The source of the majority of the information contained in this FAQ has been extracted from various technical sources, discussions with local mechanics, and postings to The Saab Network. Since so any people have contributed to this topic in The Saab Network Archives, it proved impractical to give credit to all the appropriate individuals. This is in NO way the definitive answer on how to perform the procedure. Rather, it is the summation of several different sources of information in an attempt to provide a resource for people to draw upon when attempting to perform a repair themselves. The suggestions given are intended to enhance your general understanding of your car's operation. The repairs suggested and the methods described, though they may have worked for others, may or may not work for your car due to variations in models, equipment, and other unknown factors. When all else fails, you can take your car to your trusted local SAAB mechanic and have them do the repair for you.
Neither I nor Applied Materials, Inc. assumes responsibility for the accuracy of the information contained here.
Having said all that
I have found that fixing the fuel injection system is not some sort of black art, but a logical progression of mechanical and electrical checks, each of which produces more information to tell you what is right and what isn't. To that end, if you approach any job on your Saab in a logical way and understand how the systems function, you can understand what is malfunctioning and avoid the shotgun approach to car repair (which can end up costing the shade tree mechanic more than if the car was taken to a qualified Saab mechanic in the first place).
Overview of L/LH-Jetronic Fuel Injection Systems
The Bosch L-Jetronic and LH-Jetronic (Luftmassenmesser Hitzdraht) is a pulsed injection type system. In pulsed injection, all air entering the engine first flows through an air-mass meter. The air-mass meter measures the air, which indicates engine load, and converts that measurement into an electrical signal to the control unit (ECU). The LH-Jetronic is a fuel injection system, not an engine management system. While most L-Jetronic cars have electronic ignition, they do not have electronic management of ignition timing.
Beginning in about 1980 (earlier in
The deceleration function was moved to the ECU in the 2.4 system. The dashpot was designed to provide a controlled deceleration to 875 rpm in 3 to 6 seconds from an engine rpm of approximately 3,000.
The two engine conditions which make a primary contribution to the basic flow rate are load and speed. Additionally, compensation variables (such as starting, cold operation, and special load conditions) and precision compensation variables (tweaking the other compensations for overrun or coasting and mixture control for emission reduction) are added to, or subtracted from the basic flow rate to achieve smooth operation. Cold start can pose a special problem because the fuel does not properly vaporize and even if it is adequately vaporized, some fuel condenses on the cold parts of the engine before it can be burned. In cold start conditions, the engine requires extra fuel for starting so that, in spite of vaporization and condensation problems, the engine still receives a combustible air-fuel mixture.
In measuring the volume of the air passing through the intake manifold into the engine, the air-mass meter provides a measure of engine load. Engine speed is measured in the tachometer by counting the ignition pulses in the primary circuit. The LH-Jetronic rpm signal comes from the primary ignition pulse, the current that turns the coil on and off. For most cars that is a signal which comes from the negative terminal of the ignition coil.
The ECU receives information from the following sensors:
Ignition System and Ignition Timing
All the testing should also be based on the fact that the other systems, including the ignition, are in proper working order (after all, these are relatively inexpensive fixes). The plugs should be gapped at 0.024'-0.028' (0.6-0.7 mm) and tightened to a torque of 20.7 ft-lbs. (28 NM). The plug wires should be checked to assure that resistance is within 2,000 to 4,000 ohms. Inspect the distributor cap and replace it if the contacts are burnt or tracer lines are apparent. Measure rotor resistance (1,000 ohms) and if it is out of range or the contact is burnt, replace it by breaking off the old rotor and pressing the new one in place with the appropriate glue (Loc Tite seems to work well).
Any testing of the ignition system would be incomplete without also checking the timing. Since there are different settings for different models, markets, and years, you should be able to find the correct timing for your car by checking the sticker on the fender. Ignition timing is measured at the instant the spark plugs fire, in terms of degrees of crankshaft rotation relative to the piston on its compression stroke and before or after top dead center. When the engine is idling, the spark is timed to occur just before the piston reaches top dead center so that combustion can be completed by the time the piston reaches a little past top dead center. At higher engine speeds, there is less time for the air/fuel mixture to ignite, burn, and deliver its power to the piston. Consequently, at higher engine speeds, the spark must be delivered earlier in the cycle.
To check your timing, the first thing you are going to need is a strobe timing light. The induction pick-up should be attached to the number one plug wire. On the 900, the number one cylinder is at the firewall end of the engine and on the 9000, it is opposite of the distributor. If you still can't figure it out, pull the spark plug inspection plate and read the cylinder number stamped by each plug on the head.
