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TECHNICAL MANUAL
L-JETRONIC
FUEL INJECTION SYSTEM
PORSCHE 912E
1
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TABLE OF CONTENTS 1.0 Introduction to L-Jetronic Fuel Injection 2.0 L-Jetronic Fuel Injection Theory of Operation 3.0 L-Jetronic Component Identification 4.0 Operating Conditions 5.0 Troubleshooting the Porsche 912E Fuel Injection System 6.0 Component Location 7.0 System Diagrams and Reference Tables 8.0 Bibliography 2
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1.0 L-JETRONIC INTRODUCTION The Bosch L-Jetronic fuel injection system used in the Porsche 912E is often referred to as the Air Flow Controlled or AFC fuel injection system. Originally, Volkswagen adapted the L-Jetronic for the Type 4 air cooled 411 engines sold in the United States due to more stringent emissions and fuel consumption regulations. It was also used in the Porsche 914 with the 1.8L engine and ultimately used in the Porsche 912E during the 1976 model year. The L-Jetronic w
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3.0 PORSCHE 912E L-JETRONIC COMPONENTS The following table provides a quick reference for the fuel injection components used in the 912E. FUEL INJECTION COMPONENTS FOR THE PORSCHE 912E (1) Component Part Number Fuel Pump 923 601 111 00 0 580 463 010 Fuel Filter 923 110 176 00 0 450 905 001 0 450 905 062 Fuel Regulator 022 906 035 0 280 160 200 Fuel Injector Valves 923 606 109 00 0 280 150 105 Thermo-time Switch 923 605 101 00 0 280 130 214 Cold Start Valve 923 606 107 00 0
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The following diagram provides an overview of how the fuel injection components are integrated. AIR FLOW ELECTRONIC CONTROL UNIT INJECTORS METER CYL 1 Injection Pulse CYL 2 Amount of Air Draw In Temperature of Air Drawn In CYL 3 CYL 4 THROTTLE Engine Load (WOT) PUMP and SWITCH POWER RELAYS AUX AIR REGULATOR Excess Air for Starting IGNITION Engine Speed STARTER DISTRIBUTOR IGNITION SWITCH TEMPERATURE Engine Temperature SENSOR II Excess Fuel for Starting Battery COLD START THERMO-TIME VALVE
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COLD START: When the ignition switch is initially turned on during the Cold Start phase, it energizes the dual relay set which provides battery voltage to the control unit and injection valves. When the starter is engaged, the relay set provides battery voltage to the electric fuel pump, the cold start valve, the thermo-time switch, and the auxiliary air regulator. During cold start and all other load conditions, the fuel pressure regulator found in the 912E is able to maintain an equal
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air during the idle period. This additional air is obtained via the auxiliary air regulator that provides a bypass around the throttle valve and allows air to enter directly into the manifold. This bypass route avoids any deflection by the throttle valve. However, the additional intake air that is provided through the auxiliary air device, does get detected by the air-flow sensor, which results in additional fuel being supplied at the fuel injectors. The compensation for the additional
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provides good acceleration response. If sudden acceleration is required during the warm up phase of the engine, the acceleration enrichment provided by the over-swing of the sensor flap may not be sufficient. If this condition is encountered, then the electronic control unit also detects the speed of the deflection of the sensor flap and adjusts the fuel accordingly. ADAPTATION TO TEMPERATURE: Compensation for variations in air temperature must be made. This is due to the fact that
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Failures in the L-Jetronics fuel injection system can be categorized into five major categories. The components associated with the potential failures are identified and measurements are provided which will allow the verification of each component’s operating condition. Please refer to the system diagrams provided in section 7.0 to assist in isolating the problem area. FAILURE CATEGORIES • Engine does not start • Acceleration not smooth • Engine misses • Rough idle • Engine sta
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ENGINE DOES NOT START (continued) Possible Problem Procedure 1. Connect a pressure gauge to the tee found on the driver’s side of the fuel rail. Disconnect the wire from Cold Start Valve terminal 1 of the ignition coil. Operate starter briefly to build up fuel pressure. Disconnect electrical plug to the cold start valve and apply 12 volts to the pin 45 and ground to pin 46 of the cold start valve. The pressure gauge should drop slowly indicating the valve is (1) opening. 2. A
