Fuel Injector

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The electronic fuel injection system wasdesigned and built by Bosch and has been usedsuccessfully on several VW models and othercars.

Text of Fuel Injector

  • [All content copyright Clymer Publications, Overland Park Kansas, 1972. Used with permission.)

    PDF version of this document prepared by Greg Merritt ( gregm@vwtype3.org) , 1998.This document may be viewed in HTML format on the World Wide Web at http://www.vwtype3.org/

    CHAPTER SEVEN

    FUEL INJECTION

    The electronic fuel injection system wasdesigned and built by Bosch and has been usedsuccessfully on several VW models and othercars.Complete service information for the fuel

    injected engine is beyond the scope of thismanual. Because of special knowledge andelectronic test fixtures required, some servicescannot be performed by the home mechanic. Theprocedures in this chapter permit you to isolateand repair a majority of mechanical troubles, butsome mechanical and all electrical unitprocedures should be performed by the dealer.

    BASIC PRINCIPLESThe heart of the fuel injection system is the

    control unit, a small electronic "computer."Various sensors transmit information to thecontrol unit concerning air temperature, enginetemperature, engine speed, and engine load. Thecontrol unit uses this information to determinethe exact amount of fuel to be distributed to thecylinders. Figure 1 is a simplified diagram ofthe entire fuel injection system; refer to it for thefollowing descriptions. Figures 2 and 3 areelectrical diagrams which may be helpful whentroubleshooting.

    Air System

    Components of the air system are shown inFigure 4 . Clean air from the oil bath aircleaner

    enters the intake air distributor past a throttle valveto the combustion chambers. The throttle valvecontrols the flow of air.

    During idle, the throttle valve is completelyclosed. A bypass around the throttle valve permitsa small amount of air, controlled by the idle speedscrew, to pass. When the engine is cold (below122F), the auxiliary air regulator bypasses a largeramount of air around the throttle valve. This extraair mixes with extra fuel injected for cold starts. Asthe engine warms up, the air regulator graduallycloses; when the engine is fully warmed up (above122 F), the regulator closes completely.

    A pressure switch (1968 & 1969 only) andpressure sensor connect to the intake air distributorto sample manifold pressure and determine engineload.

    Fuel System

    Figure 5 is a simplified drawing of the fuelsystem. The fuel pump draws fuel from thefront-mounted fuel tank through a filter and deliversit to 4 injectors. A pressure regulator on the returnline maintains fuel pressure at 28 psi. Excess fuelfrom the regulator returns to the fuel tank.

    The control unit controls the fuel pump througha relay, not shown in Figure 1 or Figure 4. Whenthe ignition switch is turned on, the

  • [All content copyright Clymer Publications, Overland Park Kansas, 1972. Used with permission.)

    PDF version of this document prepared by Greg Merritt ( gregm@vwtype3.org) , 1998.This document may be viewed in HTML format on the World Wide Web at http://www.vwtype3.org/

    1. Fuel tank2. Electric fuel pump3. Fuel filter4. Fuel pressure regulator5. Pressure sensor6. Air intake distributor7. Cylinder head8. Fuel injectors9. Fuel loop line10. Connection for fuel line

    (cold starting)

    11.Ignition distributor with triggercontacts

    12. Electronic control unit13. Throttle valve switch with

    power mixture enrichment15. Cold starting jet16. Electro-magnetic valve

    (cold starting)17. Temperature switch

    (cold starting)18. Auxiliary air regulator

    A and B. Signals from pressure sensorC and D. Signals from trigger contactsE and F. Signals from temperature sensors(warming up phase)G. Signal from throttle valve switch (fuelshut off on overrun)G1. Power mixture enrichment1. Signal from starter (term 50)J. Signal to injectors 1 and 4K. Signal to injectors 2 and 3

  • [All content copyright Clymer Publications, Overland Park Kansas, 1972. Used with permission.)

    PDF version of this document prepared by Greg Merritt ( gregm@vwtype3.org) , 1998.This document may be viewed in HTML format on the World Wide Web at http://www.vwtype3.org/

  • [All content copyright Clymer Publications, Overland Park Kansas, 1972. Used with permission.)

    PDF version of this document prepared by Greg Merritt ( gregm@vwtype3.org) , 1998.This document may be viewed in HTML format on the World Wide Web at http://www.vwtype3.org/

    1. Electric fuelpump8. Injector

    12.Pressure sensor19. Throttle valve switch with

    mixture enrichment20.Ignition distributor with trigger

    contacts

    21. Thermo (temperature) switch22. Control unit23. Temperature sensor in air intake

    distributor24. Temperature sensor in

    cylinder head25. Coldstart valve

    27. From starter terminal 5028. Pump relay29. Main relay30. To ignition terminal 15 (fuse box)31. To terminal 30 (fuse box)32. To battery + terminalT1, T2, and T3: Wire connectors

  • [All content copyright Clymer Publications, Overland Park Kansas, 1972. Used with permission.)

