Porsche 912E Users Manual TECHNICAL FI

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TECHNICAL MANUAL

L-JETRONIC

FUEL INJECTION SYSTEM

PORSCHE 912E

2

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

3

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

was a further development of the D-Jetronic found in the Porsche 914 1.7L and

2.0L engines. The original L-Jetronic found in the 914 1.8L engine was further

refined which resulted in the system installed in the 912E. The L-Jetronic

principles were later adapted for use in the Porsche 911 series models.

2.0 L-JETRONIC DESIGN APPROACH

The design approach used by Bosch for the L-Jetronic attempted to overcome

some of the limitations found in previous fuel injection systems as well as to meet

the more restrictive emission standards being regulated in the United States.

The design of the L-Jetronic injection system automatically takes into account all

changes in the engine which can occur during the service life of the vehicle

(abrasion, deposits in combustion chamber, changes in valve adjustment, etc.)

Uniform good quality of exhaust gases is therefore assured. In the L-Jetronic,

part of the exhaust gas can be re-circulated to lower the temperature in the

combustion chamber. The air-flow sensor measures only the fresh air drawn into

the engine and the control unit determines the quantity of fuel required only for

the quantity of fresh air. A supplementary mechanism for mixture enrichment

during acceleration is not required because the signal transmitted by the air-flow

sensor precedes charging of the cylinders. In addition, idle stability is improved.

The design of the L-Jetronic allows for fewer compensating adjustments during

the operating cycle of the fuel injection process as compared to the D-Jetronic.

This approach provides a more accurate fuel - air ratio that optimizes

performance under varying requirements. This is due to use of air-flow sensing

which takes direct account of a multitude of factors that influence the fuel

requirements of the engine.

4

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

Component Part Number

(1)

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 0 280 170 029

Air Flow Sensor 923 606 111 00 0 280 201 006

Auxiliary Air Regulator 022 906 045A 0 280 140 101

Temperature Sensor II 022 906 041 0 311 906 041A 0 280 130 012

Throttle Valve Switch 022 906 111 D 0 280 120 201

Electronic Control Unit 923 618 101 00 0 280 000 134

Injection Resistor Pack 0 280 159 001

Dual Relay 923 615 101 00 0 332 514 103/104/120

(1) Part numbers beginning with 923 are Porsche part numbers. Part numbers

beginning with 022 are Volkswagen part numbers. Part numbers beginning with

0, followed by a 3 digit number (0 311 906 041A) are Bosch part numbers.

The components listed in the table can be functionally divided into three major

systems that are integrated to provide the correct injection pulse based on

specific operating conditions. The major systems are the fuel system, sensors,

and control unit. The fuel system provides the gasoline from the tank to the

injection valves, creates the pressure necessary for injection and maintains the

pressure at a constant value. The fuel system includes the pump, filter, pressure

regulator, cold start valve, and injectors. The sensors detect the various

parameters necessary to assure the correct injection pulse duration. The most

important parameter is the quantity of air that is used by the engine. The intake

air flow sensor provides this value as well as the temperature of the air entering

the engine. The throttle valve switch determines the position of the throttle (either

idle or wide open), and the temperature sensor determines the engine operating

temperature. Two additional devices, the thermo-time switch and the auxiliary air

regulator, are used during the starting phase to control fuel enrichment and

amount of air drawn into the engine. All of the parameters generated by the

sensors are then provided to the electronic control unit. The electronic control

unit processes these inputs and the input from the ignition distributor, which

represents engine speed, and provides the correct injector fuel pulse duration.

5

The following diagram provides an overview of how the fuel injection components

are integrated.

4.0 OPERATING CONDITIONS

There are six major operating conditions that the components of the L-Jetronics

fuel injection system must detect and provide inputs to the control unit in order to

provide the correct fuel-air mixture. These operating conditions are:

1. Cold Start

2. Warm Up

3. Idle During Warm Up

4. Load Adaptation

5. Temperature Adaptation

6. Coasting Adaptation

AIR FLOW

METER

ELECTRONIC CONTROL UNIT

CYL 1

CYL 2

CYL 3

CYL 4

AUX AIR REGULATOR

COLD START

VALVE

IGNITION

DISTRIBUTOR

THROTTLE

SWITCH

IGNITION

SWITCH

Battery

PUMP and

POWER RELAYS

INJECTORS

Amount of Air Draw In

Temperature of Air Drawn In

Engine Load (WOT)

Engine Speed

TEMPERATURE

SENSOR II

THERMO-TIME

SWITCH

Engine Temperature

Engine Temperature/Time

Excess Fuel for Starting

Excess Air for Starting

Injection

Pulse

STARTER

6

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 difference in pressure on all the injector valves regardless of load

conditions. This is due to a pressure diaphragm in the fuel regulator that is

controlled by the pressure levels in the intake manifold.