When pistons number one and number four are at top dead center, the crankshaft (i.e., the '0' mark on the flywheel) will be in line with the mark on the clutch cover or end plate if the clutch cover has been removed. Disconnect the vacuum control hose at the vacuum chamber on the distributor and plug the hose (this allows for a true base timing reading). Loosen the distributor tightening bracket bolt. Focus the light of the timing light strobe on the flywheel. The rotation of the flywheel and the pattern of the strobe light will indicate the current timing setting. If your timing light has an advance setting, you will target a '0' mark on the flywheel. If you do not have an advance, you will target a setting equivalent to your car's specific timing advance setting. The mark on the flywheel should hold steady (within 1-2 degrees) and not bounce around. If it does bounce around, this is an indication that something is wrong (possibly the distributor). You should also test that the vacuum advance is working by applying vacuum to the distributor.
Under light engine load conditions, there is a high vacuum in the intake manifold caused by the restriction of the partially closed throttle valve. Consequently, there is a smaller amount of air/fuel mixture delivered to the combustion chamber. Because of the lower ratio of fuel to air, the mixture will not burn as rapidly; therefore, ignition must take place early in the cycle.
To provide additional spark advance control based on intake manifold pressures, a vacuum advance mechanism is incorporated in the distributor. It contains a spring-loaded diaphragm which rotates the breaker plate assembly in the distributor. The vacuum diaphragm is connected to the intake manifold. When the engine is at idle, intake manifold vacuum is high and the ignition timing is advanced. When the engine is accelerated, intake manifold vacuum drops, retarding the ignition timing of spark plug firing.
Engine Temperature / NTC Sensor
One of the least known sensors is the engine temperature sensor (aka NTC II). It is a semiconductor resistor, also know as a thermistor, and the NTC stands for Negative Temperature Coefficient. This means that the sensor's resistance goes down as the temperature goes up. If the ECU applies a fixed voltage to the NTC resistor, it will receive a smaller signal back as input from a cold resistor with higher resistance than from a warm one. This sensor is screwed into the water passage on the intake side of the head between intake runner #2 and #3 on the 900s and just under runner # 2 on the 9000s. Test the sensor by probing the two contact points with a multimeter to measure the resistance which should be:
Throttle Body and Switch
A dirty or gummed up throttle body can contribute to an erratic idle situation. This can easily be cleaned once or twice a year. Cleaning it only requires that you remove the intake hose to the throttle body and move it out of the way. To prevent any unwanted items from falling in the intake hose, it should be blocked with a clean rag. Using a can of carburetor cleaner, open the throttle butterfly with one hand and spray cleaner just beyond it. Wipe out any gunk that is preventing the butterfly from operating properly.
The throttle position switch provides the ECU with information on the position of the throttle: idling speed, partial throttle, or wide open throttle. In the LH 2.4.2, the position switch is replaced by a throttle position sensor. The switch has three pins of which the center provides the ground connection. When the throttle is closed (idling speed or engine braking), the microswitch closes and ground is obtained from the idling speed connection/pin. When the throttle is activated, the microswitch opens and the ground from the idling speed connection/pin is lost. When the throttle angle is greater than 72 degrees, the wide open throttle switch closes and ground is obtained from the full-load pin. After an ECU calculation is performed, full-load enrichment is provided to the fuel-air mixture. (See the FAQ on Adjusting Basic Idle)
If the accelerator is floored when the engine is started at temperatures below -4F (-20C), the ECU issues an order to reduce the amount of fuel injected into the engine. This prevents the engine from becoming flooded. In the LH 2.4, 2.4.1, and 2.4.2 only, repeated attempts to start the engine will cause the enrichment cycle to not be activated. This prevents exceptionally generous enrichment and a consequently flooded engine.
Idle Air Control Valve
From model year 1986 and later, the auxiliary air valve is replaced by an idle air control valve (AIC) which also compensates for momentary load increases at idling speed. The AIC is bolted to a bracket mounted on the intake manifold. The valve allows a controlled flow of air to bypass the throttle butterfly. The volume of the air is determined by the degree of opening of the idle control valve. The AIC consists of a slide valve with a reversible motor. The motor has two windings and maintains a continuous reciprocating action, turning the slide through a maximum angle of 90 degrees. The motor receives signals from the ECU.
