VOLKSWAGEN CORRADO 1990 Repair Manual

E - THEORY/OPERATION - DIGIFANT
Article Text
1990 Volkswagen Corrado
For Volkswagen Technical Site: http://vw.belcom.ru
Copyright © 1998 Mitchell Repair Information Company, LLC
Thursday, March 23, 2000 09:43PM
ARTICLE BEGINNING
1990 ENGINE PERFORMANCE
Theory & Operation - Digifant II
Volkswagen; Cabriolet, Corrado, Fox, Golf GL/GTI,
Jetta, Vanagon
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.
AIR INDUCTION SYSTEM
SUPERCHARGER (CORRADO)
The supercharger works like an air pump. Driven constantly
off the crankshaft by a toothed belt the supercharger compresses air.
This compressed or boosted air is cooled by an intercooler before
entering intake manifold.
Excess boost air is controlled by a mechanically operated
boost control valve and idle stabilizer. The boost control valve moves
in the opposite direction of the throttle valve. As throttle valve
opens boost control closes, directing most of the boost air back to
the supercharger intake. See Fig. 1.Fig. 1: Identifying Supercharger System Components
Courtesy of Volkswagen United States, Inc.

E - THEORY/OPERATION - DIGIFANT
Article Text (p. 2)
1990 Volkswagen Corrado
For Volkswagen Technical Site: http://vw.belcom.ru
Copyright © 1998 Mitchell Repair Information Company, LLC
Thursday, March 23, 2000 09:43PM
COMPUTERIZED ENGINE CONTROLS
The Bosch AFC Digifant II system is a computer-controlled
fuel injection system. The system does not use cold start injector or
thermo time switch for cold start enrichment. Different sensors and
switches, along with Electronic Control Unit (ECU), regulate fuel
injection and ignition timing.
ELECTRONIC CONTROL UNIT
These units cannot be repaired. The ECU controls all engine
operations, and limits maximum engine speed. The ECU receives
information from various input devices.
ECU LOCATIONS TABLE
ECU LOCATIONÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄApplication Location
Except Vanagon ..................... Behind Glove Box Passenger Side
or Under Engine Compartment Hood,
Behind Plenum Panel on Driver's Side
Vanagon ........................ On Right Side of Engine Compartment
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄFig. 2: Identifying Digifant II System Components
Courtesy of Volkswagen United States, Inc.
IDLE STABILIZATION CONTROL UNIT (VANAGON)
The idle stabilization control unit is located in front of
the right-hand taillight assembly. If engine idle speed differs from
the value stored in the idle stabilization control unit, the idle
stabilizer valve adjusts the volume of air entering the engine at
idle. The idle stabilization control unit receives information from

E - THEORY/OPERATION - DIGIFANT
Article Text (p. 3)
1990 Volkswagen Corrado
For Volkswagen Technical Site: http://vw.belcom.ru
Copyright © 1998 Mitchell Repair Information Company, LLC
Thursday, March 23, 2000 09:43PM
the following:
* Coolant Temperature Sensor
* ECU Control Relay
* ECU
* Oxygen Sensor
* Power Steering Oil Pressure Switch
NOTE: Components are grouped into 2 categories. The first category
covers INPUT DEVICES, which control or produce voltage
signals monitored by the control unit. The second category
covers OUTPUT SIGNALS, which are components controlled by
the control unit.
INPUT DEVICES
AIRFLOW SENSOR (EXCEPT CORRADO)
All intake air is drawn through the airflow sensor. The
airflow sensor contains a tunnel with a measuring flap and dampening
flap. The measuring flap swings with intake air stream against
pressure of a spiral spring and is connected to a potentiometer.
The potentiometer transmits an electrical signal determined
by measuring flap position to inform ECU of engine load. At idle, the
measuring flap is almost closed due to spring pressure. See Fig. 3.
The potentiometer within the airflow sensor prevents loss of
engine power during engine load or sudden acceleration (along with
engine speed and coolant temperature) by signaling the ECU of
necessary enrichment and timing requirements.
The airflow sensor contains an intake air temperature sensor.
An adjustable idle air by-pass screw influences CO levels at low
engine speeds. A tamper-proof plug is installed over this screw.Fig. 3: Cross-Sectional View of Airflow Meter
Courtesy of Volkswagen United States, Inc.

