idle speed control
- ignition system

 Monitoring inputs/outputs, checking plausibility and storing faults

 Formation of substitute values for "emergency operation" if no signal received

 Automatic detection of manual limit stop

 Self-adaptation of mixture formation (only with TWC)

 Automatic detection of vehicle version

Function

The PEC control unit (N3/4) is supplied with voltage directly by circuit 15 and 30.

By means of an installed pressure sensor, the PEC control unit (N3/4) detects the intake manifold pressure as main quantity to be measured for determining the injection time. It detects further input signals and actuates the outputs according to the operating conditions.

Input signals of:

R16/5Reference resistor coupling PEC [PMS]

- CO potentiometer (R33) (without TWC)

- OO22 sensor (G3/2) (only TWC)

- Crankshaft position sensor (L5)

- Intake manifold pressure (PEC pressure sensor control unit (N3/6))

- Starter lock-out and back-up lamp switch (S16/3)

- Idle speed control actuator (M16/6)

- ABS control unit (N30) (vehicle speed signal)

- Compressor cutout control unit (N6) (AC compressor cut-in signal)

- Cruise control control unit (N4) (cruise control signal "ON")

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Outputs:

- Fuel injection valves (Y62)

- O2 sensor heater (only TWC)

- Fuel pump relay (K27)

- Partial intake manifold preheater relay (K3/1) (only TWC)

- Idle speed control actuator (M16/6)

- TN signal (engine rpm signal)

- Switchover valve upshift delay (Y3/3) (TWC and AT)

- Ignition coils (T1/1, T1/2)

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Diagnosis/fault storage

All the inputs and outputs are checked for plausibility during operation. Any faults detected are stored in the fault memory. The PEC control unit (N3/6) differentiates the following fault output:

- Faults which are currently present.

- Faults which were present during a certain period

- Short time interruptions (loose contact), which have occurred more than 5 times during a drive.

A drive is performed for the PEC control unit (N3/6), if :

- speed > 4 km/h

- engine speed > 700 rpm

- ignition "OFF" >30 seconds

The faults remain stored when ignition is "OFF", however the faults are erased after disconnecting the vehicle battery.

Faults are automatically erased in the fault memory, if the fault does no longer occur within 19 consecutive drives. If a fault reoccurs for example after 6 correct drives, then the 19 consecutive drives are again required for automatically erasing the fault.

Stored faults can be read out and erased at the diagnostic data link connector (X11/4) socket 8 with:

- Hand-held tester (HHT)

- Pulse counter

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Note for ignition system

The PEC control unit (N3/6) monitors continuously the primary voltage and primary current flow of the ignition coils (T1/1, T1/2) when engine is running and can on the basis of this detect above a number of approx. 50 ignitions if faults are present in :

- ignition coil (monitoring primary current)

- secondary ignition circuit (monitoring primary voltage)

If a fault is detected, the corresponding fuel injection valves (Y62) are no longer actuated in pairs, in order to protect the catalytic converter from thermal overload.

If the fault no longer reoccurs above a number of approx. 255 ignitions, an actuation of the fuel injection valves (Y62) is performed, if:

- engine speed was >2500 rpm

- vehicle in the overrun (deceleration fuel shutoff)

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Formation of substitute values for "emergency mode" if no signal received

If input signals are not received, the PEC control unit (N3/6) forms a substitute value on the basis of other signals in order to maintain system operability.

Example:

If the pressure sensor in the PEC control unit (N3/6) fails, a load signal is formed in line with the engine speed and throttle valve position and is used for determining the injection time.

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Automatic detection of manual limit stop of the closed throttle valve

In order to assure a trouble-free function of the idle speed control, it is required that the PEC control unit (N3/6) detects the manual limit stop of the closed throttle valve.

The detection is performed when ignition is"ON" (90 seconds, selector lever in P/N). While the throttle valve is drawn onto the limit stop through the actuator motor integrated in the idle speed control actuator (M16/6), the PEC control unit (N3/6) detects via the current increase of the actuator motor, in connection with the actual value potentiometer (throttle valve angle), the manual limit stop and stores this value.

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Self-adaptation of mixture formation (only TWC)

If the PEC control unit (N3/6) detects a variation of lambda =1 in the lambda closed-loop operation on the basis of the signal supplied by the O2 sensor (G3/2), the mixture is continuously corrected by altering the injection time until lambda=1 is restored. It is only possible, however, to operate within a limited correction range. The correction is stored by the PEC control unit (N3/6). 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 in the open-loop mode is neither too "rich" nor too "lean".

Self-adaptation is performed in:

- Idle speed

- Partial-load range

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Note

The stored values of the self adaptation can be displayed at the diagnostic data link connector (X11/4) socket 8 with the Hand-Held tester (HHT). It is necessary to reset the values after carrying out repair work to exhaust-relevant components. Resetting can also be performed, if the PEC control unit (N3/6) is disconnected.

The display of the stored values of the self adaptation at the Hand-Held Tester (HHT) is in:

- idle speed in ms and means, that this value for determining the injection time is added to or substracted from the basic injection time in the performance map.

- partial-load range as factor and means, that this factor for determining the injection time is multiplied with the determined engine load.

e. g.:
Effective air mass rate: 12 kg/h
display: 1.11
use for determining the injection time of the calculated air mass rate: 13.32 kg/h

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Automatic detection of vehicle version

The PEC control unit (N3/6) detects during first startup on the basis of the assignment of the inputs, which vehicle version is present, stores it and activates the performance maps corresponding to the vehicle version.

Possible vehicle versions:

- Manual/automatic transmission

-TWC/without TWC

Note

The detection of the vehicle version can be reactivated at the diagnostic data link connector (X11/4) socket 8 with the Hand-Held Tester (HHT);
at the same time:

- Fault memory erased,

- Resetting of stored values from the the self adaptation of the mixture formation (only TWC),

- Carry out detection of the manual stop of the closed throttle valve.

The detection of the vehicle version can also be activated, if the PEC control unit (N3/6) is disconnected and reconnected.