Dme System Porsche 944 I

DESCRIPTION

The 944 models are equipped with the Bosch Digital Motor Electronic (DME) engine control system. The DME system uses various data sensors to monitor intake air volume, engine speed, crankshaft position, coolant temperature, intake air temperature, and throttle position switch. Signals from these sensors, as well as a start signal and oxygen sensor signal, are sent to the control unit.

The Control Unit (CU) is a microcomputer. It is the "brain" of the DME system. Using information obtained from data sensors, the CU determines the correct amount of fuel and optimum ignition timing.

The 944 DME control unit switches from open loop to closed loop operation when the coolant temperature is above 113°F (45°C), and when the oxygen sensor temperature is above 480°F (250°C).

OPERATION

The DME system consists of 4 sub-systems: Fuel Control, Data Sensors, Control Unit and Spark Timing.

FUEL CONTROL

The 944 series are equipped with the Bosch Airflow Controlled (AFC) fuel injection system. The DME unit has a cycled voltage signal which is sent to the air regulating valve. The CU generates control signals for the fuel pump relay, auxiliary air valve and the cylinder port injectors. These devices control cold idle, curb idle speed and mixture, air/fuel ratio and fuel supply. Additional fuel required for cold starting is supplied by the fuel injectors.

The AFC system is electronically controlled by the CU, which is programmed to regulate fuel injection based upon information received from various data sensors. It also compares the specific data (stored in computer memory) for the engine. The airflow sensor has a stepped control curve for the air sensor position, instead of a linear control curve, which is adapted to the DME control unit.

SPARK CONTROL

Spark control allows the CU to determine the exact instant that ignition is required, based upon information received from data sensors.

At the optimum time, the CU breaks the primary circuit of the ignition coil, producing a high voltage at coil center tower. This voltage surge fires the spark plug at the proper time for most efficient combustion, eliminating the need for vacuum and/or centrifugal advance.

DATA SENSORS

Each sensor furnishes electronic impulses to the CU. Using this information, the CU computes spark timing, and correct amount of fuel necessary to maintain proper engine operation.

The function of each sensor is closely related in maintaining proper engine operation. Operation of each sensor is as follows

Oxygen Sensor

This sensor is mounted in engine exhaust stream, in front of catalytic converter. It supplies a low voltage (under .5 volt) when fuel mixture is lean (too much oxygen) and a higher voltage (up to one volt) when fuel mixture is rich (not enough oxygen).

The oxygen sensor must be hot to function properly, and to allow CU to accept its electrical signals. The oxygen sensor measures quantity of oxygen only.

All Calif. models are equipped with a special electrically heated oxygen sensor. This oxygen sensor reaches operating temperature sooner and also begins to function earlier. The heated oxygen sensor has 3 wires, 2 for the heater element (power and ground), and a single wire for the oxygen sensor signal.

The heating begins with ignition on (via fuel pump and DME relay terminal No. 87). The plugs from the sensor to the wiring harness are located near the flywheel sensor plugs (speed and reference mark).

Note. No attempt should be made to measure oxygen sensor voltage output. Current drain of conventional voltmeter may permanently damage sensor, shift sensor calibration range and/or render sensor unusable. DO NOT connect jumper wire, test leads or other electrical connectors to sensor. Use these devices only on CU side of harness after disconnecting sensor.

Reference Mark Sensor

The reference mark sensor is located on crankcase flange. This sensor detects crankshaft position in relation to TDC, and sends this signal to the CU. It is triggered by a bolt cemented into the flywheel.

Speed Sensor

The speed sensor is mounted on an adjustable bracket with the reference mark sensor. The speed sensor measures engine speed by counting the teeth on the starter ring gear. The speed sensor sends 2 voltage pulses to the control unit for each tooth that passes.

Temperature Sensor II

This sensor is located in in the coolant stream of the intake manifold. This sensor supplies coolant temperature information to the CU. This information affects the air/fuel ratio (as engine temperature varies with time during a cold start) and spark timing.

Intake Air Temperature Sensor I

This sensor is located in the air stream of the airflow meter, and supplies incoming air temperature information to the CU. The CU uses this along with other information in regulating the fuel injection rate.

