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posticon Bosch L-Jetronic Fuel Injection.


If you got it don't replace it fix it emoticon

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6/5/2003, 11:48 pm Link to this post Send Email to 75westy   Send PM to 75westy
 
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Re: Bosch L-Jetronic Fuel Injection.


Air Flow Meter

On the fuel-injected Volkswagen air-cooled engine, the air flow meter can be found attached to the air cleaner housing.

The air flow meter is nothing more than a fancy variable resistor. There is a flap inside that deflects in relation to how much air is entering the engine. This flap is attached to a wiper arm which wipes across a resistive track. The ECU sends a fixed voltage signal to the air flow meter. As the flap opens, the electrical resistance of the air flow meter increases, giving the ECU a measurement of how much air is entering the engine. The air flow meter can be tested electrically by attaching the leads of an ohmmeter to the proper terminals and measuring the resistance offered by the air flow meter as the flap is moved through its travel. The ECU uses the data supplied by the air flow meter to help determine how much fuel to inject.

The air flow meter also contains a temperature sensor for the intake air. This sensor is a semiconductor that varies resistance with temperature in an inverse relationship; that is, the higher the temperature, the lower the resistance. This information also helps the ECU determine how much fuel to inject, as intake air volume (supplied by the flap) and intake air temperature (supplied by the sensor) can be used to calculate intake air mass, whereupon the computer calculates the correct amount of fuel for that given mass of air. The computer sends the temperature sensor a fixed voltage and measures the resistance, which indicates intake air temperature. As you might expect, the temperature sensor can be tested by attaching an ohmmeter to the proper terminals on the air flow meter and measuring the resistance at various temperatures.

The air flow meter also contains a fuel pump safety switch. The system is designed so that the fuel pump only operates when the starter is cranking or the engine is running. If the vehicle were to be in an accident, for example, and the engine stopped, the air flow meter would shut off the fuel pump to prevent a fire. This is accomplished through a set of contacts. When the air flow meter flap is open any amount, the fuel pump runs and when the flap closes completely, the fuel pump shuts off. Naturally, this can be tested also with an ohmmeter or just by observation. With the ignition on, if you deflect the flap by hand, you should hear the fuel pump activate.

And last but not least, the air flow meter provides the provision for idle mixture adjustment. There is a screw in the housing of the air flow meter that is used as a valve for an air bypass channel. The air flowing through this channel is not measured by the air flow meter and so affects the fuel/air mixture. Turning the screw clockwise blocks off the channel and so richens the mixture. Turning the screw counterclockwise opens the passage and so leans the mixture, as the extra air is not measured by the air flow meter and so the computer does not compensate for it

---
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6/5/2003, 11:49 pm Link to this post Send Email to 75westy   Send PM to 75westy
 
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Re: Bosch L-Jetronic Fuel Injection.


Fuel Pump


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The fuel pump is mounted in various places on the different VWs that have fuel injection. On the Bus, the fuel pump is located underneath the vehicle, near the left rear wheel. On the Vanagon, it is located on the passenger side middle of the van, mounted to the frame. On the Beetle and Super Beetle it is probably mounted at the front under the gas tank. In all models it is a cylindrical device mounted to the car with shock-absorbing mounts and in line with the fuel line from the gas tank.


 
The fuel injection system works at a higher pressure than the carbureted engines do. Therefore, the fuel injection system uses a special high pressure electric fuel pump as opposed to the mechanical pump used on the carbureted engines. The mechanical pump put out 3-5 psi of pressure while the fuel injection system operates at 28 psi.

The pump is a roller-cell pump. The fuel enters and is pressurized by the rotating motion of the roller and the elliptical pressure chamber. It's very similar in operation to a rotary engine. The gasoline then flows through the rest of the pump where it is used to cool the pump motor and then flows out of the pump into the fuel line. The pump also has a pressure limiting valve (basically a spring-loaded stopper) that returns fuel to the tank if pressure in the lines goes over a set limit. There is also a check valve on the output side of the pump which closes when the pump stops to keep the fuel lines pressurized when the engine is not running. This allows quicker restarts and serves to prevent vapor lock.

