speed switch repair...

derek murray · derek murray's Garage

As I was packing up my speedswitch I discovered a broken connection which is shown on the pictures below.

How easy is this break to fix and how would one go about doing it. Although it still may need cornpanzers handiwork it would be nice to see if this could be repaired locally first.

Tony911Z · Tony911Z's Garage

Derek-

You'll have to find an electronics specialist with acess to cross reference lists. It looks like a transistor and if has markings that cross reference to an available transistor it should only take about ten minutes to desolder and replace your blown transistor.

Early_S_Man · 11-13-2002, 01:37 PM

Derek,

Looks like a fairly easy repair!

That transistor isn't blown ... the gold-plated iron wire just rusted, and the vibration caused the wire to fracture! The easiest repair would be to install a socket on the board, after clipping the other two leads flush with the board. Unless rust causes one of the other two leads to fracture up near the case ... that transisitor should still be usable. If it does prove to be unusable, a 2N2222A or ECG-123A general-purpose NPN switching transistor should work fine.

derek murray · derek murray's Garage

Warren / Tony,

Thanks for your advice. I will let you know how things turn out.

Porschedude · 11-14-2002, 04:23 PM

What is this PCB off of? I had a similar problem with my MFI Solenoid Control Unit. It is the device that activates the rack solenoid full lean when you're above 1500 rpm and the throttle closed, so that un-needed fuel isn't dumped into the engine while coasting down hill. I called Bosch and after transferring around a bit, I got a guy that was willing to send me the schematic at no charge. With the schematic I was able to test the individual transistors, PNP or NPN, but as far as the values, No way. I ended up pulling one at a time and after I was finished re-soldering all in place, I gave it a try. Magically the thing worked and it lasted until I sold the car some 14 years later. I guess it was a cold solder joint.

Anyone interested in the schematic?

Keith

JSDSKI · 11-14-2002, 05:19 PM

Keith, it would be great if you can get it scanned into the MFI archives on this board so everyone would have access to it.
If you don't have a scanner, you can mail to me and I'll get it scanned in and returned to you. Thanks for the offer.

derek murray · derek murray's Garage

Keith,

The speed switch (aka rpm transducer) is from my MFI system and is the same piece of kit you describe.

Please post your schematic, if possible.

Porschedude · 11-14-2002, 07:14 PM

I'll track it down and scan it. I'll find it scan it and post it.

Keith

Porschedude · 11-17-2002, 05:28 PM

I posted the scanned pictures, here is the link:

http://www.picturetrail.com/gallery/view?p=999&gid=1364565&uid=778009

Keith

JSDSKI · 11-17-2002, 07:12 PM

Thanks Keith, saved on computer and printed for MFI looseleaf...

derek murray · derek murray's Garage

Thanks to this board and $5.00 worth of soldering (which isn't pretty BTW) I now have a working rpm transducer. I will do a little glue gun stabilization and move on to the next item on the "to do" list.

Thanks again, everyone...

Early_S_Man · 11-18-2002, 01:51 PM

Sorry, guys ... but, that isn't the correct speed switch or schematic! It is for a 1966 NSU!

I posted a reverse-engineered schematic with all component values in the 'MFI Warning' thread last year ... and corrected the R10 value typo to the correct 220K Ohm value in a recent thread ...

Porschedude · 04:53 AM

Eary_S_Man,

Pardon my disagreement, but did you happen to look at the middle of the page where is says ".001". From what I read on the posted item, it that the .001, .007, .008 are all the same schematic, with different values. The combination of the electronic components would relate to the functional numbers.

What did you find that makes you say that this schematic is wrong?

I'm sure all of us would like to here what you have found to be true.

Keith

Early_S_Man · 11-18-2002, 05:01 PM

Keith,

The Bosch speed switch used on 911's has a much more complex circuit with either five transistors or four transistors and a Zener diode.

If you look at the circuit diagram I posted ... you will note the circuit has more components, and greater complexity than the circuit you posted. Board members have used my diagram to repair several speed switches in the past year or so.

I just examined the third document you posted, and though the Bosch part number is correct for the 911 MFI speed switch ...the circuit is still incorrect! It may have been an early developmental version used on the '66 906E race cars ... but it ISN'T the circuit used on production 911 MFI engines from August 1968!

derek murray · derek murray's Garage

Quote:

Originally posted by derek murray
Thanks to this board and $5.00 worth of soldering (which isn't pretty BTW) I now have a working rpm transducer. I will do a little glue gun stabilization and move on to the next item on the "to do" list.

Update... I removed the "working" transducer and very carefully stabilized the unit with a glue gun... reinstalled it and, of course, it no longer works... I have had it with this, so off to Cornpanzer it shall go... (cornpanzer: an email is on its way)

The upside is I have now removed the complete console and will clean all the contacts and detail (a PO applied undercoating "everywhere").

jeff m · 11-21-2002, 09:41 AM

Derek,
Exactly the same thing happened to me.
I fixed my board, the components were breaking as I tested them.
I fixed it, I was so happy (even a little proud)
Called the wife to tell her the good news, installed it in the car, it worked......once!
My wife is standing there saying "I thought you said it was supposed to......."
Anyway I set up my tester ready for another delve into the board,
Mine turned out to the socket in which the speed switch sits.
This may not be the answer to your problem.
But I suggest you that you give it another try.
It is very likely that the SS is good.

Get some three foot leads with croc clips on each end.
Take the cover off the SS and connect it up remotely.
First check the basics, the 12-14V from the fuse, the ground the ground from the points, then the output to fuel sol (via micro sw)

Only when you are sure should you assume it is still the SS.
Remember you have probably pulled that SS from it's socket quite a few times, the tension in those thirty year old contacts might need a little help. Another pos is the pins on the board they can also suffer from a lot of install/uninstall.

Jeff
911T

Cornpanzer · 11-21-2002, 10:57 AM

Quote:

Originally posted by derek murray

Update... I removed the "working" transducer and very carefully stabilized the unit with a glue gun... reinstalled it and, of course, it no longer works... I have had it with this, so off to Cornpanzer it shall go... (cornpanzer: an email is on its way)

Arrrggghhh....exactly why I hate electronics

If anyone else is interested in this service, my brother-in-law is a 70s era electronics wiz and he has access to any replacement componants needed to repair the board. The cost is very reasonable and simply offers him a little "hobby money" so that he doesnt need to tap the family $$$

Dave Conklin