Engine Controls - Theory & Operation Audi 90 B4

INTRODUCTION

This article covers basic description and operation of engine performance-related systems and components. Read this article before diagnosing vehicles or systems with which you are not completely familiar.

MPI ENGINE MANAGEMENT SYSTEM

On later models, Multipoint Fuel Injection (MPI) system has been modified for increased capabilities. With a new module it has greater processing capacity, increased memory storage and a permanent diagnostic trouble code memory. The modified MPI system now has fully adaptive oxygen sensor control. A new mass airflow sensor is used without a CO adjustment potentiometer. CO adjustment is not possible on later models.

One control module is used for all applications worldwide. New control modules must be coded with the V.A.G. 1551 scan tool when installed in vehicle.

Input and output signals from various sensors, switches and signaling devices are constantly monitored for engine control and faults. If faults occur, they are stored in memory. Faults can be displayed by a flashing 4-digit code sequence from an LED located in the instrument cluster.

CONTROL UNIT

The control unit receives signals from mass airflow sensor, oxygen sensor(s), coolant temperature sensor, throttle position sensor, closed throttle position switch, knock sensor(s), oxygen sensor(s), crankshaft position sensor, engine speed sensor, Hall Effect sensor, EGR sensors and other sensors specific to the individual system. The control unit uses data from these sensors to vary air/fuel mixture and ignition timing to maintain optimum performance, fuel economy and emission standards.

The Engine Control Module (MPI ECM) is new. A 16-bit micro-processor is used which processes up to twice as much information as the previous 8-bit version. System memory capacity has been increased from 48 K-byte to 60 K-byte.

The MPI ECM module has same 4 connectors (64 pins total) as other MPI systems. Since terminal functions differ, this control module must NOT be installed in earlier MPI systems. (Scheme 1)

Note. Components are grouped into 2 categories: input (sensor) devices and output (actuator) signals. INPUT DEVICES are components that control or produce voltage signals monitored by control unit. OUTPUT SIGNALS are components controlled by MPI ECM.

Scheme 1

Crankshaft Position (CKP) Sensor

Sensor identifies TDC No. 3 cylinder. A notch, 62° BTDC in counterweight of crankshaft for No. 3 cylinder is used as the reference point. The notch, along with the reference sensor, generates one signal per crankshaft revolution. The signal from the reference sensor along with the signal from the Hall Effect sensor are used during starting to identify TDC No. 3 cylinder ignition point.

Engine Coolant Temperature (ECT) Sensor

Coolant temperature sensor is a negative temperature coefficient sensor. Coolant temperature information is used by the MPI ECM as a correction factor for the following

1. Cold start enrichment.
2. Correction to injection and ignition timing for cold engine.
3. Idle speed control.
4. Deceleration fuel shut-off.

Coolant temperature sensor also is used to activate certain systems at a predetermined temperature such as

1. Oxygen sensor control.
2. Knock control.
3. Exhaust gas recirculation.

Engine Speed Sensor

Sensor is an inductive pick-up located on the left side of engine block by the flywheel. MPI ECM receives one voltage pulse per ring gear tooth (135). The voltage signal is used by MPI ECM to determine engine speed and to calculate ignition and injection firing points.

Engine speed sensor is adjusted at factory. If sensor bracket is removed, mark position of bracket on cylinder block for reassembly reference.

Exhaust Gas Recirculation (EGR) Temperature Sensor

EGR temperature sensor is located on outlet side of EGR valve. Sensor is used to inform MPI ECM that EGR valve is functioning.

Sensor measures temperature of recirculated exhaust gas. EGR sensor is for fault recognition only.

Hall Effect Sensor

A separate unit, the Hall Effect sensor is now located at end of left cylinder head camshaft. Signals from the crankshaft reference sensor and the Hall Effect sensor signal are used to identify ignition TDC of No. 3 cylinder. When starting engine, the first ignition and injection points are triggered by the MPI ECM after receiving both signals.

