Order Online or Call: 888-280-7799 Porsche | BMW | Mercedes | Audi | Volkswagen | Saab | Volvo | Mini Search Pelican Parts: Enter Description View Cart | Project List | Order Status | Help Pelican Parts Technical BBS > 1 - Porsche Technical Forums > Porsche 911 Technical Forum MFI simplification thread (1st draft) User Name User Name Remember Me? Password Log in Register Garage Projects Blogs FAQ Community Calendar Today's Posts Search Page 1 of 2 1 2 Next » LinkBack Thread Tools Rate Thread Author Thread Autodidactic user Join Date: May 2003 Location: Summerfield, NC Posts: 1,206 David E. Clark MFI simplification thread (1st draft) The Bosch “timed indirect injection” or mechanical fuel injection (MFI) system used on early 911s up until the factory changed to a mechanical continuous injection system (CIS) half way through the 1973 model year is one of the most complex yet rewarding fuel delivery systems ever used on a production car. Below is an attempt to describe the various components of the MFI system in an easily understood manner. The goal is to make the operation of the system more understandable and thus assist with repair diagnosis. Please view this as an open solicitation to those with knowledge of the MFI system to suggest additions, deletions or modifications to any of these descriptions. I’d also like to add my thanks to all those who contributed to the original post which fleshed out some of these definitions. Here goes ... __________________ Please help the MFI community keep the Ultimate MFI resources thread and the Mechanical fuel injection resource index up to date. Send me a PM and I'll add your materials and suggestions. 1973 911E Targa (MFI) 02-05-2008, 01:56 PM Recommend this thread for the PelicanWiki #1 (permalink ) Autodidactic user Join Date: May 2003 Location: Summerfield, NC Posts: 1,206 David E. Clark MFI Overview MFI Overview: The Bosch Mechanical Fuel Injection system is designed to inject exact quantities of fuel in relation to inducted air to achieve better combustion of the air fuel mixture. The system is comprised of two main parts, the Injection Pump and the Control and Compensating Unit. The main elements of the Injection Pump are the pump camshaft, roller tappets, injection plungers and plunger control rack. All of these elements are lubricated by the engine lubrication system through a connecting line and filter and an oil return line to the crankcase. The Control and Compensating unit’s major elements are the contoured cam and roller sensor, centrifugal governor, shut-off solenoid, enrichment solenoid, thermostat and barometric cell. Operation: An electric fuel pump designed to deliver fuel at a rate of approximately 29 gallons per hour delivers fuel from the tank to the fuel filter. The fuel filter contains an overflow valve which regulates the fuel flow at approximately 12psi to the injection pump. The fuel pump is specifically designed to provide excess fuel capacity so that the injection pump stays a cool as possible. The excess capacity is routed back to the fuel tank via a return fuel line. The ignition pump is powered by a “spur belt” running off the left engine camshaft. The pump contains six injection plungers which are actuated by the injection pump camshaft. The plungers force fuel through six pressure lines of equal length into the injection nozzles in the cylinder heads at a pressure of approximately 210-265 psi. This system is known as “timed indirect injection” because injection is timed so that fuel is injected into the inlet valves just as they begin to open. When the car is started and the accelerator pedal pressed, air is drawn into the engine through an air cleaner then down two triple-duct velocity stacks bolted on the throttle bodies. The air/fuel mixture ratio is maintained at 14.8:1 (1 pound of fuel to 14.8 pounds of air) by a complex “compensating unit” featuring a contoured cam mounted on the control unit camshaft and moved axially and laterally by a centrifugal governor and by the accelerator linkage. A roller sensor riding on the cam relays the required fuel volume to the rack via a “control lever.” A barometric cell is included to compensate for outside air pressure, a thermostat reacts to varying engine operating temperatures and an enrichment solenoid (on pre-1971 pumps) or a “thermoswitch” (on post 1970 pumps) provides correct fuel enrichment. Finally, a shut-off solenoid cuts off fuel supply when the car is decelerating in gear. All of these devices work together to move the plunger control rack back and forth to compensate for external variations and assure that appropriate levels of fuel are provided. Here is a picture of Porsche six cylinder engine with the Bosch MFI system. Page 1 of 16 MFI simplification thread (1st draft) - Pelican Parts Technical BBS 22/03/2013 http://forums.pelicanparts.com/porsche-911-technical-forum/391224-mfi-simplificatio...
