Digifant Cec System Volkswagen Jetta II

DESCRIPTION & OPERATION

Note. This article also covers Digifant II system which is used on Golf, Golf GT and Jetta models.

The Bosch AFC Digifant system is a modified AFC system. The Digifant system does not use cold start injector and thermo time switch for cold start enrichment. Different sensors and switches along with Electronic Control Unit (ECU) regulate fuel injection operation. For further information on operation, see BOSCH AFC in the FUEL SYSTEMS section.

AIRFLOW METER

All intake air is drawn through the airflow meter. The meter contains a tunnel with a measuring flap and dampening flap. The measuring flap swings in with 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 the spring pressure.

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

An idle air by-pass receives airflow through a small hole, the size of which is controlled by the idle mixture screw. This adjustable air by-pass influences CO levels at low engine speeds.

CONTROL UNIT

The control unit is located on left rear corner of engine compartment on Golf, Golf GT and Jetta models. On Vanagon, control unit may be located in trunk floor behind the rear seat or in the equipment box in cargo area. These units cannot be repaired. The control unit limits maximum engine speed to about 5500-5800 RPM. The control unit receives information from the following

1. Air quantity from intake sensor.
2. Air temperature from temperature sensor (I).
3. Engine temperature from temperature sensor (II).
4. Exhaust oxygen content from O2 sensor.
5. Position of throttle valve from switch for full throttle enrichment.

The control unit uses this information to determine amount of fuel to be injected.

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 (Digifant). The fuel pump is sealed unit.

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 PRESSURE REGULATOR

The pressure regulator consists of a sealed, spring loaded diaphragm with a connection for intake manifold vacuum. Fuel pressure is maintained at about 36 psi (2.5 Bar) 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.

FULL THROTTLE ENRICHMENT SWITCH

The full throttle enrichment switch supplies the control unit with information to increase amount of fuel injected at full throttle.

IDLE STABILIZER

This valve is used on models with Digifant system. It is used to control both cold engine operation and warm engine idle speed.

ON-BOARD DIAGNOSTIC SYSTEM (DIGIFANT II - CALIFORNIA ONLY)

"On-Board Diagnostic System" recognizes both ignition and fuel injection system fault. The control unit for the Digifant II system is equipped with an erasable permanent memory which stores and displays any faults that could cause the vehicle to fail an exhaust emission inspection.

Fault codes appear on indicator switch labeled "Check", located on instrument panel. If a fault is detected and stored in the control unit, fault light will illuminate after one minute.

THROTTLE VALVE SWITCH

Note. DO NOT connect a test light to throttle valve switch connectors if control unit is connected.

This switch supplies control unit 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.

SYSTEM CHECK

Electrical system functions may be checked at multi-pin connector of ECU. Turn ignition off. Disconnect connector from ECU. Using either ohmmeter or voltmeter, check system between terminals as noted. (Scheme 4) See AFC DIGIFANT SYSTEM VOLTAGE and RESISTANCE CHECKS charts.

SYSTEM DIAGNOSIS

On California models equipped with Digifant II system, an "On-Board Diagnostic System" is used. Trouble codes/fault memory can be read through the "Check" indicator switch which illuminates when a fault is detected and stored in memory or by activating fault memory. To activate fault memory/code(s), turn ignition switch to "ON" position, engine off. Depress "Check" indicator switch for at least 4 seconds. Indicator light on switch will start flashing codes. A flash code consists of 4 flash impulse groups, with a maximum of 4 flash impulse per group. Between each impulse group there is a 2.5 second pause between codes.

To erase fault codes from memory, turn ignition switch to "OFF" position, press and hold down indicator switch and turn ignition switch to "ON" position. Continue depressing switch for about 5 seconds and turn ignition switch off. Test drive vehicle for a minimum of 10 minutes. Recheck codes and correct if necessary. Repeat fault erasing procedure until memory is cleared.

FAULT/TROUBLE CODES - DIGIFANT II

Scheme 4

GOLF, GOLF GT & JETTA AFC DIGIFANT SYSTEM RESISTANCE CHECKS (1)

GOLF, GOLF GT & JETTA AFC DIGIFANT SYSTEM VOLTAGE CHECKS (1)

VANAGON AFC DIGIFANT SYSTEM RESISTANCE CHECKS (1)

VANAGON AFC DIGIFANT SYSTEM VOLTAGE CHECKS (1)

Note. Terminal numbers referred to in testing procedure are found on connector housing on side of airflow meter assembly.

Airflow Sensor Potentiometer

1. Turn ignition switch to "OFF" position. Remove high voltage wire from distributor ignition coil and connect to ground. Disconnect Digifant II control unit connector.
2. Connect ohmmeter leads between terminals No. 6 and 17 of control until connector. Resistance should be within 500-1000 ohms. If not okay, check sensor wiring harness.
3. Connect ohmmeter leads between terminals No. 17 and 21 of control unit connector. Move airflow sensor pressure flap or actuate starter. Resistance should change. If not okay, replace potentiometer.

