MFI Simplification Project - Help wanted!
 

In my never-ending effort to simplify the mysteries of the Bosch/Porsche MFI system and help us all to be better equipped to diagnose and repair problems with the system, I think it might be helpful to have plain English explanations of how each part of the system works and how it relates to the other parts. The definitions in the Factory Workshop Manual are OK but I think we might all benefit from simpler (or at least better written) explanations. I figure that if we understand exactly how everything is supposed to work we'll be better equipped to diagnose problems when they occur. The various parts I have identified for this project are:

1. Barometric Cell

2. Cold Start Solenoid

3. Control Unit

4. Enrichment Solenoid

5. Fuel Pump

6. Injection Pump
a. Control Rack
b. Camshaft
c. Contoured Cam
d. Roller Tappet
e. Centrifugal Governor

7. Shut-off Solenoid

8. Thermostat

Of course I am open to suggestions - including “no need for this project.” Please consider this an open solicitation to those versed in the MFI system to take a crack at some of these “plain English” definitions. Since it’s my idea I’ll start with the first on the list:

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 servicable 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 below.

http://forums.pelicanparts.com/uploa...1201286868.jpg

bump for more info.

Cold Start Solenoid
 

OK guys, I don't want to write all of these myself! Here's what I've come up with for number 2:

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 identified in the diagram below.

http://forums.pelicanparts.com/uploa...1201216225.jpg

I know that lots of you know more about MFI than I do. Please contribute. SmileWavy

http://forums.pelicanparts.com/uploa...1201188364.gif

Anyone want to contribute?

I don't have MFI, but I thin a basic 'theory of operation' paragraph would be nice. Then follow up with the deviations from basic operation (starting; cold start; and changes in pressure, etc.).

This might also be a good place to post the links to the numerous threads on MFI.

I worked on mine enough to know the basics. Is there something missing? The thing wouldn't return to 900 RPM if the micro switch didn't close on deceleration (along with the fuel shut off). Isn't there something in the system that knows the RPM (other than the flyweights)?

The whole thing is a nightmare, AFAIC, but I did enjoy the song it produced. I'd still take EFI over anything. I know, this doesn't help your thread, but the 6 individual EFI units on carb or MFI bodies fascinates me.

I agree that a "theory of operation" introduction will be helpful. My plan for this thread is to get all the definitions, explanations and theories of operation sections written and then I will edit them all into one cohesive document and post that to the Ultimate MFI resources thread.

I've attempted to gather most of the useful MFI threads in the MFI Message Board Index. Any additions or suggestions are always appreciated. :D I've been rewriting this Index to make it alphabetical by topic but, as you might imagine, gathering all the links and properly posting and cross referencing them takes time.

I should have a definition of "control unit" posted here by tonight. That leaves lots of other definitions ripe for the taking!

David, expect some contributions here from me in the near future, I have lots rattling in my head and I know where to find a lot of interesting information. As an evolution of this, we need to assign out test projects e.g. what is the thermostat rod change with temperature to various MFI types to report back the data.

In the open heart surgery thread, there was some info on the travel of the baro cell plunger with pressure, that should be appended to the description.

Control Unit
 

Here's my try at # 3:

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.

http://forums.pelicanparts.com/uploa...1201217400.jpg

David,
Just to clarify a few points. The MFI is not proportional control. It is a very non-linear system. A doubling of speed does not result in a doubling of fuel, because of the non-linear effects of the intake, tuned runners, etc.. The same is true for the throttle position. The governor in your picture is really a mechanical transfer function that converts rotary motion(engine speed)into an axial displacement of the follower on the space cam. The other input is throttle position, which rotates the space cam. These two independent inputs result in a unique mapping on the space cam, then modified by the barometric cell and thermostat to result in a rack displacement=fuel delivered. Paul.

Paul,

Thanks for the reply and the clarification of the relationship between engine speed and throttle position. My goal is to simplify these definitions. The Control Unit is a complex system and I'm sure I may have simplified too much! That said, I have always understood "proportional control" to mean that the system has the ability to expand or decrease inputs in proportion to its needs. Said in the simplest way I know, the fuel delivered is in proportion to the engine's needs. Thus, when the car is accelerating a proportional control fuel system has the ability to deliver appropriate amounts of fuel to allow the driver to reach the desired speed notwithstanding whether the car is going uphill or downhill or is fully loaded or not. I agree that this is absolutely a non-linear system. This is what I meant in the above definition. If this is incorrect, please help clarify. In addition, I would be happy if you would cut and paste your clarifications into my definition above so that it is more complete (keeping in mind the goal of simple, easy to understand, explanations). As I said earlier, there are many people on this board that know a lot more about MFI than I do!