    PDF version of this document prepared by Greg Merritt ( gregm@vwtype3.org) , 1998.This document may be viewed in HTML format on the World Wide Web at http://www.vwtype3.org/

    1. Fuel tank2. Fuel pump3. Filter

    4. Pressure regulator5. Fuel loop line6. Electro-magnetic injectors

    7. Screw for pressure gaugeconnection8. Damper

  • [All content copyright Clymer Publications, Overland Park Kansas, 1972. Used with permission.)

    PDF version of this document prepared by Greg Merritt ( gregm@vwtype3.org) , 1998.This document may be viewed in HTML format on the World Wide Web at http://www.vwtype3.org/

    control unit turns the fuel pump on for 1-2seconds; this prevents flooding the engine if aninjector or the cold start valve is defective. Thecontrol unit turns the fuel pump back on when thestarter is on or the engine is running.

    The control unit also controls theelectromagnetic fuel injectors. The injectors areturned on in pairs. The control unit sends anelectrical pulse to Injectors l and 4 simultaneously,and then another pulse to Injectors 2 and 3simultaneously. On 1968-1971 models, Injectors 1and 3 inject when the respective intake valve isopen, while Injectors 2 and 4 inject when therespective intake valve is closed. On 1972 models,all injection takes place behind closed intakevalves. Fuel is effectively stored in the intake portsof cylinders until the intake valve opens.

    Figure 6 shows a cutaway of an injector. Thecontrol unit sends an electrical pulse to themagnetic winding in the injector. The pulse causesthe needle to move off its seat and inject fuel. Thenthe return spring reseats the needle at the end of thepulse. The time the injector is open varies fromabout 2-10 milliseconds (0.0020.010 seconds).

    The duration of the electrical pulses to theinjectors depends mainly on engine speed andengine load. Special contacts in the ignitiondistributor provide electrical signals to the controlunit indicating the engine speed. The pressuresensor generates electrical signals which indicatepressure (vacuum) in the intake air distributor andtherefore the engine load. The control unitprocesses the two electrical signals and determinesthe duration of electrical pulses to the injectors.

    The rate of acceleration and deceleration have aneffect on 1970-1972 models. When the driverdepresses the accelerator, the throttle valve switchtransmits an electrical signal to the control unitindicating how fast the pedal is being depressed.The control unit uses this information to computehow many extra injections are needed and howlong injection duration should be. When the drivereases up on the accelerator pedal, the throttle valveswitch transmits another signal to the control unit.On 1970 and 1971 models, if engine speed isabove 1,800 rpm, the control unit actually shutsoff all injec-

    1. Filter2. Magnetic winding3. Return spring4. Magnetic armature5. Sealing needle

    tion until the speed drops to about 1,250 rpm. Atthis point, injection begins again, permitting asmooth transition to idle speed if the car continuesto decelerate. Naturally, if the driver depresses theaccelerator below 1,800 rpm and before reaching1,250 rpm, the control unit resumes injectionimmediately. This method of leaning mixtures hasbeen discontinued in favor of an air injectionsystem on 1972 models. Air injection is describedin a later section in this chapter.

    Different means are used to signal enrichmentfor full throttle acceleration. On 1968 and 1969models, a pressure switch connected to the intakeair distributor signals when pressure approachesatmospheric pressure (wide-open

  • [All content copyright Clymer Publications, Overland Park Kansas, 1972. Used with permission.)

    PDF version of this document prepared by Greg Merritt ( gregm@vwtype3.org) , 1998.This document may be viewed in HTML format on the World Wide Web at http://www.vwtype3.org/

    throttle valve). On 1970 and 1971 models, thepressure switch is included in the pressure sensor.In 1972, VW installed a new throttle valve switchwith extra contacts to signal full throttle.

    In addition to engine speed and engine loadsignals, the control unit receives signals fromtemperature sensors. On 1968 and 1969 models,one sensor monitors crankcase temperature, whileanother monitors cylinder head temperature. On1970-1972 models, one sensor monitorstemperature of incoming air while anothermonitors cylinder head temperature as in previousyears. The control unit "computes" the effect thesesensors should have on the injectors. When thesensors detect cold temperatures, the control unitincreases injection duration.

    Cold Start System

    When the engine is cold, but above 5 F(15C) on 1968 and 1969 models, or 50F(10C) on 1970-1972 models, the control un