Porsche Automobile Parts 912e User Manual

TECHNICAL MANUAL  
L-JETRONIC  
FUEL INJECTION SYSTEM  
PORSCHE 912E  
1
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.  
3
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  
Fuel Pump  
Fuel Filter  
Part Number (1)  
0 580 463 010  
0 450 905 001 0 450 905 062  
0 280 160 200  
923 601 111 00  
923 110 176 00  
022 906 035  
Fuel Regulator  
Fuel Injector Valves  
Thermo-time Switch  
Cold Start Valve  
923 606 109 00  
923 605 101 00  
923 606 107 00  
923 606 111 00  
022 906 045A  
022 906 041  
022 906 111 D  
923 618 101 00  
0 280 159 001  
923 615 101 00  
0 280 150 105  
0 280 130 214  
0 280 170 029  
0 280 201 006  
Air Flow Sensor  
Auxiliary Air Regulator  
Temperature Sensor II  
Throttle Valve Switch  
Electronic Control Unit  
Injection Resistor Pack  
Dual Relay  
0 280 140 101  
0 311 906 041A 0 280 130 012  
0 280 120 201  
0 280 000 134  
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.  
4
The following diagram provides an overview of how the fuel injection components  
are integrated.  
AIR FLOW  
METER  
ELECTRONIC CONTROL UNIT  
INJECTORS  
CYL 1  
Injection  
Pulse  
CYL 2  
Amount of Air Draw In  
Temperature of Air Drawn In  
CYL 3  
CYL 4  
THROTTLE  
PUMP and  
Engine Load (WOT)  
SWITCH  
POWER RELAYS  
AUX AIR REGULATOR  
Excess Air for Starting  
IGNITION  
DISTRIBUTOR  
Engine Speed  
STARTER  
IGNITION  
SWITCH  
TEMPERATURE  
SENSOR II  
Engine Temperature  
Excess Fuel for Starting  
Battery  
COLD START  
VALVE  
THERMO-TIME  
SWITCH  
Engine Temperature/Time  
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.  
2.  
3.  
4.  
5.  
6.  
Cold Start  
Warm Up  
Idle During Warm Up  
Load Adaptation  
Temperature Adaptation  
Coasting Adaptation  
5
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  
6
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.  
8
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  
Engine misses  
Engine starts and dies  
Acceleration not smooth  
Rough idle  
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.  
Fuel Pressure or  
Fuel Pump  
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.  
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.  
9
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  
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)  
Cold Start Valve  
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.  
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.  
1. Remove the auxiliary air device and connect an  
ohmmeter to the contacts of the device. Reading  
should indicate 30 ohms.  
Thermo-Time  
Switch  
Auxiliary Air  
Device  
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.  
10  
ENGINE DOES NOT START (continued)  
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.  
Air Flow  
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  
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.  
Air Flow Sensor  
Pump Contacts  
Cold Start  
Valve  
Verify that the cold start valve is not  
leaking and is functioning properly  
ROUGH ENGINE IDLE  
Possible Problem  
Procedure  
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.  
Throttle  
Valve  
2. Remove the throttle valve clamps.  
3. Inspect for bent throttle linkage.  
4. Reset the adjusting screw at the  
throttle valve  
11  
ROUGH ENGINE IDLE (continued)  
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.  
Throttle Valve  
Switch  
a. Pins 18 and 3 - 0 ohms with  
throttle valve wide open  
Set the idle speed to 925 +/- 50 rpms  
with the idle screw on the throttle  
housing. (1)  
Idle Speed  
Setting  
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  
Exhaust Gas  
Mixture  
counterclockwise. Perform this step in  
multiple sequences to get the CO  
readings to an acceptable level.(1)  
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.  
Air Intake  
System  
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.  
Auxiliary Air  
Device  
4. The valve may be visually inspected  
with the engine running by using a  
mirror and a flashlight.  
1. Check the cold start valve for leaks  
by clamping the fuel hose shut and  
determine if the engine starts running  
evenly.  