Idle Switch & Throttle Valve Potentiometer

Idle switch and throttle potentiometer are located in a common housing on the bottom of the throttle housing.

Idle switch closes at about 1.5 degrees before primary throttle plate closes. When switch is closed, a ground signal is supplied to MPI ECM. MPI ECM uses this ground signal to activate the following functions

1. Idle stabilization.
2. Deceleration fuel shut-off with engine warm and above 1500 RPM. Fuel supply is reactivated when engine speed falls below 1200 RPM.
3. Special ignition map for deceleration.

Knock Sensor(s)

One knock sensor is installed on each head under intake manifold. The use of 2 sensors makes it possible for the system to sense more accurately and be more responsive. See DETONATION RETARD OPERATION under IGNITION CONTROL .

Mass Airflow Sensor

Mass airflow sensor is a new design. As a result of the fully adaptive oxygen sensor control, the CO adjustment potentiometer has been eliminated. The sensor housing is now a single casting with the air bypass positioned in the center of the housing. The characteristic curve of this sensor is different from the previous design. Mass airflow sensor MUST NOT be installed on earlier version of the MPI system.

Oxygen (O2) Sensor(s)

O2 sensor is a heat-type sensor which measures oxygen content of exhaust gases. MPI ECM uses data from O2 sensor(s) to determine air/fuel mixture. Sensors are positioned in exhaust system, ahead of catalytic converter.

Fuel/air mixture is regulated separately for right and left cylinder banks. Each cylinder bank has its own oxygen sensor and catalytic converter. Using 2 oxygen sensors allows MPI ECM to monitor oxygen content in each cylinder bank individually.

The MPI ECM regulates the amount of fuel injected per cylinder bank based on voltage signals received from the respective oxygen sensors.

Throttle Valve Potentiometer

Throttle valve potentiometer is connected to throttle shaft and is supplied five volts by MPI ECM. MPI ECM receives a varying voltage signal from potentiometer as throttle position changes. Voltage signal supplied MPI ECM is used to determine position of throttle plates and speed of throttle movement. This data is used for

1. Acceleration enrichment.
2. Full throttle enrichment.

Throttle valve potentiometer is also used as a substitute for the mass airflow sensor.

Additional Signals

MPI ECM manages a wide variety of information. The control unit is connected to other electronic units or system components in vehicle by means of data lines.

These extra signals are used to exchange information between the different systems. For example, the following components are connected to the MPI ECM

1. Automatic transmission ECU.
2. Climate control ECU.
3. Compressor ON signal, manual climate control system.

Canister Shutoff Solenoid

See EVAPORATIVE EMISSIONS under EMISSION SYSTEMS.

Malfunction Indicator Light (CHECK ENGINE)

See SELF-DIAGNOSTIC SYSTEM .

Fuel Injectors

Battery voltage is supplied to injectors via a 12 amp circuit breaker located in the electronic control box (above MPI ECM).

Power supplied to injectors does not use external resistors. Due to sequential fuel injection, a separate power output stage is provided for each injector.

MPI ECM controls opening time of injectors by supplying a ground for each injector.

Idle Speed Control

See IDLE CONTROL under FUEL SYSTEM.

Ignition Coil

See IGNITION CONTROL under IGNITION SYSTEM.

Fuel Pump & Relay

The fuel pump assembly, located in fuel tank, is equipped with a pressure damper at the suction end. Fuel pump is activated during start-up and when engine is running. Fuel pump relay switches off fuel pump in absence of signal from MPI ECM and provides voltage for cold start valve.

Fuel Pressure Regulator

Fuel pressure regulator maintains constant pressure at the injectors.

FUEL PUMP PERFORMANCE

INJECTOR RESISTANCE

Injector resistance is 15-17 ohms.

An idle stabilizer valve is used to control engine idle speed. It regulates flow of air to engine when throttle plates and idle switch are closed. Idle stabilizer valve is mounted on throttle valve housing.