David,
Any control system relies on feedback. In MFI there is no feedback. It is strictly open loop. A feedback parameter would be exhaust oxygen content via a sensor. If the measured parameter gets too high, a proportional control system then makes a proportional adjustment to an input parameter, which then changes the fuel output, for example. The MFI system doesn't know what is the appropriate amount of fuel to give, it just responds to the inputs(speed, throttle, temp and baro) and returns a fuel quantity. Paul.

# 4:

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.


http://forums.pelicanparts.com/uploa...1201285501.jpg
http://forums.pelicanparts.com/uploa...1201285518.jpg

Theory of Operation
 

Please take a look at this overview summary and let me know if it makes sense (especially in conjunction with the individual element summaries). It's surprisingly difficult to simplify a system as complex as MFI into a few paragraphs! Any suggestions or corrections are appreciated.

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 injection 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 psi 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.

Yes, connected to the ignition coil (on the relay panel) is an "RPM transducer" which "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. Hope this helps and thanks for any help you can provide for this project.

Does anybody have a picture of this? Does anyone still sell this part?

I am having a problem with my MFI, where just out of the blue it stalls randomly and then you can start it right back up and its fine, or sometimes when you run the engine, the RPM doesn't go quite down to 950 and then stalls. Figured this could be a suspect of the problem, since it has to do with the RPM sometimes.

Here you go, it's number 3 and/or 5 in this diagram.

http://forums.pelicanparts.com/uploa...1201317163.jpg

Here's a schematic thanks to Warren Hall - "Early S. Man" (may he rest in peace). :(

http://forums.pelicanparts.com/uploa...1201317196.jpg

From your brief description, this doesn't sound like your problem. Usually if the RPM transducer is faulty you will get lots of backfire. Since you didn't mention this, I'm assuming that you are not getting backfire. I'd start by checking the more common culprits for stalling like a faulty coil, points/plugs fouled or not properly gapped or pitted, fuel supply interruption (clogged filter or screen, etc) or intermittent vacuum leaks. Hope this helps and thanks for any help you can provide for this project.

Thanks David! I'm gonna look into the problem tomorrow and check things like you said, fuel filter, inginition timing, injection timing, etc. Correct, I do not have any backfiring at this time, thankfully. So I believe what you said is true, based on all that I have read. Maybe this can help some others out too, I hope.

By the way, this thread is a good idea guys, and I hope to contribute as much as I can.

A little bit our of order but here is my attempt at # 8.

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. The thermostat is feed by a hose from the driver's side heat exchanger with hot air. While hot air is being feed to the thermostat, the metal discs spread apart from each other and a rod that the discs ride on, is pushed into the injection pump, leaning out the mixture. This is because you do not need as much fuel after the engine warms up. These discs should be cleaned up every now and then since they can get all cruded up. DO NOT MIX THE DISCS UP THOUGH. Put them back on, in the order they came off. Make sure you are getting hot air to the thermostat! Some racing applications replace this with a screw so that the mixture can be changed much faster.

Thanks! The thermostat is number 1 in the diagram of the Ignition pump above. Here is a detailed schematic of the thermostat:

http://forums.pelicanparts.com/uploa...1201367276.jpg

Anthony,

I agree that all the experts say don't mix up the thermal disks but do you (or anyone else) know why? They seem identical to me. I realize that the spacers are different (one thicker than the other) and that the shim must not be left out but the disks look identical to me. If they are put back correctly paired against each other I don't understand why they need to go back in the exact order they came out?

Shut off solenoid
 

David and Milt got me thinking about the Shut Off Solenoid. Is there a way the RPM transducer/speed switch can be set up for a pointless/CDI type distributor like the 3.0 SC dist? My MFI SS engine is twin plugged and using an Andial converted SC distributor and I want to be able to use the shut off solenoid.

Mark,

I haven't read about anyone doing this but I don't see why it wouldn't work. The RPM transducer gets it's signal in the early cars from pulses emitted from the coil so you should be able to integrate a speed sensing switch to any coil or other electronic RPM pickup device. I'd study the early wiring diagrams to replicate the way the RPM Transducer was set up to work with the shut-off solenoid on those cars.