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David E. Clark MFI simplification thread (1st draft)
The Bosch “timed indirect injection” or mechanical fuel injection (MFI) system used on early 911s up until the
factory changed to a mechanical continuous injection system (CIS) half way through the 1973 model year is one of the most complex yet rewarding fuel delivery systems ever used on a production car. Below is an attempt to
describe the various components of the MFI system in an easily understood manner. The goal is to make the
operation of the system more understandable and thus assist with repair diagnosis. Please view this as an open
solicitation to those with knowledge of the MFI system to suggest additions, deletions or modifications
to any of these descriptions. I’d also like to add my thanks to all those who contributed to the original post which fleshed out some of these definitions. Here goes ...
__________________
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1973 911E Targa (MFI)
02-05-2008, 01:56 PM Recommend this thread for the PelicanWiki #1 (permalink)
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David E. Clark MFI Overview
MFI Overview: The Bosch Mechanical Fuel Injection system is designed to inject exact quantities of fuel in relation to
inducted air to achieve better combustion of the air fuel mixture. The system is comprised of two main parts, the Injection Pump and the Control and Compensating Unit. The main elements of the Injection Pump are the pump
camshaft, roller tappets, injection plungers and plunger control rack. All of these elements are lubricated by the
engine lubrication system through a connecting line and filter and an oil return line to the crankcase. The Control and
Compensating unit’s major elements are the contoured cam and roller sensor, centrifugal governor, shut-off solenoid,
enrichment solenoid, thermostat and barometric cell.
Operation: An electric fuel pump designed to deliver fuel at a rate of approximately 29 gallons per hour delivers fuel
from the tank to the fuel filter. The fuel filter contains an overflow valve which regulates the fuel flow at approximately
12psi to the injection pump. The fuel pump is specifically designed to provide excess fuel capacity so that the injection
pump stays a cool as possible. The excess capacity is routed back to the fuel tank via a return fuel line. The ignition
pump is powered by a “spur belt” running off the left engine camshaft. The pump contains six injection plungers which are actuated by the injection pump camshaft. The plungers force fuel through six pressure lines of equal length into
the injection nozzles in the cylinder heads at a pressure of approximately 210-265 psi. This system is known as “timed
indirect injection” because injection is timed so that fuel is injected into the inlet valves just as they begin to open.
When the car is started and the accelerator pedal pressed, air is drawn into the engine through an air cleaner then down two triple-duct velocity stacks bolted on the throttle bodies. The air/fuel mixture ratio is maintained at 14.8:1 (1
pound of fuel to 14.8 pounds of air) by a complex “compensating unit” featuring a contoured cam mounted on the
control unit camshaft and moved axially and laterally by a centrifugal governor and by the accelerator linkage. A roller
sensor riding on the cam relays the required fuel volume to the rack via a “control lever.” A barometric cell is included
to compensate for outside air pressure, a thermostat reacts to varying engine operating temperatures and an enrichment solenoid (on pre-1971 pumps) or a “thermoswitch” (on post 1970 pumps) provides correct fuel
enrichment. Finally, a shut-off solenoid cuts off fuel supply when the car is decelerating in gear. All of these devices
work together to move the plunger control rack back and forth to compensate for external variations and assure that
appropriate levels of fuel are provided.
Here is a picture of Porsche six cylinder engine with the Bosch MFI system.
Page 1 of 16MFI simplification thread (1st draft) - Pelican Parts Technical BBS
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1973 911E Targa (MFI)
Last edited by David E. Clark; 02-26-2009 at 03:03 PM.. Reason: Changed "14.8 psi" in paragragh 3 to "14.8:1 (1 pound of fuel to 14.8 pounds of air)".