Vapor lock, for the curious, is what happens when the fuel in the lines evaporates to a vapor instead of being a liquid. Vapors don't pump very well, so vapor lock can prevent an engine from being started. This is usually only a problem in hot weather with carbureted vehicles. The problem is largely non-existent in fuel-injected vehicles.

The fuel pump is initially powered through the double relay when the key is turned to the start position. The pump operates as long as the starter operates. When the starter is turned off, the double relay stops supplying 12v to the pump, but by then the air flow meter fuel pump safety switch has kicked in, allowing the fuel pump to continue running. In the event of an accident where the engine stalls, the fuel pump, thanks to the safety switch in the air flow meter, will stop pumping fuel and the check valve in the pump will close, preventing siphoning from the gas tank.

---
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6/5/2003, 11:50 pm Link to this post Send Email to 75westy   Send PM to 75westy
 
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Re: Bosch L-Jetronic Fuel Injection.


Fuel Filter


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I think the fuel filter is worth mentioning here as a fuel injection component mainly because clean fuel is extremely important to a fuel injection system. Dirty fuel can lead to clogged injectors and all kinds of headaches.


 
The fuel filter is mounted near the pump, in the fuel line upstream of the pump, i.e. the fuel flows through the filter before it flows through the pump. This is important not only to make sure that the pump gets clean fuel but because a filter on the pressure side can be dangerous. If the filter were to become clogged and fuel flow was significantly restricted, theoretically fuel pressure could build to dangerous levels and rupture fuel lines, causing gas leaks and possible fire hazards. In practice, this probably wouldn't happen as the pressure limiter in the fuel pump would prevent it, but engineers don't like to tempt fate and it's considered bad practice to put a fuel filter on the pressure side of the system.

The filter is a somewhat large rectangular plastic box with a filter element designed to catch even very small particles. There is a paper filter element and immediately after that is a strainer. Charles O. Probst, in his Bosch fuel injection book, mentions that the paper filter has a medium pore size of 10 micrometers.

The filters on our beloved Volkswagens have to be replaced every so often. You usually know when you have to replace it by how the van starts bucking and jerking at high rpm, meaning fuel starvation due to a clogged filter. Of course, other things can cause those symptoms, but a clogged filter is always a possibility.

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6/5/2003, 11:52 pm Link to this post Send Email to 75westy   Send PM to 75westy
 
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Re: Bosch L-Jetronic Fuel Injection.


Fuel Pressure Regulator


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To locate the fuel pressure regulator, follow the fuel rail around your engine. The fuel rail is the pipe that carries the fuel to all the injectors and the cold start valve. It comes in on one side of the engine and goes out on the other. The pressure regulator is the last thing the fuel gets to on the way out of the engine compartment.

The fuel pressure regulator is a cylindrical metal object connected to the fuel rail and mounted to the firewall. It also has a vacuum connection to it.


 
The regulator's job is to keep the pressure in the fuel system to a constant 28 psi. It does this with a spring-loaded diaphragm that controls a valve. The valve, when opened by excessive pressure in the fuel lines, uncovers a fuel line that returns excess fuel to the fuel tank.

The vacuum connection is there to help reduce emissions during deceleration. During deceleration, the vacuum connection serves to open the fuel return valve wider, which reduces pressure in the system and prevents excess hydrocarbon emissions due to less fuel being injected as a result of the lower pressure in the system.

This is a very nicely designed device in that it provides for an automatic self-adjusting pressure regulator and emissions control device all rolled into one. The simplicity of the design also contributes to its reliability; the fuel pressure regulator rarely, if ever, goes bad. There's just not much to go wrong with it.

Unfortunately, it's not particularly tuneable either. With Bosch's older FI system, D-Jetronic, the fuel pressure regulator was adjustable. The L-Jetronic pressure regulator is not.



--------------------------------------------------------------------------------

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Re: Bosch L-Jetronic Fuel Injection.