Cold Start  
Valve  
12  
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.  
Fuel Injection  
Valves  
3. Measure the resistance of the coil of  
each injection valve. Should be 2-3  
ohms.  
Fuel Pressure  
Air Flow  
Verify the fuel pressure is correct.  
Verify the air flow sensor is functioning  
properly.  
Sensor  
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  
Sensor II  
Temperature Range Value  
680F  
140F  
2k – 3k ohms  
7.5k – 12k ohms  
250 – 400 ohms  
1760F  
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  
Throttle Valve  
Switch  
Procedure  
1. Verify function of throttle valve.  
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.  
Air Intake  
System  
Auxiliary Air Device  
Air Flow Sensor  
1. Verify function of auxiliary air device.  
1. Verify function of air flow sensor.  
1. If the concentration is too high,  
adjust the bypass screw in the air flow  
sensor ½ turn counterclockwise and  
measure again.  
Exhaust Gas Mixture  
13  
1. Check for engine hesitations caused  
by loose contacts by moving the wiring  
harnesses and watching for changes in  
the rpms.  
Connectors  
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  
condition.  
2. Verify that there are no voltage  
drops due to high resistance contacts  
in the relays or wiring.  
Voltage  
Supply  
1. Verify the cold start valve functions,  
paying careful attention for leaks when  
running.  
Cold Start Valve  
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.  
Alternator  
and Regulator  
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  
Cold Start Valve  
1. Remove one valve at a time and  
inject fuel into a graduated container.  
2. Measure each injection valve for 30  
seconds.  
Fuel Quantity  
Injection Valves  
3. Compare the results and determine  
if any valves are not providing  
consistent quantities of fuel.  
1. Verify operation of air flow sensor  
1.Connect tachometer and operate  
engine.  
Air Flow Sensor  
Control  
Unit  
2. Move the multiple plug connector  
and tap on the control unit.  
3. Determine if engine hesitates during  
this process.  
14  
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  
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  
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  
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.  
18  
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  
88b  
Side  
88c  
86  
88a  
88z  
86b  
86c  
85  
88y  
86a  
88d  
Fuel Pump &  
Ignition Side  
19  
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  
88b  
12 Volts from Battery. This line is not fused.  
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  
86  
12 volts to Auxiliary Air Valve  
12 Volts to Cold Start Valve, thermo time switch, and  
pin 4 of ECU  
86a  
86c  
85  
12 Volts from Ignition Switch (pin 50)  
12 Volts from Ignition Switch (pin 15)  
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.  
20  
12Vdc from Ignition  
Switch (Pin 15)  
(Red/Black)  
(Purple/Black)  
Tachometer  
12Vdc from Ignition  
Switch  
Electronic Fuel Injection Unit  
27 14 15  
12Vdc from  
Ignition  
Switch  
(Pin 15)  
4
20  
34 10  
6
8
7
9
32  
33 13  
18  
3
2
5
16 17  
1
(Pin 50)  
(Yellow)  
12Vdc from  
Fuse on Rear  
Fuse Box  
(red/black)  
(red)  
(red)  
12Vdc from  
Battery  
Pump Relay  
Contacts  
18  
3
2
88y 86a  
86 86b 86c 88z  
6
7
8
9
36  
Throttle  
Valve  
Switch  
Temperature  
Sensor I  
39  
Air Flow  
Meter  
1
Temperature  
Sensor II  
88a  
15  
88d 88c 85  
88b  
27  
4
Ignition  
Coil  
Injector  
Valves  
Auxiliary Air  
Valve  
Thermo Time  
Switch  
Distributor  
G
Cold Start  
Valve  
Fuel Pump  
6
3
4
1
W
(black/yellow)  
2
Limiting  
Resistors  
Negative  
Terminal on  
Battery  
21  
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.  