If the engine starts successfully, then battery voltage is maintained to the fuel

pump and auxiliary air regulator through contacts in the air flow sensor. If the

starting effort is not successful, then the battery voltage is removed from the fuel

pump to prevent cylinder flooding.

Additional fuel is provided to the engine during the initial period of the cold start

phase. This is necessary due to condensation of the fuel-air mixture as a result of

the cold engine. This cold start enrichment is based upon temperature

conditions and is accomplished by extending the duration period of the actual

injector valve opening time based on calculations by the electronic control unit.

Under certain temperature conditions, cold start enrichment is supplemented by

directly injecting atomized fuel from the cold start valve into the intake manifold

behind the throttle body. The length of time that cold start valve enrichment

occurs is controlled by the thermo-time switch. Several factors determine how

long the duration is. The length of time the cold start valve provides additional

fuel-air enrichment is dependent on temperature of the engine, outside air

temperature, and the heating element inside the thermo-time switch. Normally,

the cold start valve does not provide additional enrichment when the engine is

warm. The 912E thermo-time switch is designed to stay energized for 8 seconds

at temperatures below 350C or 950F. This is considerably different from

previous L-Jetronic fuel injection systems. In the 914 1.8L L-Jetronic system, the

cold start valve was not utilized at temperatures above 130C or 550F.

WARM UP: Considerable fuel enrichment is required for a period of time after a

cold start. For the first 30 seconds, up to 60% additional fuel will be provided to

facilitate the initial warm up period. The actual percentage varies based upon

the temperature. After the first 30 seconds, only a small amount of enrichment

is being provided. The amount of fuel enrichment is controlled by the

temperature sensor (Temperature Sensor II) installed in the cylinder head. The

electronic control unit calculates the amount of enrichment based upon the

Temperature Sensor II input.

IDLE DURING WARM-UP: When an engine is cold it has more frictional

resistance and requires compensation to overcome this condition. To adjust for

the frictional resistance, the L-Jetronic system is designed to provide additional

7

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 air

provides more air-fuel mixture during warm up. During a cold start, the auxiliary

air device will be wide open. As the engine temperature increases, the size of

the opening decreases and eventually closes altogether. Additionally, the

auxiliary air device has an internal heater that is designed to provide a limited

opening time. The auxiliary air device should remain closed after normal engine

operating temperature is reached due to ambient heat from the engine.

LOAD ADAPTATION: The 912E engine continuously encounters varied engine

loading conditions. These can be broken down into four categories; idle, partial

load, full load, and acceleration. Air flow sensing takes the varied conditions into

account and provides inputs to the electronic control unit that provides the correct

injection duration for the load encountered.

The sensors and the electronic control unit normally control the idle load

condition. The air-flow sensor is designed to allow a small amount of air to

bypass the air flow sensor. The size of this opening can be adjusted to

compensate for lean conditions. This can be accomplished by adjusting the idle-

mixture screw found on the front of the air flow sensor. This adjustment does not

provide additional fuel to the engine, only air. Specific steps for this adjustment

are provided in Section 5.0. The 912E throttle valve switch does not provide an

input to the electronic control unit during the idle load condition as previously

found in the 914 1.8L L-Jetronic Fuel Injection System.

The majority of the time the engine encounters partial load conditions. The

electronic control unit has an internal program for this condition and provides the

correct injection pulse duration when normal driving conditions are detected by

the sensors. If a full load condition is detected, the mixture is enriched to provide

maximum output from the engine. The full load condition is detected by the

throttle valve switch that is connected to the throttle valve shaft. When the

accelerator pedal is fully depressed, a contact in the throttle valve switch is made

and this condition is detected and processed by the electronic control unit.