Idle stabilizer valve is operated by a cycled DC voltage (duty cycle). The MPI ECM supplies cycled voltage to idle stabilizer to regulate idle speed with a warm engine between 700-800 RPM. The duty cycle will vary depending on engine load and engine conditions.

Since the idle system is adaptive, no adjustments are necessary.

IGNITION CONTROL

Ignition system for the V6 is distributorless. Since there are no moving parts, no noise is generated to interfere with knock sensor system. The system is easier to maintain, as no routine adjustments are necessary.

The ignition part of the MPI system consists of

1. Power output stage.
2. Three double-ended ignition coils.
3. MPI ECM.

Ignition Coil(s)

The MPI ECM operates each double ended ignition coil via the power output stage. Power stage is located on bulkhead behind engine.

When ignition coil fires, spark is supplied to 2 cylinders simultaneously. One plug fires during compression stroke and ignites fuel mixture while the other plug fires during the exhaust stroke.

The automatic transmission ECU signals each gearshift simultaneously to the MPI ECM. The MPI ECM responds by retarding ignition timing point during the gear change, reducing engine torque during the shift, making gear changes more comfortable.

Detonation Retard Operation

Knock sensors consist of piezoelectric crystals encased in metal and plastic housings located on each cylinder head, under intake manifold. Vibrations in engine cause crystal to generate small amounts of voltage. This voltage signal is used by MPI ECM to determine necessary timing retardation.

One knock sensor is installed on each head under intake manifold. The use of 2 sensors makes it possible for the system to sense more accurately and be more responsive.

When knocking occurs, ignition timing is retarded until knocking is eliminated. Since engine knock limits vary from cylinder to cylinder, knock regulation is cylinder selective.

Knock control starts when coolant temperature is 104°F (40°C) or above. Ignition angle of knocking cylinder is retarded in steps to a maximum of 12° or until knocking stops.

If cylinder knock continues, MPI ECM will switch from premium fuel ignition map to ignition map for regular fuel. This allows ignition timing to be retarded up to 3 additional degrees.

Intake Manifold Change-Over Valve

Six flaps in the multi-path intake manifold are opened and closed by the intake manifold change-over valve. Change-over valve is a vacuum servo electronically activated by the MPI ECM.

Valve is located under mass airflow sensor and operates according to engine speed. Above 4100 RPM, change-over valve opens the 6 flaps in the intake manifold.

The valve receives 12 volts from fuse box under passenger's footwell. MPI ECM supplies a ground signal at the appropriate engine speed.

Evaporative Emissions System

When vehicle is not in operation, a charcoal canister stores fumes produced through evaporation in the fuel tank and other related system components. During engine operation, fumes are purged from canister and consumed in the combustion process.

Canister shutoff solenoid is located on side of carbon canister. When engine is started, canister shutoff solenoid is energized, allowing stored vapors to be purged from canister.

Shutoff solenoid valve is de-energized when engine is turned off. This closes purge line between carbon canister and intake air boot, preventing fuel vapors which may cause hot start problem from collecting in intake tract.

Exhaust Gas Recirculation System

EGR system does not operate at idle because NOx emissions are low during this time. Control inputs for EGR system are

1. Engine speed.
2. Engine load.
3. Signal from idle switch.

Exhaust gas recirculation is controlled by ignition map in the MPI ECM. This allows precise regulation of the exhaust gas recirculation flow.

MALFUNCTION INDICATOR (CHECK ENGINE) LIGHT (MIL)

All vehicles are equipped with a MIL, located on the instrument panel. Light comes on (as a bulb check) with ignition switch in ON position. Light also comes on when systems related to the emission controls are malfunctioning during normal engine operation. For additional information, see the appropriate TESTS W/CODES article in this section.

See also:
• TESTS W/CODES
• IGNITION CONTROL
• EVAPORATIVE EMISSIONS