Engine Controls - Theory & Operation Audi A6 C4

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.

MFI ENGINE MANAGEMENT SYSTEM

The Multiport Fuel Injection (MFI) system is controlled by the Powertrain Control Module (PCM). The MFI system has fully adaptive oxygen sensor control. A Mass Airflow (MAF) sensor is used without a CO adjustment potentiometer. CO adjustment is not possible.

Input and output signals from various sensors, switches and signaling devices are constantly monitored for engine control and trouble codes. If trouble code(s) occur, they are stored in memory. Trouble codes can be displayed by a flashing 4-digit code sequence from a Malfunction Indicator Light (MIL) located in the instrument cluster.

POWERTRAIN CONTROL MODULE (PCM)

The Powertrain Control Module (PCM) has memory storage and a permanent diagnostic trouble code memory. One PCM is used for all applications worldwide. New PCM must be coded with the V.A.G. 1551 scan tool when installed in vehicle.

The PCM receives signals from Mass Airflow (MAF) sensor, Heated Oxygen Sensor (HO2S), Engine Coolant Temperature (ECT) sensor, Throttle Position (TP) sensor, Closed Throttle Position (CTP) switch, Knock Sensor (KS), Crankshaft Position (CKP) sensor, engine speed (RPM) sensor, Camshaft Position (CMP) sensor and EGR sensor. (Scheme 1) PCM uses data from these sensors to vary air/fuel mixture and ignition timing to maintain optimum performance, fuel economy and emission standards.

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 PCM. OUTPUT SIGNALS are components controlled by PCM.

Scheme 1

Camshaft Position (CMP) Sensor

CMP sensor is located at the end of left cylinder head camshaft. Signals from the crankshaft position sensor and the CMP sensor are used to identify ignition TDC of No. 3 cylinder. When starting engine, the first ignition and injection points are triggered by the PCM after receiving both signals.

Closed Throttle Position (CTP) Switch

CTP switch is located in a common housing with the TP sensor on the bottom of the throttle housing. CTP switch closes at about 1.5 degrees before primary throttle plates close. When CTP switch is closed, a ground signal is supplied to PCM. PCM uses this ground signal to activate the following functions

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

Crankshaft Position (CKP) Sensor

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

EGR Temperature Sensor

EGR temperature sensor is located on outlet side of EGR valve. Sensor is used to inform PCM that EGR valve is functioning. Sensor measures temperature of recirculated exhaust gas.

Engine Coolant Temperature (ECT) Sensor

ECT sensor is a negative temperature coefficient sensor. Sensor resistance decreases as coolant temperature increases. Coolant temperature information is used by the PCM 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.

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

1. Heated oxygen sensor control.
2. Knock sensor control.
3. EGR operation.

Engine Speed (RPM) Sensor

RPM sensor is an inductive pick-up located on the left side of engine block by the flywheel. PCM receives one voltage pulse per ring gear tooth (135). The voltage signal is used by PCM to determine engine speed and to calculate ignition and injection firing points. PCM will not allow engine to start if RPM sensor signal is not received within 5 seconds. RPM sensor is adjusted at factory. If sensor bracket is removed, mark position of bracket on cylinder block for reassembly reference.

Heated Oxygen Sensor (HO2S)

HO2S is a heat-type sensor which measures oxygen content of exhaust gases. PCM uses data from HO2S to determine air/fuel mixture. Sensors are positioned in exhaust system, ahead of the left and right catalytic converters.

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 HO2S allows PCM to monitor oxygen content in each cylinder bank individually.

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

Knock Sensor (KS)

One KS is installed on each head under intake manifold. Using 2 sensors makes it possible for PCM to sense cylinder knock more accurately. KS I is located on right cylinder bank and monitors cylinders No. 1, 2 and 3. KS II is located on left cylinder bank and monitors cylinders No. 4, 5 and 6. See DETONATION RETARD OPERATION under IGNITION CONTROL under IGNITION SYSTEM.

Mass Airflow (MAF) Sensor

As a result of the fully adaptive oxygen sensor control, the CO adjustment potentiometer has been eliminated. The sensor housing is a single casting with the air by-pass positioned in the center of the housing.

Note. Closed Throttle Position (CTP) switch and Throttle Position (TP) sensor are located in a common housing on the bottom of the throttle housing.

Throttle Position (TP) Sensor

TP sensor is located in a common housing with closed TP switch on the bottom of the throttle housing. TP sensor is connected to throttle shaft and is supplied 5 volts by PCM. PCM receives a varying voltage signal from TP potentiometer as throttle position changes. Voltage signal supplied by PCM 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 Mass Airflow (MAF) sensor.

Additional Signals

PCM manages a wide variety of information. The PCM 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 PCM

1. Automatic Transmission Control Module (TCM).
2. Climate electronic control module ECM.
3. Compressor ON signal, manual climate control system.

Canister Shutoff Solenoid

See EVAPORATIVE (EVAP) EMISSIONS under EMISSION SYSTEMS.

Double-Ended Ignition Coils

See IGNITION CONTROL under IGNITION SYSTEM.

Fuel Injectors

Battery voltage is supplied to injectors via a 12-amp circuit breaker located in the electronic control box (above PCM). Power supplied to injectors does not use external resistors. Due to sequential fuel injection, a separate power output stage is provided for each injector.

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

Idle Speed Control

See IDLE CONTROL under FUEL SYSTEM.

Malfunction Indicator Light (MIL)

See SELF-DIAGNOSTIC SYSTEM .

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 PCM.

Valve is located under mass airflow sensor and operates according to engine speed. At engine speeds greater than 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. PCM supplies a ground signal (energizing change-over valve) at the appropriate engine speed.

Fuel Pressure Regulator

Fuel pressure regulator is located on the fuel manifold on the fuel return side. Fuel pressure regulator maintains constant pressure at the injectors.

FUEL PUMP PERFORMANCE

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 PCM and provides voltage for cold start valve.

Idle Air Control (IAC) valve controls engine idle speed by regulating flow of air to engine when throttle plates and CTP switch are closed. IAC valve is mounted on throttle valve housing.

IAC valve is operated by a cycled DC (duty cycle) voltage. PCM supplies cycled voltage to IAC switch to regulate engine idle speed with a warm engine between 700 and 800 RPM. The duty cycle will vary depending on engine load and engine conditions. Since the idle system is adaptive, no adjustments are necessary.

INJECTOR RESISTANCE

Bosch injector resistance is 15-17 ohms. Siemens injector resistance is 13.5-14.5 ohms.

IGNITION CONTROL

Ignition system is distributorless (DIS). Since there are no moving parts, no vibration 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 MFI system consists of

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

Detonation Retard

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 PCM to determine necessary timing retardation.

One knock sensor is installed on each head under intake manifold. Using 2 knock sensors makes it possible for system to be more accurate and 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 degrees or until knocking stops.

If cylinder knock continues, PCM 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.

PCM 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.

Shift Retard (A/T)

The automatic Transmission Control Module (TCM) signals each gearshift simultaneously to the PCM. The PCM responds by retarding ignition timing point during the gear change, reducing engine torque during the shift, making gear changes more comfortable.

EVAP Emissions System

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

EVAP Canister Purge Control Shutoff Solenoid

Shutoff solenoid is located on side of EVAP canister. When engine is started, 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 EVAP canister and intake air boot, preventing fuel vapors which may cause hot start problem from collecting in intake tract.

Exhaust Gas Recirculation (EGR) System

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

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

EGR is controlled by ignition map in the PCM. This allows precise regulation of the EGR flow.

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 TESTS W/CODES article.

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