Airflow Sensor

This sensor is located in the air stream of the airflow meter, and supplies air volume information to the CU. The CU uses this and other information in regulating the fuel injection rate.

The airflow meter incorporates an airflow measuring plate. The airflow plate opens when the engine draws in air. The plate is connected to a potentiometer. The potentiometer transmits an electrical signal determined by position of the measuring flap, to inform the CU of engine load.

The potentiometer within the airflow meter prevents loss of engine power during sudden acceleration or deceleration by signaling the CU of necessary fuel enrichment requirements.

Throttle Switch

A contact-type throttle switch is located on the throttle body. The 944 has 2 switch contacts. The idle contact signals the CU to control idle stabilization and coasting fuel cut-off. The full throttle contact signals the CU for optimum power demand.

High Altitude Switch

Switch is mounted behind left dashboard. The high altitude switch closes over 3300 ft. (1000 m), signaling the CU to lean out the fuel mixture.

Auxiliary Air Valve

The 944 uses an auxiliary air valve to provide additional air during cold engine starts and warm-up. It is located next to throttle body. The valve consists of an electrically heated bi-metallic strip, movable disc and air by-pass channel. The heater coil on the bi-metallic strip is energized by the fuel pump relay.

Control of the valve is based upon engine temperature. The air by-pass channel is open when engine is cold and gradually closes as temperature rises. At predetermined temperatures, air by-pass channel is blocked and additional airflow stops.

Fuel Pressure Regulator

The pressure regulator is located at the end of the injection collection line. Pressure regulator maintains constant fuel pressure to the fuel injectors.

Pressure Damper

The pressure damper is located between fuel tank and injection collector tube. The damper absorbs the pressure oscillation caused by the injection cycle.

CONTROL UNIT (CU)

The CU monitors and controls all DME system functions. The CU consists of input/output devices, Central Processing Unit (CPU), power supply and memories. The CU is located under the left instrument panel. A brief description and operation of each component is as follows

Input/Output Devices

These integral devices of CU convert electrical signals received by data sensors and switches to digital signals for use by CPU.

Central Processing Unit (CPU)

Digital signals received by CPU are used to perform all mathematical computations and logic functions necessary to deliver proper air/fuel mixture. The CPU also calculates spark timing information.

Power Supply

Main source of power for the CU is from the battery, through ignition circuit and main relay.

Memories

The memory bank of the CU is programmed with specific information, which is used by the CU during open loop (spark timing and fuel injection rate). This information is also used when a sensor or other component fails in the system, allowing the vehicle to be driven in for repairs.

944 Turbo

Optimal use of turbocharging cannot be reached with the old method of charging air pressure regulation by using a charging air pressure control valve (by-pass valve), which used the charging air pressure of the turbocharger as a regulator for air quantity.

The charging air pressure control valve must open at relatively low engine speed and in partial load range, in order to carry off the hot exhaust gas that is unused. On 944 Turbo models, the engine can now be operated with higher charging air pressure especially in a partial load range.

An electro-pneumatic cycling valve (solenoid), which receives signals from the control unit. These signals operate between the knock and charging air pressure control unit and charging air pressure control valve.

If the charging air pressure leaves the nominal curve and reaches or exceeds the safety limits for more than 3 seconds because of damage on the charging air pressure control valve or in the knock and charging air pressure regulating system, the injection signal is switched off immediately. The injection of gasoline is stopped. Injection starts up again only after the sensor plate in the airflow sensor has gone back to about 3/4 of the full deflection travel by way of the engine speed and/or accelerator pedal position. Full acceleration and driving is then possible after about one minute.

The knock sensor, which is bolted on the engine block next to the cylinder head, monitors ignition knocking. The knock sensor consists of 2 seismic materials, between which a "piezo" crystal is located. Oscillation, especially engine knocking, produces forces on the crystal by way of the seismic materials and the crystal reacts to these forces with changes in the electric voltage. These changes in voltage are sent to the knock and charging air pressure control unit as electrical signals

With the trigger signal for each individual cylinder, the DME ignition signal (TD) and the signal from the knock sensor can recognize in which cylinder there has been uncontrolled combustion (knocking).