Charcoal FIlter  
Anti-Backfire Valve  
Pressure  
Regulator  
EGR  
AIr FLow  
Sensor  
Filter Box  
Auxiliary Air  
Regulator  
To Muffler  
Fan Housing  
22  
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  
18  
1
35  
19  
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  
1
2
FUNCTION  
RPM input from ignition coil  
Throttle switch position – Idle  
NOTES  
Unsure of this pin being  
used.  
3
4
Throttle switch position – Wide Open  
Throttle  
12Vdc from pin 86 of dual relay  
Source is ignition switch  
pin 50, starter operate  
position  
5
Ground  
23  
Can measure between  
pins 6 and 27 of the  
ECU connector to verify  
sensor is functioning  
Can measure between  
pins 7 and 8 to verify  
movement of air flap.  
6
7
Temperature Sensor in Air Flow Sensor  
Air Flow Sensor Input  
8
9
Air Flow Sensor Input  
Air Flow Sensor Input  
Limiting Resistor inside  
air flow sensor can be  
measured between pins  
7 -9 and 8-9  
Present when ignition  
switch is on. Dual relay  
provides path for voltage  
via pins 88z and 88a  
and closed contacts of  
relay.  
10  
12Vdc from Battery  
11  
12  
13  
Not Used  
Not Used  
Input from Temperature Sensor II  
Temperature Range  
Value  
680F 2k – 3k Ω  
140F 7.5k – 12k Ω  
1760F 250 – 400 Ω  
14  
15  
16  
17  
18  
19  
20  
Pulses to Injection Valves  
Pulses to Injection Valves  
Ground  
Ground  
Throttle Switch position Input  
Not Used  
Can measure pump  
relay contacts in Air  
Flow Sensor between  
pins 10 and 20  
Input from Pump Relay contacts  
21  
22  
23  
24  
25  
26  
27  
Not Used  
Not Used  
Not Used  
Not Used  
Not Used  
Not Used  
Can measure between  
pins 6 and 27 of the  
ECU connector to verify  
sensor is functioning  
Temperature Sensor in Air Flow Sensor  
24  
28  
29  
30  
31  
32  
33  
34  
35  
Not Used  
Not Used  
Not Used  
Not Used  
Pulses to Injection Valves  
Pulses to Injection Valves  
Auxiliary Air Valve  
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.  
W G  
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  
6
9
8
7
39  
25  
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.  
NC  
18  
3
26  
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  
Thermo-time Switch  
Cold Start Valve  
2 – 3 ohm coil resistance  
Below 950F = 0 ohms Above 950F = Infinite  
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  
Function  
Terminal Contacts  
Measurements  
Infinite = ∞  
Ohms = Ω  
Air Flow  
Sensor  
ECU  
Plug  
Air Flow plus  
limiting resistor  
Air Flow without  
limiting resistor  
6 to 9  
6 to 9  
200 – 400 Ω  
7 to 8  
8 to 7  
120 –200 Ω  
Temperature  
Sensor in AFS  
6 to 27  
6 to 27  
N/A  
2k at room temp  
Fuel Pump  
Contacts  
36 to 39  
with AFS flap  
closed, 0 with  
AFS flap open  
Auxiliary Air Regulator  
Temperature Sensor II  
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.  
Value  
Temperature Range  
680 F  
140F  
2k – 3k Ω  
7.5k – 12k Ω  
1760F  
250 – 400 Ω  
(Troubleshooting Hint – Disconnect the sender unit wire at the  
disconnect near the distributor and substitute with resistors at specific  
values (2.5K 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 with throttle valve wide open  
27  
The following table provides a quick reference for the Porsche 912E tune-up and  
adjustment parameters.  
TUNE - UP PARAMETERS  
Idle Speed  
875 – 975 rpm  
270 BTDC @ 3,500 rpm  
0.7mm/0.028 in.  
0.4mm/0.016 in.  
440 - 500  
Ignition Timing  
Spark Plug Gap  
Breaker Point Gap  
Dwell Angle  
Valve Clearance  
Intake  
0.15mm/0.006 in.  
0.20mm/0.008 in.  
1-4-3-2  
Exhaust  
Firing Order  
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  
28  

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