Sudden acceleration could result in variations in the fuel-air mixture that could

impact performance. Acceleration enrichment is required to avoid fuel-air

mixture problems. The additional fuel above the requirements detected by the air

flow sensor is provided to the engine as a result of the design of the sensor flap

in the air flow sensor. Sudden acceleration causes the sensor flap to swing

beyond its full open position for a short period of time before it returns to the

normal position. This over-swing of the sensor flap is detected by the electronic

control unit and results in an increase the quantity of fuel to the engine and

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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 the density of the air will

affect the efficiency of combustion. Colder air is denser and provides a more

efficient combustion process than warmer air when the same throttle valve

position is used. The air flow sensor has a temperature sensor (Temperature

Sensor I) in the intake which measures the temperature of the air drawn into the

engine. The temperature measurement is provided to the electronic control unit

that compensates for the various temperatures by adjusting the amount of fuel

provided for the combustion process.

COASTING ADAPTATION: During normal driving conditions, the operator will

often lift his foot completely off of the accelerator pedal and coast. The electronic

control unit detects this operating condition and the result is that the injector

valves will be closed above a certain speed and temperature. If a load condition

is sensed again, or if the speed sinks below a preset value, then the injection

process returns to normal operation.

5.0 TROUBLESHOOTING THE PORSCHE 912E FUEL INJECTION

This section is designed to assist the Porsche 912E owner in locating faults in

the L-Jetronic system, isolating the component involved and testing the

component for correct function. The tools required will include a fuel pressure

gauge and fittings, a tachometer, a CO meter and volt-ohmmeter. The following

safety and maintenance tips are provided:

SAFETY/MAINTENANCE TIPS

Never jump the battery to start the car.

Never start the engine without battery cables firmly connected.

Always remove cables from battery before charging.

Never remove cables from battery with engine running.

Never remove or attach wiring harness plug to Control Unit with the ignition on or

with battery connected.

When turning the engine over to check compression, unplug the red cable from

the battery to the relays. Remember, there is not a fuse on this line

Before testing the L-Jetronic system, make sure the timing, dwell and spark plug

gaps are within specification.

Make sure all vacuum hoses and fuel hoses are connected and in good working

order.

Check for air leaks in associated with the air filter and connecting hose.

9

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 starts and dies

ENGINE DOES NOT START

Possible Problem Procedure

1. Check the fuel pressure by connecting a pressure

gauge with an adapter to the tee found on the driver’s

side of the fuel rail.

2. Disconnect the vacuum hose between the air

distributor and pressure regulator.

3. Turn ignition switch to the on position and open the

air flow sensor valve slightly to start the fuel pump.

4. Fuel pressure should read 35 +/- 1.4 psi with vacuum

hose disconnected. If reading is not correct, proceed

with step 5.

5. Turn the ignition switch to the start position and listen

for the fuel pump. If pump can be heard, then check for

blockage in the fuel filter and fuel line. If pump does not

operate or cannot hear the pump, then verify fuel filter is

good and proceed with step 6.

6. Unscrew the dual relay set and measure for 12 volts

at pin 88d while cranking the engine. If voltage is not

present, proceed with step 7. If voltage is present, then

verify the continuity between pin 88d and the fuel pump,

or verify voltage is present at the fuel pump.

7. Verify that 12 volts is present on the double relay at

relay terminals 88y at all times and at 86a while

cranking the engine. If voltage is not present at 86a

while cranking the engine, then go to stop 9.

8. If voltage is not present at 88y, then check for

continuity between regulator, one side of the fuses (S23,

S24) in the rear relay panel and then to pin 88y on the

dual relay. If voltage is present at 88y, go to step 9.

Fuel Pressure or

Fuel Pump

9. Verify continuity between pin 50 of the ignition switch

and pin 88a of the dual relay. If continuity is good, then

the ignition switch may be bad.

10

ENGINE DOES NOT START (continued)

Possible Problem Procedure

Cold Start Valve

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. The pressure

gauge should drop slowly indicating the valve is

opening. (1)

2. An alternate method for verifying the operation of the

cold start valve would be to remove it from the manifold

and operate the starter and allow the fuel to spray into

a container. It should spray for a maximum of 20

seconds. In order to accomplish this the temperature

must be below 500F or the thermo-time switch must be

disconnected and voltage applied to pin 45 with pin 46

grounded.

3. Verify that there is continuity between pin 45 of the

Cold Start Valve and pin G of the thermo-time switch,

and pin 46 of the Cold Start Valve and pin W of the

thermo-time switch.]