With the next ignition pulse from the DME control unit for this cylinder the "TD" signal will be delayed in the knock control unit and fed back as a "TD" signal, so that the ignition for the pertinent cylinder is retarded 3°. If knocking occurs again, the ignition of the cylinder will be retarded again by 3°. If this then stops the engine knocking, the ignition will be advanced to the correct value in steps of .3° per 100 ignition pulses.

If knock pulses continue, the charging air pressure will then be taken back in steps of (.03-.05°). Taking back the charging air pressure is accomplished by changing the rectangular signals to the cycling valve and therefore increasing the pressure on the diaphragm of the charging air pressure control valve.

The control unit of the knock regulator has a memory for the disturbances occurring during operation. The knock regulation system can be checked with a test adapter and LED in the case of assumed or determined disturbance or malfunction of components in the knock regulation system. Insufficient engine power can be low charging air pressure, engine knock or ignition timing not conforming with the test values.

When the test adapter is connected to the diagnosis plug, the engine speed is now increased to more than 1500 RPM. If the LED remains dark, there is no fault in the knock regulation system or its components. If the LED is on continuously, there is a problem. The problem can be found by dropping the engine speed to a value below 1500 RPM (idle speed). The LED will now put out interrupted light signals, which are easy to recognized as flashing codes.

SPEED SENSOR CLEARANCE

Note. Adjusting speed sensor automatically adjusts reference mark sensor. They cannot be adjusted separately.

1. The speed sensor bracket is mounted on the crankcase flange with 2 bolts. To adjust clearance, loosen bolts and turn sensor holder. Clearance should be .030-034" (.75-.85 mm).
2. To adjust clearance with engine installed in vehicle, remove speed sensor. Using a depth gauge, measure distance from sensor holder's upper surface to tooth head on starter ring gear.
3. Measure length of speed sensor. Subtract speed sensor length from holder-to-flywheel tooth distance. Difference should be .030-.034" (.75-.85 mm).
4. If not to specification, loosen screws and turn holder until holder-to-flywheel tooth distance is equal to the length of the sensor plus the specified clearance. Tighten screws, and install speed sensor in holder.

944 Only

Position throttle against the idle stop. Loosen throttle switch and turn until idle contacts are closed. Tighten throttle switch. Ensure throttle switch opens at a 1° throttle opening.

DIAGNOSIS & TESTING

Note. The DME system tests are limited. Complete testing of the DME system requires an oscilloscope, voltmeter, ohmmeter and special test leads to insert in multiple pin CU connector.

ELECTRICAL CONNECTIONS

On 944 models, check 9-pin connector above brake booster, 4-pin connector on airflow sensor, 3-pin connector on throttle switch, 35-pin connector on CU, 2-pin connector at temperature sensor II, 1-pin connector for oxygen sensor (Calif. models, check 2-pin connector for oxygen sensor heating element), and two 3-pin connectors for flywheel sensors (attached to No. 4 cylinder intake manifold).

Note. CU connector is held in place by a catch. Push catch to the right and pull off plug with a downward motion.

CU POWER SUPPLY

1. Disconnect CU connector. Connect positive voltmeter lead to terminal No. 35 of CU harness connector. Connect negative test lead to terminal No. 5 of CU harness connector. Turn ignition switch to "RUN" position. Voltmeter should read battery voltage.
2. Connect voltmeter positive test lead to terminal No. 18 of CU harness connector. Connect negative test lead to terminal No. 5 of CU harness connector. Turn ignition on. Voltmeter should read battery voltage.
3. If no voltage is available, remove DME power relay. Using a jumper wire, connect terminals No. 30 and 87. Repeat voltage checks in step 1) and 2). On 944 series, connect voltmeter between terminals No. 3 of 9-pin connector and ground. If no voltage, check battery and related circuits.

IGNITION CIRCUIT POWER CHECK

Connect positive voltmeter lead to terminal No. 1 of CU harness connector. Connect negative lead to ground. Turn ignition on, but do not start engine. Voltmeter should register battery voltage. If not, check wiring back to battery.

