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Mercedes Benz




   Fuel Injection Systems

    Automotive fuel injection began with a mechanical type system (MFI-mechanical fuel injection)
    which used an injection pump to provide a continuous and varying quantity of fuel to each cylinder
    injector. The injection pump was basically a mechanically controlled fuel distributor. The MFI fuel
    injection's two key variables of RPM and throttle position are provided by the belt drive and the
    throttle linkage. These variables effectively determine the engine's load and thereby the appropriate
    fuel quantity. This type of fuel injection system is very simple and as such is referred to as an alpha-N
    (throttle angle, & RPM) system.

    The later Bosch system, still MFI, was called a CIS (continuous injection system) version. This type
    of fuel injection system had a vacuum controlled fuel distributor. The throttle/sensor plate position was
    used to control the quantity of fuel. The K type, for emissions control, included an analog electronic
    control unit, a frequency valve to control the fuel pressure, an oil temperature switch, and an oxygen
    sensor. On the later CIS-K type, Porsche used an enrichment relay which sensed the oil temperature
    and a full throttle switch to provide additional fuel enrichment under full acceleration. The same oil
    temperature switch also provided a direct input to the CIS-K unit for cold running.

    The CIS-K type was later modified to digital electronics, microprocessor based (KE type) for better
    cold starting and emissions control, with the elimination of the frequency valve (FV), the warm-up
    regulator (WUR), and the auxiliary air regulator (AAR). An electric hydraulic actuator (EHA) unit
    replaced the frequency valve and a coolant temperature sensor replaced the oil temperature switch.
    As on the CIS-K type, the KE type utilized a cold start valve (CSV) which was controlled by the
    fuel pump relay versus the K type system CSV which was controlled by the thermal time switch.
    The CIS-KE type system was by the Mercedes Benz fuel injection system in the late 1980s.
    Performance modifications to the CIS systems were not easy for most shops and were costly.

    Early fully electronically controlled fuel injection (EFI) systems were analog computers designed
    to control the quantity of fuel to the engine by varying the time the injectors were open, where the fuel
    pressure was held constant. These included the Bosch D-Jetronic and L-Jetronic units. The Bosch
    D-Jetronic type sensed manifold vacuum as the key variable using a manifold air pressure sensor
    (MAP). The problem with using a MAP is that without any vacuum the system goes full "rich" which
    occurs at startup. Also, as the engine loses vacuum with wear, the system mixture becomes "richer"
    affecting the air/fuel ratio. The Bosch L-Jetronic type sensed intake air flow using an air flow meter
    (AFM). The D represented druck (pressure) and L represented luft (air). These systems generally
    used an auxiliary air valve (AAV) for cold idle affected by the coolant temperature versus the AAR
    which was affected by a heating element. These systems were not easy to modify for performance.

    The later systems (LH), e.g. Porsche 928 LH unit, used digital electronics to control the injection time.
    These were much more precise in the fuel control and could be easily modified for different situations;
    fuels, engines, emissions, and performance. These also differed from the early "L" type in the use of;
    a hot wire air mass sensor as opposed to the use of the AFM, and an idle stabilization valve (ISV)
    controlled by the LH unit. Performance chips were developed for these systems, but generally
    provided little improvements for the street because any excessive change in fuel mixture was offset
    by the emissions control system element (oxygen sensor input) which optimizes Lambda (an indicator
    of the air/fuel ratio). An optimum Lambda value (1.0) may differ from an optimum performance,
    i.e. maximum torque occurs at a Lambda value less than 1.0.

    The Motronic fuel injection systems integrated the ignition control function with the fuel control
    function, e.g. BMW Motronic, Porsche Motronic. Digital Motor Electronics (DME) also refers
    to the same type of engine control system, e.g. BMW DME, Porsche DME. Both names are used
    interchangeably to reference these Bosch ECM (engine control module) systems which have been
    used on many vehicles throughout the world. Motronic 1.3 added on-board diagnostics with fault
    memory. The later Motronic 1.7 used a Coil-On-Plug (COP) ignition. The early european Motronic
    vehicles, e.g. the Euro 911 3.2 Motronic, didn't utilize an O2 sensor nor a CAT and as such when
    imported as 'gray-market' cars required the addition of an O2 sensor, a CAT, a Motronic
    modification, and minor wiring changes.