Thanks to Bill in post 27
02-05-2008, 01:57 PM Recommend this thread for the PelicanWiki #2 (permalink)
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David E. Clark Barometric Cell
BAROMETRIC CELL – compensates for changes in air pressure by varying the amount of fuel injected based on the
atmospheric pressure. The barometer used on all timed indirect injection (MFI) Porsches is of the aneroid type and is
made up of thin elastic metal disks contained in a chamber which sits on the top of the injection pump. The chamber
is partially evacuated of gas and prevented from collapsing by a strong spring. The design of the chamber means that the barometer is not serviceable by a home mechanic. When the elastic metal disks are subjected to small changes in
atmospheric pressure they are designed to shrink or expand. This expansion and contraction drives a mechanical lever
inside the injection pump which alters the position of the control rack and allows the quantity of injected fuel to be
increased when the car is operated at high air pressure (for example, sea-level driving) and decreased when operated
at low air pressure (for example, mountain driving). The barometric cell is identified in the diagram of the post-1970 injection pump below.
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02-05-2008, 01:58 PM Recommend this thread for the PelicanWiki #3 (permalink)
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David E. Clark Cold Start Solenoid
COLD START SOLENOID – located on the top of the fuel filter, this device is an electromagnetic switch that
mechanically opens a circuit allowing fuel to be injected when electric current is run through it. When the starter is
engaged, an electric current goes through a thermal sensor located at the top center of the crankcase. This thermal
sensor measures engine temperature. If the engine is cold, current is allowed to pass through to the Cold Start Solenoid which activates allowing an injection of raw fuel to be sent to the intake stacks. When power is removed
from the starter, the cold start solenoid is shut off to avoid flooding the engine. The purpose of the cold start solenoid
is to supply extra fuel only when the weather is cold and the engine is harder to start. The cold start solenoid is
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1973 911E Targa (MFI)
Last edited by David E. Clark; 02-26-2009 at 02:37 PM.. Reason: Corrected the diagram to show proper flow into and out of the filter. The diagram is incorrect in volume I, page SF12 of the factory workshop manual. Thanks to Grady Clay for spotting this error!
02-05-2008, 01:58 PM Recommend this thread for the PelicanWiki #4 (permalink)
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David E. Clark Control Unit
CONTROL UNIT – is located inside the Injection Pump. The control unit consists of a contoured cam mounted on a
fixed camshaft together with a centrifugal governor that controls the speed of the engine by regulating the amount of fuel admitted so as to maintain a near constant speed whatever the load or fuel requirements. When the accelerator
is pressed the contoured cam is rotated and moved forward, back and axially on the camshaft by the centrifugal
governor (a mechanical transfer device) so that fuel delivery can be altered based on engine load or speed. Riding on
the contoured cam is a small roller sensor. This sensor maps the information received from the rotary motion of the
engine, the accelerator position, the atmospheric pressure (from the barometric cell) and the engine temperature
(from the thermostat) and transfers that fuel volume information to the control rack by moving the governor control lever. The purpose of the control unit is to supply the engine with different quantities of fuel under varying engine
speeds and loads. The control unit and its various parts are identified in the diagram below.
Page 3 of 16MFI simplification thread (1st draft) - Pelican Parts Technical BBS
Paul, screen name pjh69911, proposed this clarification:
Quote:
If this makes more sense to the board I'll be happy to change the definition I suggested or merge the two. I just
couldn't make a merger work but I'm open to suggestions! __________________
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1973 911E Targa (MFI)
Last edited by David E. Clark; 02-06-2008 at 07:14 AM.. Reason: Add picture of "Space Cam"
The heart of the control unit is the 'contoured cam'. It is a 3-dimensional cam whose radial dimension varies
both with rotation and with axial displacement. The 'governor control lever' (connected to the throttle) only acts to rotate the contoured cam. The 'centrifugal governor''s weights fly outwards with increasing speed,
causing the contoured cam to move axially. The 'sensor on the contoured cam' moves radially in response to
the rotational and axial contour variations, and thereby provides a specific output of radial displacement for
any combination of throttle position and engine speed. This radial displacement of the sensor is fed to the flat
metal plate with rectangular slots via the upper pin in the rectangular slot. The lower pin in the lower slot acts on the fuel rack. The ratio of the input upper pin to the output lower pin is influenced by the thermostat and
barometric cell, that move the pivot center of the flat slotted metal plate, further or closer to the rack lower
pin. Refer to earlier control compensating unit illustration.
