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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
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.
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
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
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.
993 Immobilizer Programming
1. Open door, insert key, and turn key to run mode. Immobilizer light should come on.
2. After about 15 seconds, the light should go off. Turn key off then on (cycle key).
3. After about 15 seconds, the light will start to flash on/off. Cycle the key to enter each
of the four digits. Wait about a second between each digit.
4. If the code is correct, the light will again begin to flash on/off. Depress the button on each remote.
5. Turn the key off, exit the vehicle, close the door, and test each remote.
6. If step #3 doesn't occur, the ignition switch is most likely bad. The immobilizer then needs to be
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