System Description

GENERAL
Porsche 968's engine has Digital Motor Electronics (DME), version DME 2.10.1, to prepare air-fuel mixture and calculate ignition timing.

DME 2.10.1 consists of following components and basic functions:

^ Control unit with 64 KByte data/program memory and 55 pin connection
^ Sequential fuel injection
^ Characteristic-map ignition with knock control
^ Ignition output stage integrated into control unit
^ Adaptive cylinder filling control with dual-winding rotary drive link
^ Actuation of variable inlet camshaft timing (VarioCam)
^ Input-signal monitoring (plausibility) and generation of substitute values
^ Air conditioning actuation
^ Diagnosis compatible with system tester 9288


SEQUENTIAL FUEL INJECTION
This system injects entire amount of fuel to each cylinder individually and once per operating cycle, in accordance with ignition timing. Fuel injection timing can by modified depending on engine load and speed.

ADAPTIVE KNOCK CONTROL
Ignition map has been divided into 8 engine-speed and 4 load ranges. When knocking occurs, ignition timing is retarded by 3° at cylinder affected and returned in increments of 0.75°. If engine-speed or load range is altered during this process, the following happens:

^ Previous range - Last ignition timing is stored (adapted).
^ New range - Ignition timing which has already been learned for this range is adopted.

This avoids unnecessary, protracted timing retardation and improves engine performance when accelerating.

CONTROL UNIT
DME unit is mounted in front passenger's footwell behind footrest board. Unit is encoded by free plug bridge, located next to DME connector, on control unit wiring loom. If plug bridge is vacant, DME control unit is regulated by lambda control. If a 1k ohm resistor is inserted in bridge, DME control unit operates without lambda control.

On model with Tiptronic transmission, engine wiring harness is different from manual transmission models. DME unit is informed that auto trans is fitted. With selector lever is in PARK or NEUTRAL, idle speed is 880 ±40 rpm. When trans is shifted into gear, idle speed is modified to 840 ±40 rpm.

To reduce shift impact when under load, engine ignition timing is briefly retarded depending on engine speed and load, when transmission shifts up or down. This reduces engine torque momentarily. This occurs if Tiptronic control unit introduces a shift process in transmission and a signal is sent to DME unit. DME briefly retards spark.

DME has several adaptive system including lambda control, cylinder filling control, and knock control. DME unit will lose its adaptive memory and any stored fault codes, if battery or control unit connector is disconnected and engine must run for about 10 minutes at normal operating temperature after the next start for control unit to adapt anew.

VARIOCAM
This feature, controlled by DME control unit, adjusts setting of intake camshaft and has an important impact on engine output resulting in excellent torque and favorable exhaust gas composition. Adjustment is made when oil temperature reaches:

^ 15° C and engine speed 1500 rpm
^ 120° C and engine speed 2000 rpm
^ 130° C and engine speed 2800 rpm

and throttle valve is open at least 8°, DME unit grounds solenoid via terminal 23. When 5500 rpm is reached, DME interrupts ground signal again.

As soon as engine exceeds threshold speed above, inlet cam is readjusted by approximately 7.5° (15° at crankshaft). Adjustment is toward greater overlap of intake and exhaust valve lift curves resulting in an increase of engine torque.

At engine speeds above 5500 rpm, intake cam is readjusted back to original setting. This improves cylinder charge and produces an increase in engine output.