SPEED SENSOR CHECK

Note. This is an alternate test when an oscilloscope is not available.

1. Secure a Fresnel lens front LED from an electronics store or use Special Test Unit (171 919 061B ). Connect a 220 ohm-1/4 watt resistor in series with one of the LED terminals.
2. Connect positive LED test lead to terminal No. 8 of CU harness connector. Connect negative LED test lead to terminal No. 27 of same connector. DO NOT start engine, but operate starter. LED will flicker dimly if speed sensor is sending a signal.

REFERENCE MARK SENSOR CHECK

Note. This is an alternate test when an oscilloscope is not available.

1. Using same LED tester as used for speed sensor, connect positive lead to CU harness connector terminal No. 25 and negative lead to terminal No. 26.
2. Operate starter, but do not start engine. If sensor is sending a signal, LED should flicker dimly.

Primary Resistance

1. With ignition switch in the "OFF" position, disconnect wires from primary terminals of ignition coil to isolate it from the system. Set ohmmeter for x1 scale.
2. Connect ohmmeter leads to 2 primary terminals. Reading should be .4-.6 ohm. If not, replace ignition coil.

Secondary Resistance

1. With the ignition switch in the "OFF" position, remove wire from coil tower. Set ohmmeter at x1000 scale. Connect ohmmeter leads to ignition coil positive terminal and coil tower.
2. Reading should be 5000-7200 ohms. If not within specifications, replace ignition coil.

IGNITION RESISTANCE CHECKS

Shielded resistance of spark plug connectors should be 3000 ohms. Shielded resistance of distributor rotor, and of all distributor cap connections should be 1000 ohms.

Power Supply (Except 944 Turbo Models)

Pull back plug seal of airflow sensor plug (plug remains connected). Connect voltmeter on terminal No. 3 and ground through back of plug. Reading should be approximately 5 1/2 to 6 1/2 volts.

Voltage Drop on Sensor Plate Potentiometer

Remove air cleaner. Connect voltmeter on terminal No. 2 and ground. Reading should be approximately 260 mVolts. Press sensor plate to full load position with a non-metallic rod. Sensor plate must move easily, without hesitation. With sensor plate in full load, reading should be approximately 4.6 volts. Turn off ignition. Pull off plug and install plug seal.

Intake Air Temperature Sensor

Pull off airflow sensor plug. Connect an ohmmeter to terminals No. 1 and 4 of airflow sensor (on control unit terminal No. 6 and 22). Reading at 32°F (0°C)= 4.4 to 6.8 k/ohms. If temperature sensor has an open circuit, should be richer mixture. At 60 to 85°F (15 to 30°C) = 1.4 to 3.6 k/ohms, At 105°F (40°C) = 1.0 to 1.3 k/ohms. If temperature sensor with grounded circuit, should be leaner mixture. It is important that intake air temperature be 60-85°F (15-35°C) for CO level adjustments.

944 Turbo Model

1. Loosen and fold down control unit bracket in passenger side kick panel. Pull out and remove plug connector on DME control unit. Remove upper section of plug connector, cutting cable band and removing fastening screw. Pull plug upper section away lengthwise.
2. Check supply voltage of airflow sensor. Connect voltmeter to terminals No. 9 and 5 (ground) on reverse side of control unit. Nominal value should be about 5 volts.
3. Remove air filter and connect voltmeter to terminals No. 7 and 5 (ground) on control unit plug. Check voltage drop on airflow sensor. Nominal voltage should be about 250-260 mV.
4. Push air flap to full load through air intake opening. Nominal value should be about 4.6 volts. Check temperature sensor (NTC 1 intake air temperature). Connect terminals No. 22 and 6 on the pulled control unit plug. Using ohmmeter, measure ohm resistance. Nominal value should be 1.4-3.6 ohms at 59-86°F (15-30°C) and 1-1.3 ohms at about 104°F (40°C). NOTE: Break on temperature sensor causes richer mixture. Short circuit on temperature sensor causes leaner mixture.
5. Repair or replace components as necessary. To install, reverse removal procedure.

