WARNING - All '69-on MFI Owners!!! Preventable Failure of $283 Speed Switch module!

[Early_S_Man]

I have learned of a second module failure since May 2001 involving breakage of transistor leads in these modules -- the most likely cause is vibration and mechanical resonance of the unsupported TO-5 and TO-18 all-metal transistor cases! The symptoms of a defective module are BACKFIRING ON THROTTLE-OFF OVERRUN!

Since NO REPLACEMENTS for these Bosch transistors are available at your local Radio Shack, I recommend that precautionary measures be taken to prevent breakage of the transistor leads immediately!

The module should be removed and disassembled by carefully bending the four aluminum tabs away from the plastic connector 'shroud.' Remove the aluminum cover. Locate the 1 large and 4 smaller shiny metal 'cans' with three wire leads -- cylindrical with a welded flange at lower edge.

What I suggest is to put a couple of 'blobs' of hot glue gun glue or 'Shoe Goo' or Plumber
s Goo' on each transitor to stabilize -- either to an adjacent component, or to the board.

The following pics may be of some help to familiarize yourself with the internal components on the board.






I will post a pic of a modified/stabilized board after I get the circuit traced out and some basic tests done with pulse generator and digital storage oscilloscope!

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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

[Wayne 962]

When I used to work at Hughes Space and Communications, we had this problem all the time on satellite equipment. The secret is to indeed bond the board with some type of adhesive. We used a grey, hard form of RTV. Is the glue gun material hard enough to temper the resonance? Maybe? I'm not sure - I don't have one of these units on hand...

-Wayne

[Early_S_Man]

Wayne,

The hardened glue has a resiliency similar to polyurethane used in anti-roll bar bushings -- it will give if pushed in with a fingernail, but it is relatively stiff in a column of 1/8" or more diameter. The 'Goo' products are similar, firmer than silicone rubber RTV compounds, not quite as stiff as the hot glue.

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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

[Early_S_Man]

I have seen some mfrs of test equipment use blobs of epoxy, but that is very difficult to remove for repair, and others have used white silicone rubber calking compound, but it is also messy to clean off. I have some F-111 terrain-following radar boards made by Texas Instruments using a clear conformal coating over all of the discrete components, but that, too, can be difficult to repair, as any military avionics technicians can testify.

What I had in mind was two or three blobs between 1/8" and 1/4" in diameter down to the board from each transistor, and run all the way up the side of each cylindrical case, making sure that it adheres to the case. The following pic should give an idea what I have in mind:




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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

[Roland Kunz]

Hello

Had seen one unit in acryl only leaving the units free to help cooling. nearly flush with the podest hight from the relay just ending under the cans from the transistors.

Preowner from that car was a Bosch employe maybe thats the reason.

Grüsse

[cegerer]

Warren, you continue to amaze me. You really should write a book. -- Curt

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1971 911T - 2.5L Big Bore

[wayne savage]

On my car I was getting a very loud backfire when I tapped the thottle after going down a hill.. scared the hell out of me and other drivers. I disconectd the micro switch and no more problems. of corse, no fuel shut of when coasting.. Is there any problem of running this way???? Wayne Savage 72S

[Oldporsche]

Warren,

In you estimation could the actions of this component be programmed into an eprom?

David Duffield

[Early_S_Man]

Wayne S. ... I don't know about 'damage' per se, until your muffler or air cleaner assembly explodes, but something isn't right about your mixture and your system! I suggest some methodical troubleshooting and getting a Gunston Gas Analyzer to be sure about your mixture.

David,

An Eprom alone wouldn't do it, a PIC, Eprom & relay, yes. Sometimes, thae analog circuit does have advantages over pure digital.

Last year, when a question came up about this module, I figured a couple of 555's and a 450-series relay driver would be the way to go, but things were too hectic to pursue it then. The thing is, ordinary consumer-grade ICs wouldn't last or be reliable under engine compartment temps in the summer, so military grade IC's would be needed, and costs go up. My thinking now is that it is an over-engineered circuit, and only the minor circuit board packaging problem is why we are even discussing it. The design has worked quite well for over 32 years, now! It certainly hasn't caused anywhere near the problems to gain a 'reputation' like the DME and fuel-pump relays of the later cars, and that speaks pretty well for the 1960's technology and engineering of the whole MFI-system!!!

For example, the largest transistor is reportedly a 1 Amp rated type, but as a relay driver is is only switching 140 mA to operate that relay ... pretty good safety margin, if you asked me!

Well, back to the 'Reverse-Engineering' grindstone ...
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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

[This message has been edited by Early_S_Man (edited 09-01-2001).]

[Kurt B]

See, now if I lived in TX, I'd have been over and Warren's place hanging out and learning stuff instead of talking to him in ICQ!
I'm sure leland and some of you other guys would be there too.

[pwd72s]

Warren...just wanted you to know that I've printed this thread, also e mailed this info to some I know who also run MFI...and thank you very much! Paul

[Early_S_Man]

For those that aren't familiar with the related May 2001 threads, or the function or operation of the Speed Swich, Speed Relay, or RPM Transducer (SAME MODULE -- factory documentation referred to this same module all THREE ways over the years) ... here are earlier threads that explain the function and some troubleshooting hints about it:

http://www.pelicanparts.com/ultimate/Forum3/HTML/009064.html

http://www.pelicanparts.com/ultimate/Forum3/HTML/009138.html

http://www.pelicanparts.com/ultimate/Forum3/HTML/009170.html

http://www.pelicanparts.com/ultimate/Forum3/HTML/009218.html

I have bumped all of them to the current page ... for casual reading.

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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

[Early_S_Man]

UPDATE --

Apparently all of these speed switches are destined to fail because of corrosion induced by a lack of sealing from the elements and the original adhesive compound used to hold the long-legged capacitors to the board and in some cases -- adjacent transistors! While contact has induced rust on every transistor in my unit, the adhesive qualities are gone and the compound had hardened to the consistency of brittle hard chocolate! Handling to trace the circuit resulted in two leads of the largest transistor fracturing due to rust underlying the gold-plated iron/steel leads! The nickel-plated case was rusted only where contact was made with the brown adhesive.

The largest Siemens transistor, type BSX45-25 seems to be quite similar to a USA-spec 2N2102 transistor and after a socket with gold-plated contacts is installed, I will replace with a vintage RCA or Motorola 2N2102.

It turns out that the emitter-base junction of one of the smaller BCY58-1X transistors is being used as a Zener diode to regulate the supply Voltage to 8.4 Volts for the timing circuits. Variations in that regulated Volatage, and the actual value of C4 require selection of the values for R5 and R6, which are unique to each unit, and replacement of D1 would require that new values be found by testing!

The circuit diagram of the RPM Transducer/Speed Switch is shown below:






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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

[98GTW]

Potentially dumb question:
Does CIS count as MFI? Would this module or its equivalent exist on a 77 2.7?
Thanks for indulging me.

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Dave
'77 911S to be
inari77s@911fanatic.com
Rochester, NY

[Early_S_Man]

No, Dave, while there some semantic justification for calling the CIS system a mechanical fuel injection system, it isn't 'the MFI' system used on selected '69 thru '73 USA cars, and ROW RS models thru '76!

So, don't worry, you don't have to add the Speed Switch to your concerns!

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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

[98GTW]

Thanks, Warren,
One less thing to worry about. :}
Only about 55 more hours before I first lay eyes on the car I bought in June !! I've lost at least one night's sleep wondering what sort of interesting problems I'll discover as I get to know the car. I suspect I'll be a regular here on the board very soon.



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Dave
'77 911S
inari77s@911fanatic.com
Rochester, NY