Fuel Injectors


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The fuel injectors (4) are found where the intake air runners connect to the cylinder heads. The injectors are mounted there, and each has a fuel and electrical connection. There are two injectors on each side of the engine.


 
The injectors are the fuel delivery vehicles for the engine, atomizing the fuel for proper and efficient combustion.

The injectors are nothing more than solenoid valves. There is a valve that is held closed by spring pressure until the control unit sends an electrical signal to the injector, which travels through a winding, creating a magnetic field, which lifts the solenoid, allowing fuel to flow through the injector. The tip on the end of the injector helps to atomize the fuel in a cone-shaped spray.

The injectors are supplied with power all the time and are activated when the control unit grounds them. The amount of fuel injected is determined by the control unit through the length of time that the control unit holds the injectors open. The injectors are all activated by the control unit at the same time, not in a sequential manner. They are activated once every crankshaft revolution. The control unit gets this information from a wire that connects to the negative terminal on the ignition coil.

The fuel pressure also affects how much fuel is injected, however the control unit has no control over or knowledge of the fuel pressure. The fuel pressure is maintained by a purely mechanical device, the fuel pressure regulator.

The injectors can be tested for fuel leakage by removing them from the intake manifold (leaving the fuel and electrical connections intact) and operating the fuel pump, measuring how much fuel leaks in a specified time period.

The injectors can be tested for the proper spray pattern by removing them from the intake manifold (leaving the electrical and fuel connections intact) and running the starter briefly, making sure the spray pattern is an even cone shape. A bad spray pattern can cause rough running due to improper fuel atomization.

The electrical connections to the injectors can also be checked with a test lamp to make sure the injectors are getting power.



--------------------------------------------------------------------------------

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6/5/2003, 11:53 pm Link to this post Send Email to 75westy   Send PM to 75westy
 
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Re: Bosch L-Jetronic Fuel Injection.


Series Resistors


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The series resistors are in the fuel injector circuit. They are mounted on the firewall (in Buses and Vanagons) on the driver's side, next to the double relay. It's a metal case surrounding four resistors, one for each injector.


 
These resistors simply limit the current flow to the injectors. I only mention them here in case someone might wonder what they are. They can be tested with an ohmmeter for the proper resistance, and that's about it.

The Bosch book by Charles Probst says that on some other L-Jetronic models these resistors were superseded and the current limiting was built into the injector windings. Later injector windings are brass while the ones used on our beloved vehicles are copper.

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6/5/2003, 11:54 pm Link to this post Send Email to 75westy   Send PM to 75westy
 
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Re: Bosch L-Jetronic Fuel Injection.


Electronic Control Unit


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The electronic control unit, or ECU, is the brain of the fuel injection system. It is a silver-colored box, mounted in various locations on different models. One way to find it is to follow the fuel injection wiring harness as all wires lead to the control unit (terminating in a large plug on the unit). In the Vanagon, the ECU is mounted in the engine compartment on the right side, close to the right taillight assembly. In the Bus, it's mounted in the engine compartment, immediately forward of the battery. In the Beetle and Super Beetle, it's mounted in the rear luggage compartment.


 
The ECU processes all the inputs from the various sensors on board and uses that information to determine the optimum amount of fuel to be injected, which it controls by varying the amount of time the fuel injectors are open.

The ECU cannot be electrically tested. The ECU should only be assumed faulty if absolutely everything else in the ignition system, fuel system, and fuel injection system has proven to be working properly.



--------------------------------------------------------------------------------

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Re: Bosch L-Jetronic Fuel Injection.


Double Relay


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The double relay is mounted to the firewall of the engine compartment on Buses and Vanagons. It is a silver box with two large white plastic connector plugs to it.


 
Here is a good description of how the double relay works, provided by the Bentley manual:

1) When the starter is operated, current from terminal 50 on the starter solenoid supplies positive (+) current to terminal 86a of the double relay, causing one of the relays -- the fuel pump relay -- to close and send battery voltage from terminal 30 on the starter to the electric fuel pump, via double relay terminals 88y and 88d.