4. Measure resistance of coil in start valve – 4 ohms.

Thermo-Time

Switch

1. Remove the connector from the cold start valve.

Connect an ohmmeter between both contacts of the

connector. The engine temperature should be below

950F. The measurement should indicate continuity (0

ohms) with a cold engine. At engine temperatures

above 950F, the ohmmeter should indicate open.

Auxiliary Air

Device

1. Remove the auxiliary air device and connect an

ohmmeter to the contacts of the device. Reading

should indicate 30 ohms.

2. Inspect the valve in the device and determine that is

wide open when in a cold condition. Apply 12 volts to

the auxiliary air device and allow the heater unit to

function. As the unit heats up, the valve should slowly

close.

11

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. This measurement is the fuel pump

contacts that engage after engine start to keep the

pump running.

Air Intake

System

1. The air intake system must be free of leaks. Check

the intake manifold, hoses and associate components

with soapy water to determine if leaks are occurring.

Note (1): This will not verify that the spray nozzle is atomizing the fuel.

ENGINE STARTS BUT THEN DIES

Possible Problem Procedure

Air Flow Sensor

Pump Contacts

Verify that the resistance between pins

36 and 39 of the Air Flow Sensor is

infinite when the flap is closed and

when the flap is opened, the resistance

goes to 0 ohms.

Cold Start

Valve

Verify that the cold start valve is not

leaking and is functioning properly

ROUGH ENGINE IDLE

Possible Problem Procedure

Throttle

Valve

1. Verify that the throttle valve is closed

and determine if it can be closed

further. If it can be closed further, then

listen for the engine speed to increase

and decrease.

2. Remove the throttle valve clamps.

3. Inspect for bent throttle linkage.

4. Reset the adjusting screw at the

throttle valve

12

ROUGH ENGINE IDLE (continued)

Throttle Valve

Switch

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. Pins 18 and 3 - 0 ohms with

throttle valve wide open

Idle Speed

Setting

Set the idle speed to 925 +/- 50 rpms

with the idle screw on the throttle

housing. (1)

Exhaust Gas

Mixture

Set the CO readings to 0.5% to 1.2%

with a CO meter. If the concentration is

too high, then turn the bypass screw in

the air flow sensor ½ turn

counterclockwise. Perform this step in

multiple sequences to get the CO

readings to an acceptable level.(1)

Air Intake

System

1. The air intake system must be free

of leaks. Check the intake manifold,

hoses and associate components with

soapy water to determine if leaks are

occurring.

Auxiliary Air

Device

1. Verify that the auxiliary air device is

closed when the engine reaches its

normal operating temperature.

2. Clamp one of the hoses coming out

of the auxiliary air device closed and

determine if the engine speed dropped.

3. If the rpms dropped, then the

auxiliary air device is not closing

completely.

4. The valve may be visually inspected

with the engine running by using a

mirror and a flashlight.

Cold Start

Valve

1. Check the cold start valve for leaks

by clamping the fuel hose shut and

determine if the engine starts running

evenly.

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Fuel Injection

Valves

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.

Fuel Pressure Verify the fuel pressure is correct.

Air Flow

Sensor

Verify the air flow sensor is functioning

properly.

Temperature

Sensor II

1. Measure the output of the

temperature sender unit under various

conditions. Following values should be

used for reference purposes, but the

value may be different due to the age

of the sensor.

Temperature Range Value

680F 2k – 3k ohms

140F 7.5k – 12k ohms

1760F 250 – 400 ohms

Note (1): Activated charcoal filter hose must be disconnected at air cleaner, the

air injection hose must be removed and the check valve must be plugged. The

adjustments should be made as quickly as possible to prevent excessive heat in

intake lines.

ACCELERATION NOT SMOOTH

Possible Problem Procedure

Throttle Valve

Switch

1. Verify function of throttle valve.

Air Intake

System

1. The air intake system must be free

of leaks. Check the intake manifold,

hoses and associate components with

soapy water to determine if leaks are

occurring.

Auxiliary Air Device 1. Verify function of auxiliary air device.

Air Flow Sensor 1. Verify function of air flow sensor.

Exhaust Gas Mixture

1. If the concentration is too high,

adjust the bypass screw in the air flow

sensor ½ turn counterclockwise and

measure again.

14

Connectors

and Wiring

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.

Voltage

Supply

1. Inspect and check that the harness

plugs for the dual relays are in good

condition.

2. Verify that there are no voltage

drops due to high resistance contacts

in the relays or wiring.