E - THEORY/OPERATION - DIGIFANT
Article Text (p. 4)
1990 Volkswagen Corrado
For Volkswagen Technical Site: http://vw.belcom.ru
Copyright © 1998 Mitchell Repair Information Company, LLC
Thursday, March 23, 2000 09:43PM
COOLANT TEMPERATURE SENSOR
Is a temperature sensitive variable resistor sensor (less
resistance as temperature increases). This sensor returns signals to
the ECU to determine amount of cold start enrichment, ignition timing
and idle stabilization during warm-up. The sensor return signal has
input to the ECU when the oxygen sensor, idle stabilization, and full
throttle enrichment functions are activated.
CO POTENTIOMETER (CORRADO)
The CO potentiometer adjusts CO mixture. Located on the
intake air duct before the throttle housing. The adjustment screw has
a tamper-proof plug. An air temperature sensor located within the
potentiometer housing is used to calculate air density.
ECU CONTROL RELAY
When energized by ignition switch, the ECU control relay
provides battery voltage to ECU.
FULL THROTTLE (FUEL) ENRICHMENT SWITCH
The full throttle enrichment switch supplies the control unit
with information to increase amount of fuel injected during full
throttle operation.
HALL EFFECT SENSOR
See ELECTRONIC IGNITION SYSTEM under IGNITION SYSTEM in this
article.
INTAKE AIR TEMPERATURE SENSOR
Intake air temperature sensor is a thermistor-type variable
resistor (resistance decreases with increase of temperature). This
sensor voltage signal varies to ECU in relation to engine air
temperature. Sensor is located inside the airflow meter.
KNOCK SENSOR(S)
Pick-up engine vibrations before knock occurs. The ECU
compensates timing as needed and controls timing advance or retard for
maximized engine performance.
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR (CORRADO)
MAP is located inside the ECU. The MAP sensor signal is used
by ECU to determine engine load and manifold boost pressure. This
signal along with RPM and intake air temperature is used to calculate
fuel injection quantity.

E - THEORY/OPERATION - DIGIFANT
Article Text (p. 5)
1990 Volkswagen Corrado
For Volkswagen Technical Site: http://vw.belcom.ru
Copyright © 1998 Mitchell Repair Information Company, LLC
Thursday, March 23, 2000 09:43PM
OXYGEN (O2) SENSOR
The O2 sensor detects oxygen content in the exhaust gas and
sends this information to the ECU. In operation, the ECU receives
signals from the O2 sensor and varies the duration during which fuel
is injected. A high voltage signal indicates a rich mixture. A low
voltage signal indicates a lean mixture.
The O2 sensor is heated electrically for rapid warm-up and
constant operating temperature. Power to the heating element is
supplied whenever ignition switch is turned to ON position.
POWER STEERING (P/S) OIL PRESSURE SWITCH
The P/S oil pressure switch signals the ECU when the power
steering load is high. The ECU then sends a voltage signal to the idle
stabilization valve to increase idle speed with power steering load.
THROTTLE VALVE SWITCH (VANAGON ONLY)
Throttle valve switch supplies ECU with information that
throttle valve is closed. If engine is above 1500 RPM with throttle
closed, fuel will be shut off to the injectors. At idle speed, this
switch signals control unit to regulate amount of fuel injected.
OUTPUT SIGNALS
NOTE: Each vehicle may be equipped with different combinations of
computer controlled components. The following listed
components may NOT be used on all models. For theory and
operation on each output component, refer to the system
indicated in brackets, to the right of each component.
* Fuel Injectors (Fuel Control)
* Idle Air Stabilizer Valve (Idle Speed)
* Ignition Coil Control (Ignition System)
FUEL DELIVERY
ELECTRIC FUEL PUMP
The fuel pump provides fuel under pressure to the fuel
pressure regulator. Power for operation during cranking mode is
provided from starter relay via the fuel pump relay. After the engine
has started, control of the fuel pump is through the ignition signal.
The fuel pump is sealed unit.
FUEL PUMP RELAY
When energized by the ignition switch and grounded by the
ECU. The fuel pump relay provides battery voltage to the fuel pumps,
injectors, idle stabilization control unit, oxygen sensor heating