2) Because the ignition is turned on when the starter is operating, ignition primary terminal 15 delivers positive (+) current to double relay terminal 86c, thereby closing the power relay that sends positive (+) current from the battery to all electrical parts of the fuel injection system with the exception of the fuel pump, the cold start valve, and the auxiliary air regulator -- which, except for the cold start valve, receive positive (+) current from the pump relay. The power relay remains closed as long as the ignition is on and will not open again until the ignition is turned off.

3) When the engine has started, the driver moves the key from the start (3) position to the running (2) position. This removes the supply of positive (+) current that was reaching double relay terminal 86a from terminal 50 of the starter -- the current that had originally closed the pump relay. However, the pump relay now receives current from the pump switch in the intake air sensor, via terminal 86b and the resistor of the double relay. Whether or not the pump operates depends on whether or not the pump switch in the intake air sensor opens or closes the pump relay.

4) When the driver stops the engine by turning off the ignition, ignition current no longer reaches double relay terminal 86c. The power relay opens, breaking all positive (+) connections between the fuel injection system and the car's electrical system.

The double relay can be tested electrically to make sure the two relays are opening and closing like they are supposed to.

Here is the test procedure for the double relay:

To Test Current Supply:

1) Detach the double relay from the tank compartment panel. Connect one test probe of a test lamp to terminal 85 of the double relay (which is grounded to the negative post of the battery).

2) Apply the second test probe to terminal 88y. Repeat the test by applying the second test probe to terminal 88z. The test lamp should light during both repetitions of the test. If not, positive (+) battery current is not reaching the terminal to which the second probe is connected.

3) With the ignition turned to the running (2) position, apply the second test probe to terminal 86c. If the test lamp fails to light, positive (+) battery current is not reaching terminal 86c from terminal 15 of the ignition coil.

4) While running the starter (ignition switch in the 3 position), again apply the second test probe to terminal 86c. The test lamp should light, indicating positive (+) battery current from the ignition coil.

5) Again running the starter, apply the second test probe to terminal 86a. The test lamp should light, indicating that positive (+) battery current is reaching the double relay from terminal 50 on the starter solenoid.

NOTE: If positive (+) battery current reaches double relay terminal 86a whether the starter is running or not, the wires to terminals 30 and 50 of the starter solenoid have accidentally been reversed. Correct the wire positions. If positive (+) battery current is reaching the double relay correctly, continue with the next test procedure.

To Test Relay Function:

1) Connect one test probe of a test lamp to terminal 85 of the double relay (which is grounded to the negative post of the battery).

2) While running the starter, apply the second test probe to terminal 88d. If the lamp does not light, the pump relay is not closing and the double relay should be replaced.

3) With the ignition turned to the running (2) position, apply the second test probe to double relay terminal 88b. If no positive (+) battery current reaches terminal 88b, the power relay is not closing and the double relay should be replaced.

(Procedure from the Bentley manual)

---
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6/6/2003, 12:11 am Link to this post Send Email to 75westy   Send PM to 75westy
 
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Re: Bosch L-Jetronic Fuel Injection.


Cylinder Head Temperature Sensor


--------------------------------------------------------------------------------

The cylinder head temperature sensor can be found screwed into the cylinder head by the number 3 cylinder. It's simply a little black metal thing with a single wire coming from it.


 
This sensor is exactly like the intake air temperature sensor in the air flow meter. It's a solid-state resistor, called a thermistor. Its resistance varies inversely with temperature. The warmer it is, the lower its resistance.

The ECU sends the sensor a fixed voltage signal and can tell the engine temperature by the resistance it measures. The sensor can be tested with an ohmmeter at various temperatures to see if it is within original specifications. If not, the sensor must be replaced.

This sensor is one of the main inputs used by the ECU to determine how much fuel to inject, so it is important that it be working properly. If the wiring breaks, for example, the ECU will see infinite ohms as the resistance measurement and will go into a "full-rich" mode where the engine will be running extremely rich and get very poor fuel mileage.

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