TECHNICAL MANUAL L-JETRONIC FUEL INJECTION SYSTEM PORSCHE 912E 1
TABLE OF CONTENTS 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.
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.
3.0 PORSCHE 912E L-JETRONIC COMPONENTS The following table provides a quick reference for the fuel injection components used in the 912E.
The following diagram provides an overview of how the fuel injection components are integrated.
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.
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.
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.
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.
ENGINE DOES NOT START (continued) Possible Problem Cold Start Valve Thermo-Time Switch Auxiliary Air Device Procedure 1. Connect a pressure gauge to the tee found on the driver’s side of the fuel rail. Disconnect the wire from 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.
ENGINE DOES NOT START (continued) Air Flow Sensor 1. Check the air-flow sensor function by removing the connection on the back of the sensor. Connect an ohmmeter to the following contacts to verify 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.
ROUGH ENGINE IDLE (continued) Throttle Valve Switch Idle Speed Setting Exhaust Gas Mixture Air Intake System Auxiliary Air Device Cold Start Valve 1. Take the cover off of the throttle valve switch and verify that the contacts are open with the throttle at idle position. If throttle is placed in wide open position, then the contacts should make and the following reading should be detected. a.
Fuel Injection Valves Fuel Pressure Air Flow Sensor Temperature Sensor II 1. Verify the operation of each injection valve by disconnecting the electrical supply to each injection valve, one at a time and determine if the engine speed 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. Verify the fuel pressure is correct. Verify the air flow sensor is functioning properly. 1.
Connectors and Wiring Voltage Supply Cold Start Valve Alternator and Regulator Fuel Quantity Cold Start Valve Fuel Quantity Injection Valves Air Flow Sensor Control Unit 1. Check for engine hesitations caused by loose contacts by moving the wiring harnesses and watching for changes in the rpms. 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 condition. 2.
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.
• 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.
• 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.
• Temperature Sensor II – Located next to the spark plug for the 3rd 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.
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.
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.
12Vdc from Ignition Switch (Pin 15) (Red/Black) Tachometer (Purple/Black) Electronic Fuel Injection Unit 88y 86a 86 86b 86c 88z 36 Auxiliary Air Valve (red/yellow) 6 7 8 (blue) 16 17 1 (blue/purple) 5 (brown) 2 (brown) 3 (brown) 18 (white) 13 (black) 33 (yellow) 32 (white/green) 15 (black/white) (green/white) (white) 14 3 2 9 Throttle Valve Switch Temperature Sensor I Air Flow Meter 1 Temperature Sensor II (brown/white) (red/white) (red/green) 88d 88c 85 88b 8
Vacuum Hose Diagram Besides air leaks around the air filter box and intake hose, vacuum leaks can also lead to poor fuel injection performance. The following diagram is provided to assist in locating the vacuum hoses in the 912E. Please note that the smog pump injection system is not shown.
The following diagrams of connector plugs are provided to assist in the isolation of fuel injection component failures. Specific characteristics of the connector plugs and reference pins have been identified in order to aid the 912E owner in taking voltage and resistance measurements.
6 Temperature Sensor in Air Flow Sensor 7 Air Flow Sensor Input 8 9 Air Flow Sensor Input Air Flow Sensor Input 10 12Vdc from Battery 11 12 13 Not Used Not Used Input from Temperature Sensor II 14 15 16 17 18 19 20 Pulses to Injection Valves Pulses to Injection Valves Ground Ground Throttle Switch position Input Not Used Input from Pump Relay contacts 21 22 23 24 25 26 27 Not Used Not Used Not Used Not Used Not Used Not Used Temperature Sensor in Air Flow Sensor Can measure between pins 6 and
Not Used Not Used Not Used Not Used Pulses to Injection Valves Pulses to Injection Valves Auxiliary Air Valve Ground 28 29 30 31 32 33 34 35 Thermo-Time Switch Connector Plug – This 2 pin plug can be used to verify the operation of the thermal time switch and the cold start valve. There is a small tab on the top of the plug that can be used to assist in identifying the connector pins.
Throttle Valve Switch Connector Plug – This 3 pin connector has only two functional pins, 18 and 3. Pin 2 is no longer used and is not even part of the plug. It is best to verify the operation of the throttle valve switch at the ECU plug due to access constraints.
The following table provides a quick reference for verifying the measurement values for each L-Jetronic fuel injection component. Quick Reference Table - Fuel Injection Component Values Fuel Injector Valves 2 – 3 ohm coil resistance Thermo-time Switch Below 950F = 0 ohms Above 950F = Infinite Cold Start Valve Remove cold start valve from manifold and place in a container to collect the fuel, reconnect electrical plug.
The following table provides a quick reference for the Porsche 912E tune-up and adjustment parameters. TUNE - UP PARAMETERS Idle Speed 875 – 975 rpm Ignition Timing 270 BTDC @ 3,500 rpm Spark Plug Gap 0.7mm/0.028 in. Breaker Point Gap 0.4mm/0.016 in. Dwell Angle 440 - 500 Valve Clearance Intake 0.15mm/0.006 in. Exhaust 0.20mm/0.008 in. Firing Order 1-4-3-2 8.0 BLIOGRAPHY Dr. Ing. H.c. F. Porsche AG 912E Workshop Manual 1975 Print No. 48 14. 21 H.