E - THEORY/OPERATION - DIGIFANT
Article Text (p. 6)
1990 Volkswagen Corrado
For Volkswagen Technical Site: http://vw.belcom.ru
Copyright © 1998 Mitchell Repair Information Company, LLC
Thursday, March 23, 2000 09:43PM
element and the power steering pressure switch.
FUEL PRESSURE REGULATOR
The fuel pressure regulator is a sealed, spring loaded
diaphragm with connection for intake manifold vacuum. Fuel pressure is
maintained at about 36 psi (2.5 kg/cmý) pressure.
A connection for intake manifold vacuum provides a constant
pressure differential which ensures that the amount of fuel injected
is solely dependent upon injector open ON time. Excess fuel is
returned to fuel tank. No service of pressure regulator is required.
The pressure regulator is located on or near fuel rail.
FUEL CONTROL
Data on engine temperature, engine speed, intake air volume,
throttle position, exhaust oxygen content and intake air temperature
are used by ECM to determine injection pulse width.
FUEL INJECTORS
A fuel rail links the fuel pressure regulator with the fuel
injectors. Each cylinder is provided with a solenoid-operated injector
which sprays fuel toward backside of each inlet valve. Each injector
is energized through the ignition coil and grounded through the ECU to
complete the circuit
Each injector is linked to a resistor (resistor may be
external or integral with injector or ECU) to reduce operating voltage
to 3 volts and to protect injectors from power surges. The ECU
controls length of time each injector is open. The ON time of the
injector governs the amount of fuel delivered. The injector delivers
1/2 the amount of fuel required for an operating cycle each time they
open (twice per cycle).
FUEL PUMP AFTER-RUN RELAY (CORRADO)
The purpose of the After-Run Fuel Pump relay system is to
reduce the chance of fuel vaporizing in fuel rail. Both transfer pump
and fuel pump are used to recirculate fuel. The after-run relay
switches the pumps on for 2 minutes after ignition is turned off or
when under hood temperature exceeds 194
ø F (90ø C) and fuel pressure
is above 17 psi (1.2 kg/cm
ý). The pumps operate a maximum of 8
minutes.
IDLE SPEED
Engine idle speed is controlled by the ECU and idle speed
control unit depending upon engine operating conditions. The ECU and
Idle speed control unit are fed information on engine operating
conditions and determine the best idle speed.
IDLE AIR STABILIZER VALVE

E - THEORY/OPERATION - DIGIFANT
Article Text (p. 7)
1990 Volkswagen Corrado
For Volkswagen Technical Site: http://vw.belcom.ru
Copyright © 1998 Mitchell Repair Information Company, LLC
Thursday, March 23, 2000 09:43PM
The idle air stabilizer valve is ECU controlled. On Vanagon
idle stabilizer valve is controlled by a separate idle stabilizer
controller. The idle air stabilizer operates to continuously maintains
engine idle at a computed engine idle speed.
IGNITION SYSTEM
The Hall Effect sending unit in the distributor, uses a
shutter window wheel mounted on the distributor shaft. The shutter
blades pass in and out of the air gap of the hall effect sender
resulting in signal pulses (engine speed signal). There is one shutter
window for each engine cylinder. Signals from distributor hall sender
are sent to the ECU.
ELECTRONIC IGNITION SYSTEM
IGNITION COIL CONTROL
Signals from distributor hall sender are sent to the ECU,
which produces a pulsating signal to the ignition coil. This computed
signal from ECU to ignition coil, control ignition timing according to
engine load (airflow sensor signal), engine speed (Hall Effect signal)
and engine coolant temperature.
IGNITION TIMING ADVANCE CONTROL SYSTEM
Timing is ECU controlled.
EMISSION SYSTEMS
EVAPORATIVE EMISSIONS SYSTEM
Fuel vapors are collected in the expansion tank. Any liquid
gasoline collect in expansion tank flows back to the fuel tank through
vent lines. See Fig. 4. Fuel vapors are drawn from tops of the
expansion tanks and flow into the carbon canister where the vapors are
stored. When the engine is not running.
After engine is started, the control valve is opened by
throttle vacuum. Fresh air is drawn into bottom of the canister. Fuel
vapors from the canister are drawn into the intake manifold.
EXHAUST GAS RECIRCULATION
Information is not available. See M - VACUUM DIAGRAMS for
system components and vacuum hose routings.