Mark,

Take a look at this schematic. It should give you a good idea of how the RPM transducer, Speed Switch and Microswitch are wired in the early cars.

http://forums.pelicanparts.com/uploa...1201379656.jpg
© Intereurope Workshop Manual, Porsche 912, 911T, 911E, 911S, page 62 (October 1972). Posted here for educational purposes only.

Fuel Pump
 

# 5:

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 # 1 in the Fuel Injection System Diagram and it's location is shown in the diagram below.

http://forums.pelicanparts.com/uploa...1201391944.jpg

About the microswitch, RPM trandsducer, and speed switch; I ran my car today and pulled the wires off of the microswitch with the car at idle, and I could smell it got much richer, since the system is designed to not deliever as much fuel when activated. I hooked it back up and it smelled better and right when I hooked it up I heard a little bang, which I think was the speed switch since the sytem was then live. Just my experience, and plus like you guys said, I don't have any backfiring. I'm pulling my injectors again and sending them to Supertec Performance to get them tested and cleaned. I just had them out and tried to clean them but I want them tested. I may need new ones.:rolleyes:


David,

I agree with you on the thermostat discs arragement. I don't understand it either, and when I first took mine off and cleaned them, I think I put them back in the correct order, but I may have mixed one or two up. I don't think I did though, but I dont' know why it would effect it either.

Also, is that fuel pump one that they used on earlier fuel injected cars? If these pumps go bad, instead of replacing with the $600+ original one, has anyone put another type of fuel pump in it's place and has not had any problems with it (as far as fuel flow, etc.)????

Anthony,

I removed the Bendix pump that I had pictured under "fuel pump" because I don't think that particular pump was ever used on the MFI system. I couldn't find a picture of the roller-cell type pump and I didn't think about the Bendix picture being confusing. :confused: As far as using a different pump, I imagine any electric pump that matches the appropriate specifications would work. I think the tricky part will be finding a substitute pump with a relatively low current draw of 3.5 amps and a 14psi bypass valve since there are lots of modern pumps that can put out in excess of 28 gallons per hour. Keep us posted if you find an acceptable substitute. Suppliers like Summit Racing or JEGS might be a good place to start your search since they supply loads of racing pumps which, by design, have to pump fuel like crazy!

David

David,

Yeah that diagram is more familiar to me.:p I am going to look for another pump because mine is old and I think I can find one that will be more reliable. Also, when I got the car, the pump was stuck, so I'm sure putting a new one on will be a good idea, cause it's probably only a matter of time with the original one. I'll let you know what I find.

David, do you think I need to have a return from the pump if I was to get one with just an in and an out, if one is coming from the injection pump and then into the fuel filter housing. Could I just leave that line in place and then get rid of the T in my line that splits it from the fuel coming from the engine, and the return from the pump, both which go into the T and to the fuel tank. So in other words, I would just have the return from the filter housing flowing into the tank. Or do I need to find a pump with all 3 directions of flow?????

Thanks! I hope this will help others, maybe if I get this to work, when other people's pumps die they can do this instead of spending over $600.

Anthony,

I don't see how you could get by with a two directional pump since you need to (1) flow excess amounts of fuel into the filter then to the injection pump and back to the filter, (2) overflow out excess fuel from the injection pump through the filter and, (3) bypass fuel (different from overflow) back to the fuel tank in case of excess pressure. If you look closely at the above fuel pump diagram you will see that the lines are directional arrows that show all three areas of fuel flow. I think a two directional pump would probably cause all kinds of problems, not the least of which would be constant flooding. :eek:

Well I have a brand new fuel pump now, that has only an inlet and an outlet. David, what do you think if I put a pressure relief valve, or a check valve of some-type that is built into a T after the outlet? This way fuel would still flow up to the filter and if there was too much pressure, some of the fuel would go back to the tank which along the way I would just hook the normal T into from the filter. Only things here is that I have to have enough flow to keep the right amount of pressure going to the filter.

I'm gonna keep looking online for a pump with a return though.....

David, So the fuel coming from the injection pump and the filter, need to meet with a return under pressure? (Isn't the fuel coming from the injection pump controled by a valve above the filter, that lets the fuel flow back to the tank. If so how is this affected by a return from a pump?) Or are you just saying it may cause flooding because the pump pressure may be too high going to the filter. So Maybe then a relief valve would do the trick?;) We both on the same page? Thanks!SmileWavy

Anthony,

I'm not promising that this will work but here's a very simple schematic of what I've come up with for a standard fuel pump:

http://forums.pelicanparts.com/uploa...1201435286.jpg

The bypass valve is in the factory pump but I don't see why it couldn't be on the outside like you suggest. It shouldn't be hard to find a relief valve rated at 14psi.