Cold Start Valve

1. Verify the cold start valve functions,

paying careful attention for leaks when

running.

Alternator

and Regulator

1. With the engine stopped, remove the

plug from the alternator. Start the

engine. If this eliminates the engine

misses, check the alternator and

regulator.

Fuel Quantity

Cold Start Valve

1. Detach the fuel line at the cold start

valve and hold the hose inside a 5 liter

container that has a graduated scale.

2. With the ignition switch in the on

position, operate the air flow sensor

flap by hand and start the fuel pump.

3. After 1 minute, the fuel quantity

should measure 1.5 – 2 liters.

4. If the quantity is not correct, check

for clogged fuel filter, pinched or

clogged lines, pressure regulator, and

fuel pump.

Fuel Quantity

Injection Valves

1. Remove one valve at a time and

inject fuel into a graduated container.

2. Measure each injection valve for 30

seconds.

3. Compare the results and determine

if any valves are not providing

consistent quantities of fuel.

Air Flow Sensor 1. Verify operation of air flow sensor

Control

Unit

1.Connect tachometer and operate

engine.

2. Move the multiple plug connector

and tap on the control unit.

3. Determine if engine hesitates during

this process.

15

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.

16

• 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.

17

• 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.

18

• 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.

19

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 component, or at a

convenient pin on an associated plug along the wiring harness. The right side of

the drawing is located adjacent to the bulkhead of the left rear engine

compartment and is attached by a bolt. Please note that on the relay itself, the

contact pins are different in size, which the drawing depicts.

Fuel Injection

Side

Fuel Pump &

Ignition Side

88b

88c

88a 86b86

88z86c 85

88y

88d

86a

20

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.

88a 12 Volts to pin 39 (pump relay contacts) of Air Flow

Sensor and 12 Volts to pin 10 of ECU. This is present

only when 12 Volts is present at pin 86c, which comes

from the ignition switch.

88c 12 volts to Auxiliary Air Valve

86 12 Volts to Cold Start Valve, thermo time switch, and

pin 4 of ECU

86a 12 Volts from Ignition Switch (pin 50)

86c 12 Volts from Ignition Switch (pin 15)

85 Ground

85a Not Used

L-Jetronic System Diagram The next diagram is a schematic drawing of the

complete L-Jetronics fuel injection system. This diagram was adapted from the

Porsche 912E Workshop Manual and has been enhanced to provide additional

information to assist in isolating fuel injection problems.

21

Electronic Fuel Injection Unit

420 3410 6 78 9 27 14 15 32 33 13 18 3 2 5 16 17 1

1

Ignition

Coil

Distributor

218 3

Throttle

Valve

Switch

Temperature

Sensor II

Injector

Valves

1436

2

Temperature

Sensor I Air Flow

Meter

27

7689

39

36

Pump Relay

Contacts

88b

88a88c 88b

G

W

Cold Start

Valve

86a 8688y

Auxiliary Air

Valve

Fuel Pump

86b 86c 88z

88d 85

12Vdc from

Battery

12Vdc from

Fuse on Rear

Fuse Box

12Vdc from

Ignition

Switch

(Pin 50)

12Vdc from Ignition

Switch

(Pin 15)

(red/black)

12Vdc from Ignition

Switch (Pin 15)

Tachometer

Negative

Terminal on

Battery

(red)

(red)

(Yellow)

(Purple/Black)

(Red/Black)

(brown/white)

(brown/white)

(green/blue)

(red/blue)

(red/yellow)

(red/yellow)

(black/yellow)

(green)

(green/white)

(greenyellow)

(green/black)

(green/red)

(white)

(red/white)

(black/white)

(white/green)

Limiting

Resistors

(yellow)

(black)

(white)

(blue)

(brown)

(brown)

(brown)

(blue/purple)

Thermo Time

Switch

(red/white)

(red/green)

4

15

22

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.

Fan Housing

Filter Box

Charcoal FIlter

To Muffler

EGR

Anti-Backfire Valve

Pressure

Regulator

Auxiliary Air

Regulator

AIr FLow

Sensor

23

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. It is most often easier to isolate

the fuel injection problem by using an access point such as a related connector

plug on another component than by attempting to remove the connector on the

suspect fuel injection component, and then trying to take measurements on the

component itself.

Electronic Control Unit Connector Plug – This 35 pin plug has a tab on the right

hand side of the plug that can be used as a reference for Pin 1. Please note that

the top row of the plug has more pins and is longer than the bottom row. The

following table may assist in using the Electronic Control Unit connector plug for

measurement purposes.