    Both of these systems are fully digital and used a programmed memory (firmware) to store the
    fuel and ignition parameters (maps) for the full range of engine operations and could provide
    sequential fuel injection with cam data. Some Motronic systems use the CIS type fuel injection
    versus the fully electronic type (L/LH) fuel injection. Later Motronic systems added a throttle
    position sensor (TPS) to provide additional air intake data and supplement the air mass sensor
    data, i.e. TPS data is a proxy for air intake when combined with RPM data. As in the MFI fuel
    injection system which uses just throttle position and RPM, a simple TPS system is also referred
    to as an alpha-N fuel injection system.

    The Motronic fuel and ignition maps could easily be changed for any condition desired for fuel
    and/or ignition control. The maps stored in the firmware (EPROMs or EEPROMs/flash devices)
    can be re-programmed in some Motronic units without disconnecting the units from the vehicle.
    Again, because of the emissions controls (O2 sensor), little could be gained by changing the fuel
    maps, i.e. without modifying the emissions controls. The only beneficial performance modifications
    that could be done for street vehicles were changes to the ignition timing maps versus providing
    additional fuel, i.e. without major engine modifications for more air flow. Furthermore, effective
    catalytic converter operation requires a slightly excess lean condition (Lambda > 1.0) on later
    model engines.

    More specifically, additional fuel without more intake air; e.g. a turbo, supercharging, porting,
    or an improved exhaust system, will generally not result in increased performance. The key is to
    maximize the torque with an optimized fuel input based on all engine load conditions given the
    engine variables of; intake air flow, RPM, and throttle position. By using these inputs and deriving
    map data empirically from dynamometer tests, since no real-time engine torque sensor exists,
    optimized fuel injection maps for the maximum torque can be determined. This is in contrast to
    ignition systems which can easily adapt the ignition timing thru the use of knock sensors with basic
    maps and thereby optimize the ignition timing continuously over all load and RPM conditions
    achieving the maximum torque.

    In the mid-1990s, the Motronic systems included the OBDII standards, on-board diagnostics,
    which further increased emissions controls and the reporting of engine malfunctions from; sensors,
    misfires, or secondary emissions controls. These late model fuel injection systems adapt to engine
    changes, e.g. air intake leaks, or fuel pressure variations, which affect the ability of the fuel injection
    system to maintain the optimum AFR (air fuel ratio) for ideal emissions control. Any changes to the
    maps, either fuel or ignition, can affect the adaptation process resulting in fault codes.

    One of the key features of OBDII is the monitoring of the fuel injection system adaptation to emissions
    control and pending engine fault codes. The OBDII standards require that the fuel injection system go
    thru a self-test at engine startup and cycle thru six or more readiness modes which includes certain
    driving conditions over various time periods. The resulting state of these modes determines the overall
    functionality of the emissions control system and whether an acceptable emissions test can be initiated.

    The readiness state of the OBDII does not initially cause a check engine light. Once the OBDII system
    has been reset, the fuel injection system must complete its self-tests before determining a "hard" fault
    and thereby causing a check engine light. The OBDII diagnostics will provide info on the readiness
    state via a basic OBDII scanner. Thus, a no check engine light condition does not necessarily indicate
    that a readiness state exists for an acceptable emissions test.

    If the fuel or ignition maps are changed in these types of mid-1990s and later systems, potential major
    problems may occur when a vehicle needs to be emissions certified (smog tested). Many states now
    directly access the OBDII data during an emissions test. Therefore, any type of chip modification to
    the later Motronic, e.g. BMW Motronic, or other OEM type systems is generally not advised for street
    use where additonal problematic emissions related issues may arise.

    Ignition Systems
    Basic automotive ignition systems used an ignition coil to store energy to produce a spark when a
    set of points opened, an inductive discharge ignition. An ignition coil is basically a transformer whose
    spark voltage results from the peak points voltage times a turns ratio (typically 100). The minimum
    required spark energy is about 60 milli-joules (mj) resulting from the square of the ignition coil
    current times the inductance of the coil divided by two. For a typical coil current of eight amps and
    an inductance of 2 milli-henries (mh), the required spark energy results. The other spark requirement
    is a spark voltage of about thirty thousand volts (30KV) for the typical automotive ignition system.
    Significantly greater values for either of these results in a stressed ignition system, e.g. a burnt rotor
    and distributor cap, or spark plug wires which breakdown. The basic points ignition system produces
    a spark pulse (oscillating) of 150 to 200 microseconds decaying in voltage.