Due to the AIC motor's continuous, limited-travel reciprocating action (noticeable only as vibration), the opening of the valve can be varied within extremely short periods of time (opening/closing in about 150-200 milliseconds). This permits the air flowing through the valve to be controlled at all times so that the volume necessary for obtaining the desired constant or increased idle speed can be achieved as required.
The AIC for the LH 2.4, 2.4.1, and 2.4.2 differs from the LH 2.2 in that the reversible motor is replaced by a solenoid. The coil receives pulse signals from the ECU. When activated, the coil overcomes a mechanical spring that keeps the valve closed. This allows a specific volume of air to flow past the throttle butterfly in the intake manifold. In the event of a fault, the opening pulses terminate and the spring pulls the valve to the end position. The air flow through the valve in this position (Limp Home) is greater than during normal operating conditions. This gives an idle speed of 1,200 to 1,500 rpm.
Air Mass Meter
The air mass meter is usually mounted some distance from the intake valves rather than directly on the engine manifold. Any air that enters the intake system between the meter and the valves is UNMEASURED AIR. Therefore, the engine gets no fuel to match that air. The result can be lean mixtures that can cause hard starting, rough idle, low CO, and stumbling. This is why people always say to check for any vacuum leaks first as this is relatively easy compared to some of the other test procedures and you may find your problem right away. Additionally, turbos have a bad habit of blowing these hose connections apart under high boost.
The air mass meter depends on the measurement of current flowing through heated wires to measure air flow. It is also known as the hot-wire sensor because of its heated wire design, hence the 'H' in LH. In the unlikely event that a wire should break, the warm engine runs, though without fuel compensation, in a 'Limp-Home' mode. For 'Limp-Home' operation, injector pulse time is fixed. For any rpm above idle, the ECU is programmed to deliver fixed pulses, typically 7.5 milliseconds.
The air mass meter works by measuring the air mass, or weight, so it requires no correction for changes in density due to temperature or altitude. The hot wire system depends on the measurement of the cooling effect of the intake air moving across the heated wires. With a small movement of air past the heated wires, the cooling effect is small. With more air moving past the heated wires, the cooling effect is greater. LH control circuits use this effect to measure how much air passes the LH hot wire.
The LH hot wire is found within the air passage tube of the air mass meter and is made of a platinum filament. The hot wire is heated to a specific temperature differential above the incoming air when the ignition is turned on (the differential is measured in degrees Celsius).
As soon as the air flows over the wire, the wire is cooled. The control circuits then apply more voltage to keep the wire at the original temperature differential (100 degrees C). For example, if the air is at freezing, 32F (0C), the wire will be heated to 212F (100C). On a hot day, if the air is at 86F (30C), the control circuits heats the wire to the same 212F (100C) difference to 266F (130C). This creates a voltage signal which the ECU monitors - the greater the air flow and wire cooling, the greater the signal. These signals are transmitted from the air mass meter to the ECU as follows:
Since the filament is located in the inlet duct, it becomes dirty and loses its sensitivity over time, which affects measurement accuracy. To ensure a clean sensor, the control system will heat the wire red hot for about one second, hot enough to burn-off any dirt. However, there is no burn-off unless the engine has run above 3000 rpm and attained a engine temperature of 150F (65.6C). This burn-off function takes place 20 seconds (4 seconds for the LH 2.2) after the engine has been switched off.
Vacuum and Related Sensors
Vacuum leaks can be caused by any loose clamp or gasket, or by any slit in the flexible intake hose or vacuum hoses. The vacuum hoses are the small black hoses running from various ports on the manifold to points on the engine such as the fuel pressure regulator, the crankcase vent fitting and/or valve, the distributor vacuum control, heater control vacuum tank, the fuel evaporation control canister, the 'hooter' valve, etc. The crankcase ventilation system evacuates crankcase gases through the throttle body.
The 'hooter' valve is notorious for leaking vacuum, however, it is a very easy item to test. Test it by connecting a vacuum hose to the nipple and draw a vacuum. If it holds, you are good. If it looses vacuum, it needs to be replaced. The purpose of the 'hooter' valve or officially known as the turbo bypass value is to shunt excess turbo boost pressure when the throttle is suddenly closed. The 'hooter' valve can upset idle by allowing bypass air into the intake manifold and interfering with vacuum at idle.