Air Temperature Sensor

1. Connect ohmmeter between terminals No. 1 and 4 of airflow sensor connector. Ohmmeter reading should be within specification depending on temperature. See «TEMPERATURE SENSOR RESISTANCE SPECIFICATIONS»(/volkswagen/jetta/ii-1984-1992/remont/testing-diagnostics/#digifant-cec-system) table.
2. Replace airflow meter if sensor is defective. If replaced, idle speed and CO level must be adjusted.

COOLANT TEMPERATURE SENSOR

1. Check coolant temperature. Using ohmmeter, check resistance of temperature sensor. Resistance will vary depending on coolant temperature.
2. Replace temperature sensor if resistance is not within specification. See «TEMPERATURE SENSOR RESISTANCE SPECIFICATIONS»(/volkswagen/jetta/ii-1984-1992/remont/testing-diagnostics/#digifant-cec-system) table.

TEMPERATURE SENSOR RESISTANCE SPECIFICATIONS

DECELERATION FUEL SHUTOFF

1. Warm engine to normal operating temperature of 176°F (80°C). Fuel shutoff will operate in these conditions; when engine RPM was above 2600, throttle is closed, coolant temperature is above 122°F (50°C) and engine RPM is between 1250 and 2600 RPM.
2. Locate double sided connector. Using Adapter (VW 1490), jump proper side of connector. (Scheme 5) Slightly accelerate engine. Note engine RPM.
3. Engine RPM should surge, then decelerate when shutoff operates. If engine RPM does not surge, check for damaged wiring, defective engine temperature sender or defective control unit.

Scheme 5

Spray Pattern

1. Remove a pair of injectors, leaving electrical plugs and fuel lines connected. Disconnect electrical plugs from remaining installed fuel injectors.
2. Hold injectors over pan. Operate starter briefly. Spray pattern must be an even, cone-shaped spray. Repeat test for remaining injectors.

Leak Test

1. Disconnect electrical connectors from injectors. Remove injectors in pairs, leaving fuel lines connected. Turn ignition on for approximately 5 seconds for fuel pump operation.
2. Note amount of fuel drops from injectors. Replace injectors if leakage exceeds more than 2 drops per minute.

Voltage Supply

1. Disconnect electrical connectors from injectors. Connect Test Light (US 1115) across plug connector terminals. CAUTION: DO NOT ground connector contacts as ECU will be damaged.
2. Test light should flicker while operating starter. If voltage is not present test wiring circuit. See «TESTING»(/volkswagen/jetta/ii-1984-1992/remont/testing-diagnostics/#digifant-cec-system) .

Using "T" fitting, connect fuel pressure gauge in fuel delivery line. Run engine at idle speed. Pressure should be 36 psi (2.6 kg/cm 2 ) on Golf, Golf GT and Jetta or 29 psi (2 kg/cm 2 ) on Vanagon with pressure regulator vacuum line connected. With vacuum line disconnected from pressure regulator, pressure should be 44 psi (3.1 kg/cm 2 ) on Golf, Golf GT and Jetta or 36 psi (2.6 kg/cm 2 ) on Vanagon.

Delivery Rate

1. Disconnect and plug fuel return line at pressure regulator. Attach hose to return line fitting and place other end in measuring container.
2. Disconnect fuel pump relay. Connect jumper wire between terminals No. 30 and 87 of relay harness connector for 30 seconds. Minimum delivery rate should be 16.9 ozs. (.5L) for 30 second period.

Operation

1. Turn ignition switch on. Idle stabilizer should vibrate and hum. If stabilizer does not operate, disconnect connector from idle stabilizer. Using Tester (VW 1325 A-1 ), check for continuity. Continuity should exist.
2. Replace idle stabilizer if no continuity exists. If continuity is obtained, reconnect connector to valve. Check electrical connections on control unit relay adapter located in front of right taillight.
3. Replace idle stabilizer control unit if electrical connections are okay.

Regulation

1. Warm engine to normal operating temperature. Ensure all accessories are off. Inspect for leaks in intake air system. Check ignition timing, idle RPM and CO level. See TUNE-UP PROCEDURES section.
2. Remove crankcase breather hose. Tightly pinch hose closed. Install Tester (VW 1315 A-1 ) and Adapter (VW 1315 A-2 ) on idle stabilizer. Operate engine at idle and note reading.
3. Reading should be approximately 410-450 mA and fluctuate. Disconnect connector for oxygen sensor. Reading should be approxi-mately 430 mA with a constant reading. Replace idle stabilizer if readings are not within specification.

KNOCK SENSOR

1. Turn ignition switch off. Disconnect control unit harness connector. Remove wiring connector from knock sensor and bridge or jump wiring connector terminals No. 4 and 5 and terminals No. 4 and 7. Ohmmeter should read continuity.
2. If no continuity is present, repair knock sensor wiring. If knocking continues, replace knock sensor.