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ENGINE DOES NOT START (continued) 1. Check the air-flow sensor function by removing the connection on the back of the sensor. Connect an Air Flow ohmmeter to the following contacts to verify sensor. Sensor Pins 6 and 9 200 – 400 ohms Pins 7 and 8 120 – 200 ohms Pins 6 and 27 2k Ohms at room temp (Temp Sensor) Pins 36 and 39. With the flap closed the resistance should be infinite. With the flap slightly open it should drop to 0 ohms. This measurement is the fuel
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ROUGH ENGINE IDLE (continued) 1. Take the cover off of the throttle valve switch and verify that the Throttle Valve contacts are open with the throttle at Switch idle position. If throttle is placed in wide open position, then the contacts should make and the following reading should be detected. a. Pins 18 and 3 - 0 ohms with throttle valve wide open Set the idle speed to 925 +/- 50 rpms Idle Speed with the idle screw on the throttle (1) Setting housing. Set the CO readin
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1. Verify the operation of each injection valve by disconnecting the electrical Fuel Injection supply to each injection valve, one at a time and determine if the engine speed Valves drops when the valve is disconnected. 2. If the speed drops, the valve is defective. 3. Measure the resistance of the coil of each injection valve. Should be 2-3 ohms. Fuel Pressure Verify the fuel pressure is correct. Air Flow Verify the air flow sensor is functioning Sensor properly. 1. Measure t
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1. Check for engine hesitations caused by loose contacts by moving the wiring Connectors harnesses and watching for changes in the rpms. and Wiring 2. Check wires 5, 6, 17, and 49 for proper continuity to ground. 1. Inspect and check that the harness plugs for the dual relays are in good Voltage condition. Supply 2. Verify that there are no voltage drops due to high resistance contacts in the relays or wiring. 1. Verify the cold start valve functions, Cold Start Valve paying
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6.0 LOCATION OF COMPONENTS • Cold start valve – Located on the throttle housing underneath the connector tube. There is a fuel line attached to the cold start valve and allows for easily recognition. • Thermo-Time Switch – Located just below the cold start valve underneath the intake pipe on the passenger’s side. It is distinguished by its long copper sensor. 15
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• Pressure regulator – This component is located below the air filter box along the back of the engine tin. Two fuel lines and the vacuum hose attachment allow easy recognition of this component. • Air flow sensor – Attached between the connecting tube and the air cleaner box. • Auxiliary air valve – It is found directly behind the oil filler tube. To remove this component, it is best to unsnap the clamp on the filler tube and remove the top assembly. 16
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• Dual Relay – Located on the bulkhead of the driver’s side of the engine compartment beside the relay plate cover. The voltage regulator and the injection resistors are collocated with this component. • Electronic Control Unit – Can be found on the bulkhead of the passenger side of the engine compartment. Easily recognized by the large connector attached to it. The wiring harness plug is removed for many of the tests described in Section 5.0. 17
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rd • Temperature Sensor II – Located next to the spark plug for the 3 cylinder behind rubber cover. • Throttle Valve Switch – Located on the flywheel side of the throttle valve. The cover is removed in this photograph to illustrate the simple contact switch which senses the wide open throttle (WOT) condition only. • The fuel pump is located underneath the front of the car behind a removable guard. 18
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7.0 SYSTEM DIAGRAMS AND REFERENCE MATERIAL This section provides numerous reference diagrams and measurement tables to assist in the diagnosis of L-Jetronic fuel injection problems. Dual Relay Drawing The following drawing is a diagram of the bottom of the dual relay located on the back left side of engine compartment. The accompanying table provides pin functions and voltage readings for the dual relay. All voltage and resistance measurements can be take either at the componen
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DUAL RELAY TERMINAL PIN FUNCTION TABLE RELAY TERMINAL PIN FUNCTION 88Y 12 Volts from Battery via the regulator and rear fuse panel. This line is not fused. 88d 12 Volts to Fuel Pump. This output to the fuel pump is available only while cranking the engine, or after the engine has started, through the pump relay contacts in the air flow sensor. 88z 12 Volts from Battery. This line is not fused. 88b 12 Volts to Limiting Resistors and Injection Valves, when ignition is on.