E - THEORY/OPERATION - DIGIFANT
Article Text (p. 8)
1990 Volkswagen Corrado
For Volkswagen Technical Site: http://vw.belcom.ru
Copyright © 1998 Mitchell Repair Information Company, LLC
Thursday, March 23, 2000 09:43PMFig. 4: Identifying Evaporative Emissions System Components
Courtesy of Volkswagen United States, Inc.
SELF-DIAGNOSTIC SYSTEM
O2 SENSOR WARNING LIGHT
All vehicles are equipped with an O2 sensor warning light
located on the instrument panel. The light will illuminate when a
mileage counter reaches 60,000 miles (on Vanagon 90,000 miles)
indicating recommended O2 sensor replacement and mileage counter
reset.
CHECK ENGINE LIGHT (CALIF MODELS)
Some California vehicles are equipped with a CHECK engine
light and rocker switch on the instrument panel. The light will
illuminate when the ignition switch is turned to the ON position (for
bulb check) and when engine management systems are malfunctioning
during normal operation with the engine running. For additional
information see appropriate SELF-DIAGNOSTICS article.
MISCELLANEOUS CONTROLS
NOTE: Although not considered true Engine Performance related
systems, some controlled devices may affect driveability if
they malfunction.

E - THEORY/OPERATION - DIGIFANT
Article Text (p. 9)
1990 Volkswagen Corrado
For Volkswagen Technical Site: http://vw.belcom.ru
Copyright © 1998 Mitchell Repair Information Company, LLC
Thursday, March 23, 2000 09:43PM
CRANKCASE VENT LINE HEATING ELEMENT
A heating element is used in the crankcase vent line to
prevent icing during cold engine operation, this element has a 5.5-mm
hole in the restrictor plate. The circuitry to operate the heating
element is protected by an in-line 5 amp fuse located in the wiring
connector box located in the engine compartment.Fig. 5: Locating Crankcase Vent Line Heating Element
Courtesy of Volkswagen United States, Inc.
COOLING FAN MOTOR
The cooling fan is either a one or two speed motor. If
vehicle is equipped with single speed motor the fan comes on at 198-
207øF (92-97øC) and off at 183-196øF (84-91øC). If equipped with a two
speed motor. The first speed of cooling fan should come on at 197
øF
(91
øC) and go off at 98øF (18øC). Second speed comes on at 203-121øF
(95-100
øC) and off at 188øF (87øC).
AFTER-RUN THERMOSWITCH (CORRADO)
An after-run switch is used to help prevent fuel
vaporization. The thermoswitch turns the cooling fan on when
temperatures in the engine compartment exceed 230
øF (110øC) and off at
217
øF (103øC).
END OF ARTICLE

ELECTRICAL COMPONENT LOCATOR
Article Text
1990 Volkswagen Corrado
For Volkswagen Technical Site: http://vw.belcom.ru
Copyright © 1998 Mitchell Repair Information Company, LLC
Thursday, March 23, 2000 09:44PM
ARTICLE BEGINNING
1990-91 VOLKSWAGEN
ELECTRICAL COMPONENT LOCATOR
Corrado
BUZZERS, RELAYS & TIMERSÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄComponent Component Location
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄABS System Anti-Lock Brake Pump
Fuse Relay In fuse/relay panel, below
left side of instrument panel.