OXYGEN SENSOR

1. Warm engine until engine oil temperature is at a minimum of 176°F (80°C). Check for leaks in exhaust system. Using Adapter (Sun 120 239 ), connect CO test equipment hose to the sampling point on left exhaust pipe.
2. Allow engine to idle for approximately 2 minutes and note CO reading. Remove hose from pressure regulator and pinch closed. This is done to check function of oxygen regulation. CO reading should increase briefly and then drop again.
3. If reading did not increase and then drop, disconnect connector at oxygen sensor. DO NOT allow oxygen sensor wire to touch ground. Hold O2 sensor wiring harness connector wire to ground. CO reading should increase. Connect O2 sensor wiring harness connector wire to alternator connector. CO reading should decrease.
4. If CO reading was incorrect, check wire connection on control unit. See «TESTING»(/volkswagen/jetta/ii-1984-1992/remont/testing-diagnostics/#digifant-cec-system) . If wiring connection is okay, replace control unit. Oxygen sensor is defective if test step 2) was faulty, but test step 3) was okay. Replace oxygen sensor.

RELAY TEST (VANAGON ONLY)

The power supply and fuel pump relays are located in the engine compartment in a plastic box on the firewall. The fuel pump relay is mounted on the right side of the box and power supply relay on the left. To test relay, use the following procedures

1. Unplug all electrical connectors to injectors. To check power to relays, turn ignition on. Use test light to check for voltage at terminals No. 30 and 85 of both relays. Power supply relay should also have voltage at terminal No. 85. If not, check wiring.
2. Terminal No. 87 of fuel pump relay should have voltage when starter is operated. If not, check wiring. If wiring is good, check relay. If relay is good, replace ECU.

Scheme 6

1. Closed throttle valve switch controls injection quantity for idle, switches off injection during deceleration fuel shutoff and ignition timing at idle. Open throttle valve switch controls fuel quantity at full throttle.
2. Disconnect wiring lead from throttle valve switch. Install ohmmeter between terminals on wiring lead. Reading should show continuity at idle and full throttle position of throttle valve switch.
3. To check switching point, open and close throttle valve slowly. When ohmmeter indicates continuity at idle stop position, measure gap between idle stop and idle adjusting screw with feeler gauge. (Scheme 6)
4. Gap should be.002-.004" (.05-.10 mm). Proper adjustment of throttle valve switch is important. Excessive gap will cause engine surge when throttled slightly.
5. Insufficient gap causes cold engine to stall at full throttle acceleration and warm engine will have no deceleration fuel shutoff. If gap requires adjustment, see «THROTTLE VALVE SWITCH»(/volkswagen/jetta/ii-1984-1992/remont/testing-diagnostics/#digifant-cec-system) under «ADJUSTMENTS»(/volkswagen/jetta/ii-1984-1992/remont/testing-diagnostics/#digifant-cec-system).

Switch Wiring

Turn ignition on. Disconnect wiring lead from throttle valve switch. Using voltmeter, check voltage between both terminals of wiring lead. Reading should be approximately 5 volts. If voltage is incorrect, check for break in wiring or defective ECU.

Full Throttle Enrichment Function

1. Warm engine until engine oil temperature is at a minimum of 176°F (80°C). Stop engine and connect CO tester and tachometer. Start engine and allow to idle for 2 minutes.
2. Remove connector from double sided connector at throttle valve switch. (Scheme 6) Slowly accelerate engine to approximately 4500 RPM. Note and record CO level reading.
3. Bridge connections of double sided connector using Adapter (VW 1490). Note CO reading. Reading should increase at least 1% over recorded CO reading.
4. If reading did not increase, check for defective wiring, engine temperature sending unit or control unit.

IDLE SPEED

See TUNE-UP PROCEDURES section.

THROTTLE VALVE

Note. Adjusting screw is preset at factory. Basic adjustment should only be made if screw position has been altered.

Basic Adjustment

1. Turn adjusting screw outward until gap exists between stop and adjusting screw. Place small piece of paper between screw and stop. Slide paper back and forth while turning screw inward.
2. When screw contacts paper, turn adjusting screw an additional 1/2 turn clockwise. Check idle speed and CO reading. See TUNE-UP PROCEDURES section.

Loosen lock bolt. Move adjusting bolt until switch-on point (measured with ohmmeter) occurs with gap of.002-.004" (.05-.10 mm) between idle stop and idle adjusting screw. (Scheme 6) DO NOT alter throttle valve adjusting screw. Tighten lock bolt.

Golf Bosch Digifant II Fuel Injection Wiring Diagram. Scheme 7

Jetta Bosch Digifant II Fuel Injection Wiring Diagram. Scheme 8

Vanagon Bosch Digifant Fuel Injection Wiring Diagram. Scheme 9

See also:
• TEMPERATURE SENSOR RESISTANCE SPECIFICATIONS