Electronic Control Unit Connector Pin Functions

PIN NUMBER FUNCTION NOTES

1 RPM input from ignition coil

2 Throttle switch position – Idle Unsure of this pin being

used.

3 Throttle switch position – Wide Open

Throttle

4 12Vdc from pin 86 of dual relay Source is ignition switch

pin 50, starter operate

position

5 Ground

1

19

18

35

24

6 Temperature Sensor in Air Flow Sensor Can measure between

pins 6 and 27 of the

ECU connector to verify

sensor is functioning

7 Air Flow Sensor Input Can measure between

pins 7 and 8 to verify

movement of air flap.

8 Air Flow Sensor Input

9 Air Flow Sensor Input Limiting Resistor inside

air flow sensor can be

measured between pins

7 -9 and 8-9

10 12Vdc from Battery Present when ignition

switch is on. Dual relay

provides path for voltage

via pins 88z and 88a

and closed contacts of

relay.

11 Not Used

12 Not Used

13 Input from Temperature Sensor II

Temperature Range

Value

680F 2k – 3k Ω

140F 7.5k – 12k Ω

1760F 250 – 400 Ω

14 Pulses to Injection Valves

15 Pulses to Injection Valves

16 Ground

17 Ground

18 Throttle Switch position Input

19 Not Used

20 Input from Pump Relay contacts Can measure pump

relay contacts in Air

Flow Sensor between

pins 10 and 20

21 Not Used

22 Not Used

23 Not Used

24 Not Used

25 Not Used

26 Not Used

27 Temperature Sensor in Air Flow Sensor Can measure between

pins 6 and 27 of the

ECU connector to verify

sensor is functioning

25

28 Not Used

29 Not Used

30 Not Used

31 Not Used

32 Pulses to Injection Valves

33 Pulses to Injection Valves

34 Auxiliary Air Valve

35 Ground

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.

By disconnecting this plug from the thermo- time switch and applying 12Vdc to

pin G and ground to pin W, the cold start valve will energize and provide

atomized fuel to the intake manifold – providing the fuel system is pressurized.

Air Flow Sensor Connector Plug – This 7 pin connector plug can be used to

verify the operation of the air flow sensor as well as verify the continuity of the L-

Jetronic wiring harness between the air flow sensor and the electronic control

unit and the dual relays. Remember, when checking the air flow sensor itself, the

pins on the sensor will be a mirror image of the plug (39 will be on your left).

27 36 393968793627

WG

26

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.

3NC 18

27

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.

Disconnect plug from thermo-time switch and apply

12Vdc to pin W and ground to pin G of the connector.

Pressurize the fuel system and observe fuel exiting the

cold start valve.

Air Flow Sensor

Auxiliary Air Regulator Pins of Aux Air Reg should read 30 Ω. Alternate check

of contacts would be to read from 88c of dual relay

and pin 35 of ECU plug. Aux Air Reg should be open

when cold. Apply 12 Vdc to pins and it should close

completely.

Temperature Sensor II

(Troubleshooting Hint – Disconnect the sender unit wire at the

disconnect near the distributor and substitute with resistors at specific

values (2.5K Ω when temp is 700F) and see if problem goes away.

Ground one side of resistor to engine block.)

Throttle Valve Switch Pins 18 to 3 - 0 Ω with throttle valve wide open

Function Terminal Contacts Measurements

Air Flow

Sensor

ECU

Plug

Infinite = ∞

Ohms = Ω

Air Flow plus

limiting resistor

6 to 9 6 to 9 200 – 400 Ω

Air Flow without

limiting resistor

7 to 8 8 to 7 120 –200 Ω

Temperature

Sensor in AFS

6 to 27 6 to 27 2k Ω at room temp

Fuel Pump

Contacts

36 to 39 N/A ∞ with AFS flap

closed, 0 Ω with

AFS flap open

Value Temperature Range

2k – 3k Ω 680 F

7.5k – 12k Ω 140F

250 – 400 Ω 1760F

28

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. Bauer

Bosch Electronic Gasoline Fuel Injection System with Lambda Closed-Loop

Control

L-Jetronic Technical Instruction

1987

G. Glocker and B. Kraus

L-Jetronic Gasoline Fuel Injection System

Bosch Technical Report 5

1975