Oxygen / Lambda Sensor
The lambda sensor (aka oxygen sensor) is essentially a small battery that generates a voltage signal based on the differential between the oxygen content of the exhaust gas and the oxygen content of the ambient air. The tip of the sensor that protrudes into the exhaust gas is hollow, so that the interior of the tip can be exposed to the ambient air. Both sides of the ceramic tip of the sensor are covered with metal electrodes that react to create a voltage only if the ambient air has a higher oxygen content than the exhaust and the ceramic material is hotter than 575F (300C). The electrolyte consists of a ceramic material, zirconium oxide, which has been temperature stabilized through the addition of a small amount of yttrium oxide. The electrolyte is in tubular form with one of the ends blanked off. The surface has been coated with platinum to make it electrically conductive.
The voltage is usually about 1 volt, but if the engine is running lean, the exhaust gas has about the same amount of oxygen as the ambient air and the sensor will generate little or no voltage. If the engine is running rich, the oxygen content of the exhaust will be much lower than the ambient air and the sensor voltage will be larger. Sometimes the voltage reading is disturbed by a poor ground situation due to corrosion. To test for a poor ground, connect a 12V bulb to the oxygen sensor body and the battery positive. The bulb should light up indicating current flow and a good ground. If it does not light up, remove the oxygen sensor and clean the threads. Alternatively, you can run an extra ground lead from the oxygen sensor's outer casing (secured with a hose clamp) to a good engine ground. Resistance for preheating is 4 ohms, + or -2 ohms (at 20C/68F). To check this, unplug the electrical connector (square, two pin connector) and use a multimeter to check the resistance across the oxygen sensor terminals.
Fuel Delivery System
The LH-Jetronic is a re-circulating fuel system in that the electric fuel pump delivers more fuel than is needed even at full throttle, so most of the fuel is returned to the tank. This design essentially eliminates the condition know as 'vapor lock' (vaporized fuel in the lines) since the fuel temperature is kept low by constant re-circulation and reducing the heat-soak from the hot engine compartment. Additionally, the fuel pump is typically located in or near the tank so that the maximum length of the fuel lines are pressurized, usually about 2.5 bar (36 psi), to reduce vapor lock. Vapor lock is the situation where, unlike liquid fuel, vaporized gas becomes trapped in the fuel lines (compressible) and the fuel pump cannot necessarily overcome the problem and deliver fresh fuel. Another point with this type of system is that you should not run the car for extended periods with a low tank of gas. The reason being that the fuel acts to cool the fuel pump. So, if you do run out of gas, just don't crank a long time or you may ruin the pump.
Fuel Rail and the contribution it makes
Another part of the fuel delivery system is the fuel rail. Other than being a source for fuel delivery, it serves to stabilize fuel pressure at the injectors. You can imagine how the pressures change rapidly in the fuel rail as the injectors pop open and closed. This can affect the amount of fuel injected. But the larger the fuel rail (usually a square box shape), the more fuel it stores and the steadier the pressure at the injectors. In smaller pipes, with small volume, pressure tends to fluctuate each time the injectors open.
Fuel Pressure Regulator
The relative fuel pressure in the fuel system is held constant by the pressure regulator. The design of the regulator is that spring pressure normally keeps the regulator valve closed. When the fuel pumps turns on, fuel pressure presses on the diaphragm to compress the spring and opens the valve, returning excess fuel to the tank. The higher the pressure, the more the diaphragm moves away from the return pipe, increasing the volume of the chamber, maintaining the desired pressure. Most systems operate on 2.5 bar (36 psi) gauge pressure, but some people have installed the 3.0 bar (44 psi) for greater fuel delivery per millisecond. For the 9000, standard fuel pressure regulators were as follows: pre-'86 Turbo-2.5 bar (36 psi), post-'87 Turbo-2.8 bar (40 psi), non-Turbo B202I '86 on and B234 '90 on use the 3.0 bar (44 psi).
For each millisecond of injector pulse time, the amount of fuel delivered through the injector tip depends on the size of the injector opening: that's a fixed factor. But fuel delivery also depends on the relative pressure - the difference between fuel pressure pushing the fuel out into the manifold and manifold absolute pressure pushing back. As you can imagine, the manifold pressure changes when the throttle opens. If the fuel pressure were constant for all manifold pressures, then at low engine loads, with the throttle partly closed, reduced manifold absolute pressure would increase fuel delivery. To keep that relative pressure constant as the throttle is opened and closed, the fuel pressure regulator is connected to the intake manifold by a vacuum hose. Manifold pressure acts on the diaphragm to hold the relative pressure constant.
