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Saab engine management Part 1by Jason Pearce

This is going to be an ongoing article in many parts. I plan to cover Saab's electronic fuel and ignition systems theory of operation. We are going to start with the Bosch LH 2.2 system of the 1985 Saab 16 valve turbo and work are way up to today's Trionic T7 on the Saab 9-5. The fuel injection systems of today are referred to as engine management systems. For instance the Saab Trionic T7 controls many functions other than fuel delivery. The Trionic system takes care of the ignition system, AC , and traction control just to name a few. Even today's systems still work on the same basic principal as back in 1985 so as you read these articles you will see the Saab fuel and ignition systems as they have evolved and integrated. I'm going to start with a basic run down of the major parts of the fuel systems.

On all of the Saab electronic fuel systems a contiguously circulating system is used. The fuel is pumped from the Tank to the fuel filter and on to the engine's fuel rail. At this point the injectors supply fuel to the engine. The left over fuel is then relieved back to the fuel tank via the fuel pressure regulator. These are some of the main advantages of this type of fuel system.

1) Low possibility of vapor lock due to the fact that the fuel does not spend much time in the engine compartment.

2) The pump used can be of greater volume insuring uninterrupted supply even under full load conditions.

The fuel pressure regulator is located on the engines fuel rail, and is not adjustable. The turbo's regulator is preset at 2.5 bar and the non turbo is set at 3.0 bar this will be stamped on the out side of the unit. The regulator has a internal diaphragm that is connected to the intake manifold with a vacuum hose. This will adjust fuel pressure from it's marked specification. This is to maintain a constant pressure drop across the injector tips no matter what the manifolds pressure is. This feature is not to enrichen the fuel mixture under load, but to keep the fuel flow even under varying manifold pressures.

The fuel injector on the Saab is very hardy I have rarely seen one go bad other than from fuel deposits. On the inside of the injector there is an plunger operated by an electrical solenoid. The plunger is held closed by spring tension until the computer applies a pulsed ground. These injectors run in the 2-18 mil second range. We will talk more about that later.

The turbo by pass valve sometimes called the hooter valve is meant to reroute turbo boost pressure back to the turbo when the throttle is suddenly closed during boost. If the valve was not there to do it's job the boost pressure would back up against the throttle plate causing jerking hesitations and noises. The valve is diaphragm operated. The valve is pulled open with manifold vacuum, when the throttle is closed the intake manifold pressure drops applying a vacuum on the valves diaphragm. This pulls open the plunger inside the valve allowing the turbo boost to recycle back to the turbo. The most common problem with this part is that the diaphragm tears causing a vacuum leak. This leads to engine stalling hunting at idle and rough running cold.


IAC valve

air mass meter
 
 

The air mass sensor is used on the LH and motronic systems. This is the main means of measuring the volume of air entering the engine. The computer then uses this information to adjust the fuel mixture by changing the time the injectors remain open in mille seconds. The air mass sensor accomplishes this job by means of a platinum wire strung inside and heated to 100°C. As air flows past the platinum wire the computer adjusts the amount of current to maintain it's temperature at 100°C. This current is passed through a resistor on it's way to the wire. The computer reads the voltage drop across this resistor and calculates the exact air mass passing through the air mass housing. The motronic version does not use a wire but a ceramic tile, the principle of operation is the same.

The Map sensor or manifold absolute pressure sensor is the device that the Trionic system uses to measure the amount of air entering the engine to adjust fuel mixture. The MAP sensor reads intake manifold pressure/vacuum to make it's calculation's on the time to keep the injector open. Trionic also uses this information to adjust and maintain the correct turbo boost. The map sensor internally has a pressure sensitive ceramic device and a circuit to compensate for temperature. The Trionic computer reads the sensor in this way, the map sensor has 3 wires 2 of those wires are power and ground from the trionic unit the third is feed back to the trionic unit. Trionic supplies a constant 5volt current depending on the pressure applied to the ceramic sensor the third wire feeds back 0 and 5 volts.

TPS or Throttle Position Switch does just what the name says, measures the position of the throttle plate. The TPS on the LH 2.2 , 2.4 , 2.4.1 only gives information that the throttle is closed or wide open anywhere in-between the computer just knows the throttle is open. The computer uses closed throttle signal to adjust fuel mixture for idle and setting the idle air control valve. At full throttle the computer enriches the fuel and cuts out the AC compressor to aide in acceleration. This switch is adjustable and should be checked when experiencing derivability problems.
On later models it is no longer a switch but a Throttle position sensor. Unlike the older tps's the sensor no longer needs to be adjusted. It also tells the computer the exact position of the throttle plate at all times.

IAC valve or the Idle Air Control valve is the device the computer uses to maintain proper idle speed. The computer uses information from the ignition on rpm speed then runs the valve at a corresponding frequency to maintain desired idle speed. This is used to compensate for cold start, AC load, and even wear on the engine.

The oxygen sensor also called the O2 sensor sits in the exhaust header pipe and measures the amount of oxygen in the exhaust. The computer then uses this information to adjust fuel mixture. The O2 sensor does this by sampling the outside air and comparing it to the level in the exhuast. The sensor creates a small voltage when there is a differential. The sensor works on a 0-1.0 volt range. When the engine is running lean the oxygen content in the exhaust is high and closer to the outside level, the voltage output will be 0-.5 volts.

 

 

(This article is meant to educate a consumer,or as a guideline for professionals. You can cause serious damage to your vehicle and/or cause yourself injury. Only those qualified should attempt repairs. I do my best to assure that the above info is correct but take no responsibility for any damages incurred.)

 

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