RPM transducer repair

[Flieger]

My RPM transducer/speed switch is faulty. How should I go about disassembly, inspection, and repair? I see that there are a few transistors that commonly break loose from their solder joints due to vibration. Can I just re-solder and then try an RTV coating?

The unit failed the test of raising the rpms to ~2000 with the hand throttle, then manually activating the microswitch while the hand throttle is still raised. The rpms did not fluctuate according to the microswitch. They dropped ~200 after a few momwents and stayed there until I released a little tension on the hand throttle and then pulled it all the way up again. All the while, my friend was pushing the microswitch and releasing it with intervals of a few seconds.

The rpms did not correlate to the microswitch and did not fall and rise again as the fuel was cut off and the idle circuit then catches it on the low end.

The engine is a 2.7 liter RS spec and it was up to temperature when the test was done. It idles a little high when warm ~1500 rpm. It pops through the exhaust on over-run after aggressive driving, in addition to the normal ticking through the intake.

I have the RPM transducer schematic from the ultimate MFI resource thread.

How should I go about fixing the problem? Any info is greatly appreciated.

[Flieger]

motivation for the helpers:

[FenderBender]

Try this For MFI Gurus. . .and friends of Warren . . .

Do a search, the answer is there somewhere.



Romove the transducer, pry up tabs, slide out circuit board, start soldering.

[Zeke]

This is all you need to know thanks to the wonderful info provided for years here by the departed and most respected of our members, Warren Hall, Jr. aka Early_S_Man.

Originally Posted by Early_S_Man

]Do a search on the following 'strings:' 'rpm switch,' 'speed relay,' 'speed switch,' and 'rpm transducer' ... to find all of the relevant postings of the past couple of years. I posted a schematic reverse-engineered from my own unit last year, and suggested an 8.2 Volt Zener diode for replacement of D1 -- one of the transistors in some units was used as a Zener at Location D1, and the board was stenciled 'E,' 'B,' & 'C.' A 2N2222A or ECG-123A can replace the small-cased TO-18 transistors in T1, T2, or T3 locations, and 2N2102, 2N2222A, or ECG-123A can replace the larger TO-5 cased T4. Several board members have repaired their faulty RPM Transducers using the info I posted.

http://forums.pelicanparts.com/showt...ht=mfi+warning

In fact, if you will search 'transducer' and his user name, you will get to "read the book."
__________________
Warren Hall, Jr.

[Flieger]

Thanks for the replies

[Flieger]

I disassembled the unit this afternoon. The contacts all appear to be solid.

I am going to clean the copper contacts and the relays. I will warm the solder joints. Hopefully this will get it to work.

I have not seen this type of 2 relay circuit layout in the photos of others' restorations. Anyone have the same thing?

[FenderBender]

???? Is that the two stage defroster relay ??

[Zeke]

those dirty connections look suspect.

[dafischer]

I don't think that's the RPM transducer...looks more like the rear defrost relay. Also, "Heckscheiben Heizregler" translates as Rear Window Heat Regulator.

______________________________
Dave

1969 911T Coupe
1972 911E Targa

[Jim727]

Originally Posted by dafischer

I don't think that's the RPM transducer...looks more like the rear defrost relay. Also, "Heckscheiben Heizregler" translates as Rear Window Heat Regulator.

Genau!

[Flieger]

Ach du Lieber

[Flieger]

This box was in the #6 defroster location in the diagram, after all. There was nothing in the #5 RPM transducer location on the diagram. I now see why the motor was backfiring on engine braking.

Anyone have a working RPM transducer? Trade for a 2 stage rear defroster. I have Lexan windows. I don't need a defroster but I would like an RPM transducer.

[Jim727]

Otto's, in Venice rebuilds them. 310-399-3221

You will be very much in their better graces if you can find something at a recycler to provide them as a replacement to rebuild!