At full throttle, manifold pressure is close to barometric, so the fuel pressure gauge reads about 2.5 bar. At idle, absolute pressure in the manifold is about 0.3 bar (0.7 bar less than barometric). Now the manifold absolute pressure pushing the pressure regulator diaphragm is only 0.3 bar instead of 1 bar. The reduced manifold pressure on the diaphragm allows it to move away from the opening, returning more fuel to the tank, and dropping the gauge fuel pressure in the distributor pipe to about 1.8 bar (2.8 absolute). The relative pressure at the injector tip is still 2.5 bar (2.8 minus 0.3 absolute). That's why fuel delivery per injector is not affected by changes in the manifold absolute pressure.
While we're on the topic of manifold pressure, we should also consider the information conveyed by reading a manifold vacuum gauge. First you need a good quality, large dial vacuum gauge (I got one made by Actron which cost me less than $20). This gauge can be connected into most any vacuum hose that normally connects to the intake manifold.
Before we get into the actual testing, we should probably take a moment to discuss how vacuum is generated and how it is measured. Put simply, vacuum is empty space and may exist as either a total or partial vacuum. The atmosphere exerts a pressure of 14.7 pounds per square inch (psi) or 1 bar on everything at sea level. If a part of the air is removed from one side of a diaphragm (partial vacuum) then a force is equal to the pressure difference times the diaphragm area. Generally, the less air (greater vacuum) in a given space, the more the atmosphere tries to get in and the more force is created.
Vacuum is commonly measured in either inches ('Hg) or centimeters (cm Hg) of Mercury. Atmospheric pressure will support a column of Mercury in a manometer gauge about 30' high or about 76cm. This is the barometric pressure in 'Hg which varies as the weather changes. Vacuum readings in 'Hg are really negative pressure readings. For example, 30'Hg vacuum would be a complete vacuum. Half of a complete vacuum would be about 15'Hg. A gasoline engine at idle usually pulls about 16' to 22'Hg of vacuum in the manifold. On deceleration, because the throttle is closed, the vacuum will increase.
As noted above, vacuum is created when air is withdrawn from a given volume. That, of course, is why vacuum is available in an engine. On the intake stroke, the piston moves down and creates a partial vacuum because the volume of the cylinder has been greatly increased. The air cannot rush through the throttle body, intake manifold, cylinder head port, and around the open intake valve fast enough to totally fill the space created when the piston moves down rapidly. This is the most common automotive vacuum supply source.
While performing vacuum testing, you should make sure that you are not contributing to any vacuum leaks or disrupting any sensor performance. With the engine idling, the following are general indicators of engine performance:
Before you do any testing on the system, you should make sure that all ECU tests are done by probing the rear of the connector by peeling back the rubber boot to gain access to the ECU connections. Also, the ECU must never be unplugged while the ignition is on or within 60 seconds of turning the ignition off. To be safe, you should avoid unplugging or removing the ECU unless absolutely necessary. The reason for this is that all ECUs are more or less sensitive to static electricity and, if handled carelessly or incorrectly, they may be damaged so seriously that they no longer work properly (by the way, dealer price is about $500 for a rebuilt one).
Integrated Fault Diagnosis - Faults that only occur intermittently are often difficult to find. The built-in memory in the LH 2.4 stores information on such faults so that they can be identified and rectified. When a fault has been detected, the CHECK ENGINE light on the instrument panel will flash indicating that a fault has been detected. Each fault has a special code consisting of a combination of short flashes. The series of five flashes can be translated using the code tables. The memory stores up to three faults. Once a fault has been rectified, it may be necessary to erase the contents of the memory to delete any additional codes for the same fault.
To pull the fault codes from the ECU memory, perform the following:
Here's how to check stuff assuming you already know how to retrieve fault
For these tests, you ground the jumper BEFORE turning on the ignition. DON'T
start the engine. When the check engine light (CEL) flashes once, open the
jumper and listen for the fuel pump to run for about 1 second.
Ground the jumper till the CEL flashes once, then open it for the next test.
After you're done with each test, ground the jumper till the CEL flashes
once, just like you're trying to read the next fault code. Each test, in this
order, displays a code like the faults:
1) (no code) fuel pump - listen, it'll run for about 1 second
2) 12411 fuel injectors - listen (unplug individually if you want)
3) 12412 AIC valve - listen, it'll open/close once per second
4) 12413 ELCD valve - listen, it'll cycle once per second
5) 12421 'drive' signal, automatic - stops flashing when you shift D to N
6) 12424 throttle closed signal - stops flashing when you just open throttle
7) 12431 throttle open signal - stops flashing when you fully open throttle
Have fun & let us know what happens. I'm kind of surprised this isn't more
prominent in the repair manuals
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