KNOCK/CHARGING PRESSURE CONTROL

The knock/charging pressure system includes a self-monitoring system, which can recognize a defect with the help of a test adapter. The defect is seen as a blink code, similar to morse code.

If a defect has been found, the engine will operate on a safety system (basic turbo boast pressure up to about 4 psi over - pressure and 6° later on timing. A defect stored in memory is erased when ignition is switched off. For this reason, the defect must read out directly after a test drive without switching off the ignition.

1. Remove plug connection on idle actuator. Switch on ignition and connect voltmeter to middle contact of plug connector and ground. Nominal value should be 12 volts.
2. Reconnect plug connector on idle actuator and connect tachometer. During stable idle, screw in throttle air circulation bolt. After a short deviation, idle speed should re-adjust itself.
3. To check the activation of the idle actuator by the control unit, connect a new or proper functioning idle actuator to the plug connector. Turn on ignition. The idle actuator should vibrate due to frequency activation.
4. Start engine and increase idle speed by screwing out air circulation screw. The idle actuator should turn in the direction of closing. After lowering idle speed, the idle actuator should turn in the direction of opening.

AIR TEMPERATURE SENSOR I

Disconnect airflow sensor plug. Connect ohmmeter to airflow sensor terminals No. 6 and 22. Compare ohmmeter readings to specifications chart.

AIR TEMPERATURE SENSOR I SPECIFICATIONS

FUEL PRESSURE

1. Locate fuel pipe cap nut at end of fuel rail. Remove cap being careful that sealing ball does not fall out. Connect a Test Pressure Gauge (P 378).
2. Start engine and maintain idle speed. Pressure gauge should show 29.4 psi (2.06 kg/cm 2 ). Disconnect vacuum line at pressure regulator. Pressure gauge should show 33.8-39.7 psi (2.37-2.78 kg/cm 2 ).
3. Pinch fuel return line of pressure regulator. Pressure gauge should show a minimum of 59.0 psi (4.13 kg/cm 2 ). If fuel pump does not meet specifications, check fuel filter or replace fuel pump and retest.
4. If engine will not start, check fuse No. 2. If okay, remove fuel pump relay "U" and bridge terminals No. 30 and 87b. Pressure gauge should show 33.8-39.7 psi (2.37-2.78 kg/cm 2 ).

Remove CU connector plug. Connect a ohmmeter between terminal No. 13 and ground. Compare ohmmeter readings to specifications.

TEMPERATURE SENSOR II SPECIFICATIONS

1. Locate the high altitude switch on left instrument panel. Disconnect the altitude switch plug. Connect an ohmmeter to switch terminals. The altitude switch should be open under 3300 ft. (1000 m).
2. Warm engine to operating temperature. Disconnect the oxygen sensor. Disconnect the high altitude switch. Remove vacuum hose from the fuel pressure regulator and plug.
3. Connect an exhaust gas analyzer to the exhaust pipe. Operate engine at 2000 RPM and note CO% level.
4. Bridge the high altitude switch harness terminals. At 2000 RPM, CO% level should be leaner. If not, check high altitude connectors. Ensure continuity of one wire to ground. The other wire should have continuity to CU terminal No. 28.

Idle (Coasting Fuel Shutoff)

1. An accelerator linkage arm on the throttle operates idle microswitch. This accelerator linkage arm is connected with drag arm of the throttle shaft.
2. A distinctive switching click will be heard when there is a play of approximately .039" (1 mm) travel between the accelerator linkage arm and drag arm. The throttle is open after overcoming this play.

Checking Idle Switch Function

Operate accelerator linkage carefully approximately .039" (1 mm). The microswitch should open without the throttle opening and there must be a definite increase in idle speed. Speed should increase 500 RPM. Ignition timing advance should be 12° BTDC. If nothing happens, check microswitch and its adjustment.

Checking Activation of Control Unit

1. With engine at idle speed, pull off plug on idle switch. Engine speed must increase considerably. Engine speed should increase 500 RPM. Ignition timing advance should be 12° BTDC.
2. If nothing happens, check wire between idle switch plug and control unit plug for breaks according to wiring diagram. If necessary, replace control unit.