    Inductive discharge systems are very simple in design requiring only one energy storage element.
    Typically, they require high currents, which can stress components and produce heat, and large coils.
    Later ignition systems replaced the points with an electronic switch. These ignition systems are called
    transistor controlled ignitions (TCI). Because of the early transistor devices, the current and its rise
    time (the time required to deliver the spark energy to the plug) of the spark were limited. This was a
    problem for firing rich mixtures or fouled plugs, or for engines under heavy load conditions.

    To resolve this problem, the capacitive discharge ignition (CDI) system was developed. This system
    has two energy storage elements plus an energy transfer element (a transformer which is an ignition
    coil with less inductance to produce the high voltage). The input energy must first be stored in a coil
    or an input transformer, then transferred to a capacitor, and then finally transferred to the ignition coil.
    The spark pulse duration at the coil output is about 100 to 200 microseconds for a typical system.
    The typical inductive discharge system (TCI) spark pulse duration is about 50 to 100 microseconds.
    The actual spark burn time in each system may range from 500 microseconds to a millisecond.

    The capacitive discharge system can produce a very fast rise time spark, but is much more complex
    and thus inherently less reliable. The Bosch CD ignition (Porsche CDI) produces the ideal spark
    energy and voltage for most applications, and because of its simplicity is the most reliable CD ignition.
    This is exemplified by the fact that many '83 Porsche 911SCs still have the OEM unit after 20 years.
    A number of after-market ignition systems use the capacitive discharge design as a replacement for
    OEM systems, but some of these systems have spark pulse durations half of the Bosch OEM units
    which results in less energy utilized for combustion. Additionally, the Bosch CDI uses a fly-back
    type of voltage converter whose output voltage is not directly a function of battery voltage
    which results in better starting spark voltage in cold weather with low battery voltage.

    The later semiconductor technologies, e.g. silicon bipolar power devices, have provided the inductive
    discharge ignition systems with comparable CDI spark rise times, energy levels, with the benefit of
    simplicity of design, better reliability, lower costs, and longer spark durations. As a result of these
    TCI improvements, most/all OEM vehicles use the inductive discharge type of ignition systems.
    The issues of reliability and cost become more significant for coil-over-plug (DIS - direct igntion
    systems) when considering a CDI versus a TCI ignition system, thus favoring TCI.

    Some after-market systems produce multiple sparks per plug firings, which has questionable value
    being localized to one area in the combustion camber and because of the multiple spark timing
    compared to the speed of the combustion process, i.e. no additional later spark energy enhances
    the combustion process. This is in contrast to the multiple sparks produced in the twin plug ignition
    systems used by Porsche and others. The effect becomes clearer when you consider that at a 1000
    RPMs an engine rotates 6 degrees every millisecond. So for every additional spark pulse which is
    delayed a millisecond after the first, the piston has moved 6 degrees. Furthermore, the complete
    combustion process occurs within 2 milliseconds of the initial ignition spark and the initial spark
    burn time can last a millisecond.

    The HC (hydrocarbons) level, a measure of unburned fuel, can be used as a comparative indicator
    of the effectiveness of an ignition system. Since the HC level is a key emissions test parameter, the
    use of multiple sparks to reduce the HC level by major OEM car manufacturers would be universal.
    Also, if the claim for added performance were true, this would be another benefit of using a multiple
    spark system. The added technical complexity and cost of utilizing multiple sparks is very minimal.
    Thus, based on these key facts, a multiple spark ignition system has little to no benefit over a single
    spark ignition system.

    Because of the added complexity of multiple spark systems, the potential reliability is further reduced
    for this type of CD ignition system. Also, the multiple sparks may cause interference problems with
    electronic systems, e.g. fuel injection, voltage regulators, because of the additional RF noise without
    proper shielding or main 12 volt supply noise. Other capacitive discharge ignition systems integrate
    the ignition coil into the spark control unit which also potentially reduces the reliability. Additionally,
    some after-market control units have been filled with a potting compound which also potentially
    causes reliability problems with varying temperatures, e.g. no ignition spark when the engine is hot
    or an intermittent running condition.

