Digifant Engine Management system

The Digifant Engine Management system is an electronic fuel injection system designed by Volkswagen A.G. in cooperation with Robert Bosch GmbH.

Digifant is the outgrowth of the Digi-jet Fuel Injection system first used on early Vanagon vehicles.



History

Digifant was introduced in 1986 on the 2.1L Vanagon Engine. This system combined digital fuel control as used in the earlier Digi-Jet systems with a new digital ignition system. Digifant as used in Golf and Jetta models simplified several functions and added knock sensor control to the ignition system. Other versions of Digifant appeared on the Fox, Corrado, Eurovan as well as later production versions of the rear engined VW Beetle, sold only in Mexico.


Features

Fuel injection control is digital electronic. It is based on the measurement engine load (this signal is provided by the Air Flow Sensor), and on engine speed (Signal provided by the hall sender in the distributor). These primary signals are compared to a map, or table of values, stored in the Engine Control Module (ECM) memory.

The amount of fuel is controlled by the injector (duration). This value is taken from a program in the ECM that has 16 points for load and 16 points for speed. These 256 primary values are then modified by coolant temperature, intake air temperature, oxygen content of the exhaust, battery voltage and throttle position to provide 65,000 possible injector duration points.

Digifant is unlike the earlier CIS and CIS-E fuel injection systems that it replaced in that fuel injectors are mounted on a common fuel rail. The fuel injectors are wired in parallel, and are supplied with Constant System Voltage. The ECM switches the Ground on and off to control duration. All injectors operate at the same time each crankshaft revolution; two complete revolutions being needed for each cylinder to receive the correct amount of fuel for each combustion cycle.

Ignition control is also digital electronic. The sensors that supply the engine load and engine speed signals for injector duration provide information about the basic ignition timing point. The signal sent to the Hall control unit is derived from a program in the ECM that is similar to the injector duration program.

There are 16 points available for load and 16 points for speed. The resulting 256 single operational points are modified by coolant temperature signals and cylinder selective knock control to achieve the optimal ignition point.

Knock control is used to allow the ignition timing to continually approach the point of detonation. This is the point where the engine will produce the most power, as well as the highest efficiency.

Additional functions of the ECM include operation of the fuel pump by closing the Ground for the fuel pump relay, and control of idle speed by a throttle plate bypass valve. The idle air control vale (previously known as an idle air stabilizer valve), receives a changing milliamp signal that varies the strength of an electro-magnet pulling open the bypass valve.

Idle speed stabilization is enhanced by a process known as Idle Speed Control (ISC). This function (previously known as Digital Idle Stabilization), allows the ECM to modify ignition timing at idle to further improve idle quality.



Inputs/Outputs - Digifant II

The 25 pin electronic control unit used in the Golf and Jetta receives inputs from the following sources:

- Hall Sending Unit (Provides Engine Speed Signal) - Air Flow Sensor (Provides Engine Load Information) - Coolant Temperature Sensor - Intake Air Temperature Sensor - Knock Sensor

Additional signals used as inputs are:

- Air Conditioner (compressor on) - Battery Voltage - Starter Signal

The anti-lock brake system, 3-speed automatic transmission and vehicle speed sensor are not linked to this system.

Outputs controlling engine operation include signals to the following:

- Fuel Injectors - Idle air control valve - Hall Control Unit - Fuel Pump Relay - Oxygen Sensor Heater



Additional Systems

The evaporative emission system is controlled by a vacuum operated mechanical carbon canister control valve.

Fuel pressure is maintained by a vacuum operated mechanical fuel pressure regulator on the fuel injector rail assembly.

Inputs and outputs are shown in the following illustration. Digifant II as used on Golf and Jetta vehicles provides the basis for this chart.



System Variants (North America Only)

In North America, Volkswagen released two versions of the Digifant Fuel Injection system. Note that this refers to A2 Golf and Jetta models only.

Digifant I was used exclusively in California market vehicles. Primary differences between Digifant I and Digifant II are as follows:

A limited number of 1987-1990 California Golf and Jetta models are equipped with an On Board Diagnostics system (OBD). These vehicles have blink code capacity to store up to 5 Diagnostic Trouble Codes (DTCs). For the most part, diagnostic troubleshooting is done with a Volkswagen special tool, known as the VAG 1598 and a Digital Multimeter. This system can also have carbon monoxide (CO), ignition timing and idle speed adjusted to baseline values.

In 1991, California Golf, Jetta, Fox, Cabriolet and Corrado vehicles were equipped with expanded OBD capabilities. These later Digifant versions have 38-pin ECMs with Rapid Data Transfer and permanent DTC memory. All Eurovans with Digifant also have rapid data transfer and permanent DTC memory. These systems use a throttle plate potentiometer to track throttle plate position in place of the idle and full throttle switches used on earlier systems.

Another characteristic of Digifant 1 equipped vehicles is a switch mount on the dashboard which has a "Check Engine" symbol.


Summary

Digifant is an engine management system designed originally to take advantage of the first generation of newly developed digital signal processing circuits. Production changes and updates were made to keep the system current with the changing California and federal emissions requirements. Updates were also made to allow integration of other vehicle systems into the scope of engine operation.

Changes in circuit technology, design and processing speed along with evolving emissions standards, resulted in the development of new engine management systems. These new system incorporated adaptive learning, enhanced and expanded diagnostics, and the ability to meet total vehicle emissions standards.



Maintenance of older Digifant Vehicles

In Volkswagen circles, Digifant has often been criticized for it's lack of performance tuning and occasional driveability issues. Most of these issues can be traced back to one of two issues:

- Bad ECM grounds - Faulty Engine Coolant Temperature Sensor (ECT)

The engine coolant temp sensor is located in the coolant flange on the front of the cylinder head.

In the following picture the Digifant Coolant Temperature sensor is clearly visible, as it is identified by a blue electrical connector.


Common issues that are indicative of a failed ECT are:

- Vehicle idles poorly - Engine sputters, might stall - Higher than normal fuel consumption

The part number for this sensor is 025-906-041-A (Always check with your dealer for the most updated part number).

When replacing this sensor, it is important to also replace the clip that holds it in position (032-121-142) and the O-Ring (N-903-168-02).

Once the new sensor has been installed, start the engine and disconnect the blue coolant temperature sensor. Rev the engine through 3000 RPM 3 times, each time allowing the throttle to close completely. This clears the Digifant ECM fault memory.