02-05-2008, 01:59 PM Recommend this thread for the PelicanWiki #5 (permalink)
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David E. Clark Enrichment Solenoid
ENRICHMENT SOLENOID – is located beside the Thermostat on pre-1971 Injection Pumps. When the ignition is activated, a time-limit relay closes the solenoid circuit for two seconds moving it beyond the full power position and
pushing the plunger on the control rack to allow maximum fuel rate delivery for starting. When engine temperature is
below 50° Fahrenheit, a thermo-limit switch keeps the circuit closed for an appropriate period of time to allow
additional fuel for starting. The purpose of the Enrichment Solenoid is to enrich the combustion mixture in cold or hot
starting conditions. The Enrichment Solenoid was removed from the Injection Pump beginning in Model year 1971. On 2.2 and 2.4 liter engines, a thermoswitch in the breather cover operates the cold start valve. The pre-1971
Enrichment Solenoid is pictured below and at # 28 in the diagram of the early Injection Pump.
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1973 911E Targa (MFI)
Last edited by David E. Clark; 02-05-2008 at 03:07 PM.. Reason: Add picture of thermo-time limit switch
02-05-2008, 02:00 PM Recommend this thread for the PelicanWiki #6 (permalink)
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David E. Clark Mechanical Fuel Injectors
FUEL INJECTORS - The six fuel injectors used in the Bosch MFI system are mechanical in operation. They are threaded into the cylinder heads behind each intake valve. The injectors operate by opening up when fuel is forced
through the hard fuel line connected to the injector and through a filtering screen at approximately 210-265 psi. The
fuel comes in from the bottom of the injector and is forced upward against a spring, where it forms a cone and is
finely atomized and sprayed out of the injector into the intake port (a picture of proper fuel atomization is shown
below). The injectors should be tested regularly to assure that they hold pressure and have a perfect spray pattern with a nozzle tester similar to the one shown below.
Below is a picture of a Bosch mechanical fuel injector for the MFI system:
Here is a picture of the appropriate nozzle spray pattern for the Bosch fuel injectors:
Page 7 of 16MFI simplification thread (1st draft) - Pelican Parts Technical BBS
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Last edited by David E. Clark; 02-06-2008 at 03:58 PM.. Reason: Fix Typo
02-05-2008, 02:00 PM Recommend this thread for the PelicanWiki #7 (permalink)
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David E. Clark Fuel Pump
FUEL PUMP – is an electric roller-cell type with a delivery capacity of up to 33 gallons per hour. The pump is
mounted on a bracket under the fuel tank. The fuel is delivered from the tank to the fuel filter. The fuel filter contains
an overflow valve which regulates the fuel flow at approximately 12psi to the injection pump. The fuel pump is
specifically designed to provide excess fuel capacity so that the excess fuel can be used to cool the injection pump. After flowing through the injection pump, the excess capacity is routed back to the fuel tank via a return fuel line. If
fuel pressure increases above 14psi, an independent bypass valve installed in the fuel pump routes excess fuel to the
fuel tank.
The Exact Fuel Pump Specifications are: Regulated pressure = 14.2 PSI, 0.8±0.2 Bar
Flow rate = 125 liters/hour
Porsche P/N = 901.608.105.00
Bosch P/N = 058097 0001
Current draw = 3.5A @ 12V RPM = 2800
The fuel pump is # 2 in the Fuel Injection System Diagram and its location is shown in the diagram below.
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Last edited by David E. Clark; 02-06-2008 at 02:20 PM.. Reason: Fix typo
02-05-2008, 02:01 PM Recommend this thread for the PelicanWiki #8 (permalink)
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David E. Clark Injection Pump
INJECTION PUMP: The MFI Injection Pump consists of two major parts: (1) the pump assembly which contains the camshaft (#19 below), the six plunger units (#s11-14 below) and the toothed control rack (aligned horizontally
behind #27 below); and (2) the control and compensating unit (described above). Each of the six plunger units
consists of a plunger and a cylinder. Each plunger has teeth which mesh with the control rack on top and a roller
tappet (#20 below) which rides up and down on one of the camshaft lobes on the bottom. As the camshaft rotates,
the movement of the lobes forces the tappet into the plunger. At the same time, the various components of the compensating unit (thermostat, barometer, enrichment solenoid, etc) are sending instructions via connecting levers to
the toothed control rack causing it to slide back and forth in the teeth of the plunger. This movement of the control
rack causes the plunger to rotate exposing a slanted "metering land" in each plunger. This metering land is designed
to close or open the inlet port to the fuel injector (see diagram below). The amount that this inlet port is open
determines the quantity of fuel to be delivered with each stroke. As a result of these complex movements, the fuel contained in the plunger chamber is forced out in appropriate quantities through the hard fuel pressure lines then
through the injectors and into the intake port.