    Since the capacitive discharge ignition coil functions basically as a transformer, the type of coil used
    is not critical versus the coil type used in an inductive discharge ignition. The coil inductance can vary
    from .5 mh to 5.0 mh and the series resistance can vary from .5 ohms to 5 ohms and not significantly
    affect the functioning of a capacitive discharge ignition. Both the inductance and the series resistance
    are critical to the final spark energy and maximum engine RPMs in an inductive discharge ignition.
    Therefore, most inductive discharge coils can be used with capacitive discharge ignitions, but not

    Both the inductive discharge system and the capacitive discharge system can be controlled digitally
    thru ignition maps and integrated with fuel injection systems. Because of the simplicity and improved
    technology of inductive discharge systems, all late model vehicles use an inductive discharge system
    (TCI) as original equipment. Later systems usually place one coil directly connected to each cylinder
    spark plug, DIS (direct ignition system). As a result of this, no after-market system can be used on
    late model non-race engines without major changes.

    Performance modifications of Motronic ignition systems, thru ignition map changes, advance the
    ignition timing by a few degrees at various RPMs. At best these changes have very limited results
    (typically 5-10 hp @ max RPM) for non-race engines without possible effects to legal emissions
    requirements, e.g. very high oxides of nitrogen (NOx). The actual results must be determined by
    use of a dynamometer to verify the before and after claims, and not by comparing crankshaft and
    wheel horsepower using assumed driveline losses, i.e. a standard before and after test. This should
    be the approach taken when evaluating any performance modification and the claims or its value.
    Furthermore, problems can occur when the ignition maps are changed to a point where pinging or
    detonation occurs at varying engine loads to achieve maximum performance.

    The tighter emissions controls introduced in the early 1990s essentially ended most performance
    modifications thru map changes, mainly because of fault code problems and emissions inspections.
    Only the pre-1990s systems can be easily modified without problematic results and even these may
    incur problems, e.g. a poor running engine at idle or other RPMs, or an emissions certification test
    failure. Specifically, advancing the timing thru map changes may significantly increase the NOx level
    above an acceptable value resulting in an emissions test failure. The NOx emissions can be very
    problematic for high performance engines, e.g. high compression, and difficult to reduce.

    Later Motronic systems included knock sensors, e.g. Porsche DME (964/968/986/993/996), to
    optimize the ignition timing for performance and emissions. Typically, the knock sensors affected the
    ignition timing by retarding it in 3 degree increments to a maximum of 9 degrees. The knock sensors
    basically eliminated any performance effects from map changes unless the knock sensors are disabled
    or the ignition maps are "pushed" beyond knock sensor control. A typical problem which may result is
    pinging at various temperatures and loads.

    Modifications of ignition maps in late model vehicles can result in engine damage (detonation) and/or
    emissions certification test failures (e.g. NOx). As a result of OBDII ('96 and later) other problems
    may occur, e.g. an incorrect fuel injection adaptation reading that becomes apparent when a vehicle
    has the OBDII port accessed during an emissions test. Therefore, ignition map changes thru chip
    modifications for non-race engines are not advised for late model ('96 & later) street legal types of

    Climate Control Systems
    Early automotive climate control systems consisted of three basic control elements; a fan speed switch,
    a temperature selector, and an air diverter slider. Later systems with air conditioning (A/C) included a
    manual switch to control the A/C compressor. These were made semi-automatic by including an inside
    temperature sensor and an analog amplifier/comparator with a relay to control the A/C compressor.

    The fully electronic climate control system used a microprocessor to basically sense; the selector
    switches on the climate control panel and the inside/outside temperature, and then to control thru
    relays/solenoids and electronic switches; the air mixing flap, the air diverters, the water valve or the
    hot air flow, the fan speed, and the A/C compressor. Later variations of this basic electronic climate
    control system allowed independent driver and passenger temperature control.

    Most climate control systems consist of a number of elements; a control head, a fan speed controller,
    small motors or vacuum actuators, temperature sensors, relays, and the A/C compressor. Usually,
    some of the elements contain electronics besides the control head. Late model units use power
    semiconductors to control the fan speed versus power resistors in older systems. Water valves
    for the heater core are controlled by vacuum or electrical solenoids. All of these elements should be
    considered as problem sources and not only the key element which is the control head.

    Relays either electro-mechanical or fully electronic generally control the A/C compressor and are
    usually separate from the control head, because the compressor clutch current is about three to
    five amps. The Porsche 928 climate control, though, integrated the A/C relay into the control head.
    Some vehicles with serpentine compressor belts utilize relays which sense the speed of the compressor
    to determine if the compressor is seized causing the belt to break. Other inputs used to control the
    compressor clutch include; the engine & compressor temperatures, the engine rpm, and the high/low
    freon pressure.