Checking Coasting Fuel Shutoff

1. With engine at speed, bridge disconnected plug of idle switch with a jumper wire. Accelerate engine to approximately 1300 RPM. Engine should begin to surge (this is coasting fuel shutoff).
2. If engine does not surge, check wire between idle switch plug and control unit plug for breaks according to wiring diagram. If necessary, replace control unit.

Full Load Enrichment

1. With engine at idle speed, pull off plug on idle switch. Engine RPM should go up to 1300 RPM and ignition timing should be 12° BTDC. Pull off plug on throttle switch (full load contact).
2. Bridge connections on plug with a jumper wire. Engine speed should drop by approximately 100 to 200 RPM and ignition timing will be retarded approximately 4 to 8°. If nothing happens, check wire between throttle switch plug and control unit plug for breaks according to wiring diagram.

1. Connect an exhaust gas analyzer to test point. Warm engine to operating temperature. Disconnect oxygen sensor wire. Note CO% level.
2. Disconnect and plug the vacuum hose from the fuel pressure regulator. The CO% level should increase. Reconnect the oxygen sensor wire. The CO level should decrease to 0.4-0.8%. If not, test the DME control.

DME CONTROL

1. Reconnect the fuel pressure regulator vacuum hose. Disconnect the oxygen sensor. Connect the oxygen sensor harness connector terminal to ground.
2. Check if the CO% level increases. If not, check wire from CU connector terminal No. 24 to oxygen sensor plug. If no problem is found, replace the oxygen sensor.

REMOVAL & INSTALLATION

Note. The removal and installation procedures are for the 944 model only.

Removal

1. Make a tool from a 4 1/2-6" long screwdriver. The screwdriver tip must be approximately 5/16" wide. Heat screwdriver and bend to a 90° angle about 3 1/2" from the end of tip.
2. Push in lower clamping hook with a screwdriver, and turn to the right (clockwise). Push in on upper clamping hook. Turn it to the right and remove cap.

Installation

1. Before installing cap, remove cable for No. 3 cylinder to provide access to clamping hook. Align distributor cap with its locking boss facing up.
2. Align clamping hooks in distributor cap so they are positioned horizontally and facing toward left side when viewed from front. Guide hooks into both slots, and engage distributor cap by turning back and forth slightly. Watch position of dust cap.
3. Turn both clamping hooks against left stop. Press in on hooks far enough so that they can be turned counterclockwise approximately 1/4 turn and engage when released.
4. Ensure distributor cap fits tightly. Ensure hooks are firmly engaged. Reinstall cable for No. 3 cylinder on distributor cap.

1. With the distributor cap removed, split the dust shield at base of rotor. Remove the dust shield. Remove the hex head bolt from rotor assembly. Pull the rotor assembly from shaft.
2. To reinstall, the push rotor assembly onto shaft. Ensure the rotor assembly bottoms on shaft. Install a new mounting screw and tighten to 35 INCH lbs. (4 N.m).

Removal (944 Turbo)

1. Remove plug connection on idle actuator. Switch on ignition and connect voltmeter to middle contact of plug connector and ground. Nominal value should be 12 volts.
2. Reconnect plug connector on idle actuator and connect tachometer. During stable idle, screw in throttle air circulation bolt. After a short deviation, idle speed should re-adjust itself.
3. To check the activation of the idle actuator by the control unit, connect a new or proper functioning idle actuator to the plug connector. Turn on ignition. The idle actuator should vibrate due to frequency activation.
4. Start engine and increase idle speed by screwing out air circulation screw. The idle actuator should turn in the direction of closing. After lowering idle speed, the idle actuator should turn in the direction of opening.

To install, reverse removal procedure. When installing idle actuator, note airflow direction. Make sure to position vacuum hoses and hose clamps correctly. Always replace seals for intake air distributor and hollow-core bolt.

Wiring Diagram for Porsche 944 & 944 Turbo. Scheme 5

Poses Online