pre-1971 MFI Injection Pump
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Last edited by David E. Clark; 02-07-2008 at 01:36 AM.. Reason: Fix Typo
02-05-2008, 02:01 PM Recommend this thread for the PelicanWiki #9 (permalink)
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David E. Clark Thermostat
THERMOSTAT - The thermostat is located on the front of the injection pump and is one of the compensation units on
the pump along with the barometric cell and the enrichment solenoid. Its purpose is to compensate for various engine operating temperatures. The unit consists of several pairs of thermal expansion discs which shrink or expand based on
engine operating temperature. The thermostat receives its heat signals from a double walled hose connected to the
driver's side heat exchanger. When the engine is cold, the control rack is set to fully enrich the fuel mixture for easier
starting. As the engine warms up, hot air is feed through the hose to the thermostat. This causes the discs to expand
moving a lever in the injection pump which alters the control rack to lean out the mixture. Thermostat has no effect on the control rack after a temperature of approximately 113°. The thermostat is number 1 in the diagram of the
Ignition pump above. Here is a detailed schematic of the thermostat and the expansion disks:
Schematic drawing of MFI Thermostat.
Page 10 of 16MFI simplification thread (1st draft) - Pelican Parts Technical BBS
Copied from the MFI thermostat spacers thread. Posted here for educational purposes only. __________________
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Last edited by David E. Clark; 02-07-2008 at 01:40 AM.. Reason: Label diagram
02-05-2008, 02:02 PM Recommend this thread for the PelicanWiki #10 (permalink)
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David E. Clark Shut-Off Solenoid, Micro-Switch and RPM Transducer
SHUT-OFF SOLENOID, MICRO-SWITCH AND RPM TRANSDUCER CIRCUIT: The shut-off solenoid is part of the control and compensation unit (on the lower right in the diagram below). Its function is to move the control rack to
the fuel off position when the vehicle is decelerating in gear. The shut-off solenoid is controlled by a micro-switch in
the accelerator linkage and an RPM Transducer on the relay panel which receives its RPM signal from the distributor.
The RPM transducer senses engine speed. It is supposed to activate the shut-off solenoid cutting off the fuel supply
when the throttle is closed (for example when the car is decelerating) and engine speed is above 1,500 RPM. If everything is working correctly, the solenoid is supposed to release when the RPM transducer senses engine speed
dropping below 1,300 RPM regardless of throttle position. This allows the fuel supply to resume and the car to idle
correctly. If engine speed is again increased, the solenoid circuit is interrupted by the micro-switch allowing the RPM
transducer to again become activated at 1,500 RPM.
Here's a schematic of the RPM transducer thanks to Warren Hall - "Early S. Man" (may he rest in peace) and Mike
Gillies of Brisbane, Australia.
Finally, below is a simplified wiring diagram and description for the micro-switch and RPM transducer (#5 in the schematic). * Provided by Pelican member Jim727.
Pin 1: Output to shutoff solenoid;
Pin 2: Power from engine fusebox;
Pin 3: Ground and pin 31 of time limit switch; Pin 4: CD Ignition and distributor.
The RPM Transducer (also known as the "Speed Switch") receives a signal proportional to the engine RPM from the
distributor (contact 1 on component 1 in the diagram); this signal is also used to trigger the CD Ignition pulses
(contact "C" on component 3 in the diagram). Circuitry in the RPM Transducer bridges contact 1 to contact 2 (on component 5 in the diagram) when engine RPM is above 1500 and opens the connection when engine RPM is at or
below 1300 and falling. The final link in the chain is the microswitch which allows the output of the RPM Transducer to
energize the shutoff solenoid when the throttle is closed and the above conditions are met.