I think the filter is designed to operate at a fuel flow of 12psi. If it gets above 14psi without "relief" I think the filter will be unable to continue flowing causing flooding in the filter or (just guessing here) the "overflow" will go down the only available route which is back to the injection pump again flooding or, worse, it will just leak out creating a serious fire risk. :eek:

Hopefully, someone who has tried this will chime in -- maybe this deserves it's own thread?

David,

It's worth a shot...and yeah I'll start a thread on this subject alone. Let's get back onto the main subject of this thread...sorry for getting off of it.

I'll do the Fuel Injectors next...let me just get a picture of them...

Fuel Injectors- The fuel injectors that are part of the MFI system are also mechanical. They operate by opening up when fuel is pushed through the line of that injector at around 213-256 psi. A Very Very high pressure! The fuel comes down around the bottom of the injector and pushs it upwards against a spring, and then the fuel forms a cone and is sprayed out of the injector. These are located in the head, behind the intake valve. There is always fuel sitting in the line, even if it is not under pressure, because of this the injectors need not to leak. They should be checked if your injectors are old. Supertech Performance, and Pacific Fuel Injection will both test your injectors. If you need new ones, these are still available for around $180. These injectors were made by Bosch and can not be repaired. There are also screens in the injectors that can get blocked if you don't have clean fuel.

Below is a picture of a set of original fuel injectors for the MFI system

http://forums.pelicanparts.com/uploa...1201489683.jpg

David,
I was rereading your thread and thought a revised text might further clarify the control unit's function. The picture is somewhat confusing as the governor control regulator does not directly connect to the fuel rack. It only acts to rotate the space cam. Anyway, here's what I propose for the text:

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.

Hope this helps. Paul.

Thanks Paul, I know that helped me some!SmileWavy

There are still a lot more parts to be identified!

Injection Pump
 

OK, here's my attempt at describing the Injection Pump. Once all these parts are successfully described, I will edit this post into one comprehensive thread describing the entire MFI system. I'll post that to a new thread so it's more readable and to solicit corrections before making it final. Hopefully, once we've finalized the thread, a moderator can delete the prior threads to avoid confusion. We'll see ... :D

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 (#s 11-14 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.


http://forums.pelicanparts.com/uploa...1201285518.jpg
pre-1971 MFI Injection Pump
© Dr. Ing. h.c. F. Porsche A.G. Posted here for educational purposes only.

http://forums.pelicanparts.com/uploa...1202165988.jpg
Pump Plunger and Cylinder showing fuel delivery.
© Dr. Ing. h.c. F. Porsche A.G. Posted here for educational purposes only.

David, this is a great idea. I'm a bit too new to the MFI cult to be able to speak with any depth of knowledge, but I'm learning and will chip in knowledge permitting.

Jim

OK, here is the final major definition. When I get home tonight, I'll try to edit this thread into one comprehensive post and resubmit. Please take a look and get back to me with any additions, corrections or deletions. Thanks!

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 ignition coil. 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.

http://forums.pelicanparts.com/uploa...1201217400.jpg
© Glenn’s Diesel and Gasoline Fuel Injection Manual. Posted here for educational purposes only.

The RPM Transducer is #5 in the diagram below and wired through #30b on the terminal.
http://forums.pelicanparts.com/uploa...1201317163.jpg
© Dr. Ing. h.c. F. Porsche A.G. Posted here for educational purposes only.

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.
http://forums.pelicanparts.com/uploa...1201317196.jpg

Finally, here is a simplified wiring diagram for the micro-switch and RPM transducer.
http://forums.pelicanparts.com/uploa...1201379656.jpg
© Intereurope Workshop Manual, Porsche 912, 911T, 911E, 911S, page 62 (October 1972). Posted here for educational purposes only.

David -

I have recently had to troubleshoot and replace my RPM Transducer, and I think there is a change that needs to be made in the text.

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 (not coil).

How about also including a pic of the time limit switch for reference?

Jim

Good catch Jim,

The RPM Transducer gets its RPM signal from the coil but to say it is "connected to it" is confusing. I modified the sentence so that it is less confusing. I'm at work and I can't locate a picture of the micro-switch. That is what you are referring to as a time-limit switch, isn't it? (We all use so many diffferent names)! I'll see what I can find when I get home.