    Fan speed control in most climate control systems utilizes some form of a servo amplifier. The output
    of the amplifier provides a variable ground which varies the fan motor current, as the input voltage
    from the control head is varied. The motor current is sensed and provides feedback to stabilize the
    actual motor speed based on the selected motor speed in the control head. The fan speed control
    unit is usually placed within the fan blower housing to provide cooling for it. In some applications,
    the fan speed control unit may have a large heatsink and be attached to the body, e.g. the Porsche
    964/993. Because of the fan motor current (about 10 amps or more) and the power (25 watts or
    more) being dissipated by the amplifier, the fan motor amplifier may fail keeping the fan motor from
    turning off.

    Some climate control systems, e.g. BMW, utilize the control head only as a control input source
    with the main controller located elsewhere. Porsche on the 928 climate control system had two key
    elements; the control head and a servo unit that had an electric motor to adjust the mixing flap and
    control vacuum solenoids. Mercedes Benz on many climate control systems had all the electronics
    integrated into the control head. Porsche CCU (964/993) also had all the electronics integrated into
    a control head with only small motors and solenoids being external as in the Mercedes Benz system.
    Again, the control head may not be the only source of problems and thus other elements must also
    be diagnosed, e.g. servo motors or vacuum solenoids for the flaps.

    Diagnosis of the fully electronic climate control system was fairly difficult until a self-diagnosing
    mode was included in the late '80s and early '90s. Thru the use of special testers, error codes could
    be read and system elements, e.g. the temperature sensors, could be measured and output elements
    actuated. These early self-diagnosing systems usually required a separate/special tester. Later climate
    control systems included self-diagnosing modes which were accessed as part of a main dedicated
    automotive tester e.g. the Porsche Hammer, & the BMW MODIC. Without some form of climate
    control tester, troubleshooting is very difficult beyond testing for power and measuring sensors.

    The climate control unit can be problematic not only from its operation in affecting the cabin
    temperature, but other areas also. Since the late model climate control unit may turn fans, the A/C
    compressor & solenoids on/off, and affect the engine idle, this unit may be the source of other
    problems, e.g. an excessive standby current (ignition key off) from the climate control unit or its
    elements draining the battery (Porsche 964/993). Therefore, the diagnosis of possible unrelated
    problems should include the climate control unit as a problem source.

    The fully electronic climate control system has not changed significantly in ten years since the early
    '90s. The current trend is to replace most of the simple mechanical elements (vacuum solenoids) with
    small motors and the relays with electronic switches. Reduction of wiring, as with other automotive
    electronics thru some form of a controller area network (CAN) and distributed electronic control,
    will continue to occur.

    Cruise Control Systems
    All cruise control systems basically function the same, where the desired speed is set via a switch
    or lever and then stored in an ECU either in an analog form or a digital form as used in most post
    '80s vehicles. The set speed is then compared to the actual speed to produce a differential speed
    value that is used to either increase or decrease the engine RPMs, thus increasing or decreasing the
    vehicle's speed to maintain the desired speed.

    Most early vehicles (pre '80s) used a vacuum servo unit to actuate the throttle linkage to vary the
    engine RPMs. This system had only the vehicle's speed as its feedback element. The later systems
    used a motor with a variable resistor that provided an additional throttle position feedback that
    increased control and speed stability. The early 911 cruise control with the vacuum actuator had
    a failure mode where the speed would oscillate (surge) 3 to 5 mph. This resulted from the highly
    intermittent analog ECU circuit board and its poor mounting being affected by vibrations. The 911
    used the same 928 cruise control until 1988.

    To simpify and increase the reliability of the cruise control ECU, the ECUs of the late '80s
    utilized a microprocessor. The use of a microprocessor allowed for some self-diagnostics.
    With the advent of drive-by-wire throttle control systems, the cruise control actuator was eliminated
    and the throttle body actuator could now replace its function. Since the vehicle speed, throttle
    position, and engine RPM variables were already being used by the ECM, the later vehicle cruise
    control systems became very simple eliminating a dedicated ECU and integrating that function into
    the engine ECM, resulting in a more reliable system.

    Besides the vehicle's speed input, the brake pedal and clutch pedal (standard transmission) status is
    used to enable/disable the system. Additionally, all systems have a on/off switch. Some later 'smart'
    systems (adaptive cruise control) may integrate the vehicle's proximity sensors to control the speed
    when approaching another vehicle, maintaining a safe distance for braking.


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