Page 12 of 16MFI simplification thread (1st draft) - Pelican Parts Technical BBS
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Last edited by David E. Clark; 02-06-2008 at 03:55 PM.. Reason: Correct wiring description for RPM Transducer
02-06-2008, 05:05 AM Recommend this thread for the PelicanWiki #11 (permalink)
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MJHanna Very nice. Thanks for all the work to put this info together.
02-06-2008, 07:06 AM Recommend this thread for the PelicanWiki #12 (permalink)
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Jim727 David -
It still says "...RPM Transducer on the relay panel which receives its RPM signal from the ignition coil". Your diagram
indicates othewise.
Jim
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02-06-2008, 12:57 PM Recommend this thread for the PelicanWiki #13 (permalink)
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sballard In the "Fuel System" figure, the fuel pump in the diagram is #2, not #1
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1972 911t
1999 328is
02-06-2008, 01:21 PM Recommend this thread for the PelicanWiki #14 (permalink)
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David E. Clark Quote:
Jim,
Can you please explain what you are talking about in more detail? As I understand the system, the RPM Transducer
gets it's RPM signal from the ground connection on the coil. I agree that it was confusing to say that they are
"connected" but unless I've been misinformed (or read my reference materials wrong) the RPM signal is from the coil. This is what I'm trying to say anyway! Can you think of a better (or less confusing) way to say this?
I found this description of the RPM Transducer on the Question and Answer page but it doesn't help with this issue:
Quote:
Originally Posted by Jim727
David -
It still says "...RPM Transducer on the relay panel which receives its RPM signal from the ignition coil". Your
diagram indicates othewise.
Jim
Page 13 of 16MFI simplification thread (1st draft) - Pelican Parts Technical BBS
Thanks Steve. Even I should be able to tell the difference between the fuel pump and the fuel tank!
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What the heck is this thing called the RPM transducer?
The rpm transducer is used to activate the shut-off solenoid on the MFI pump. It prevents the pump from
continuing to pump fuel into the engine when coasting at high speed, which will result in backfiring and
other unpleasantness.
The rpm transducer puts out 12v above about 1600 rpm, and 0v below it. It’s connected to the MFI shut-off
solenoid through a microswitch that’s activated by the accelerator linkage—the one on the left side of the
engine, attached to the input stacks. With your foot off the accelerator, the switch is closed, and if the
engine speed is above 1600 rpm, i.e., you’re coasting at speed, then 12v is applied to the relay and it shuts
off the pump. If you’re below 1600 rpm, even though the switch is closed, i.e., you’re idling, the solenoid is not activated because the sensor is putting out 0v, and fuel is pumped to the engine.
The transistor leads corrode and break over time. Virtually any generic NPN, like a 2N2222, will work.
In the "Fuel System" figure, the fuel pump in the diagram is #2, not #1
02-06-2008, 02:43 PM Recommend this thread for the PelicanWiki #15 (permalink)
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Jim727 Pin 1: Output to shutoff solenoid;
Pin 2: Power from engine fusebox;
Pin 3: Ground and pin 31 of time limit switch;
Pin 4: CD Ignition and distributor.
The RPM Transducer (also known as the "Speed Switch" receives a signal proportional to the engine RPM from the
distributor (contact 1 on component 1 in the diagram); this signal is also used to trigger the CD Ignition pulses
(contact "C" on component 3 in the diagram). Circuitry in the RPM Transducer bridges contact 1 to contact 2 (on
component 5 in the diagram) when engine RPM is above 1500 and opens the connection when engine RPM is at or
below 1300 and falling. The final link in the chain is the microswitch which allows the output of the RPM Transducer to energize the shutoff solenoid when the throttle is closed and the above conditions are met.
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02-06-2008, 03:33 PM Recommend this thread for the PelicanWiki #16 (permalink)
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David E. Clark Great explanation! Thanks. I made the changes. Is it clearer now?
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Jim727 Works for me. Good stuff.
Jim
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David E. Clark
Page 14 of 16MFI simplification thread (1st draft) - Pelican Parts Technical BBS
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Page 16 of 16MFI simplification thread (1st draft) - Pelican Parts Technical BBS