MFI RPM Transducer Problems

[Biff]

I'm continuing to work out bugs in MFI system from 20 year storage on '72T. I've got it to run pretty good after new rebuilt distributor, fuel pump, mfi pump, plug wires, ect. ect. A recent back fire problem lead me to investigate rpm transducer which I found a transistor detached bouncing around inside the aluminum case. Had an electrical engineer friend replace with new transistor, plugged it back in and readjusted the throttle switch. The rpm transducer now works but the car no longer runs below 2000 rpm....it doesn't idle. There must be something else wrong with the rpm transducer?? After putting in transducer back in Thursday evening my wife called me while I was away Friday and told me there was a smell of smoke in garage (she said it smelled electrical) so she disconnected neg. ground strap on battery...good wifey!! Any thoughts??

[Flieger]

Can you describe you symptoms below 2000 rpm?

I repaired my transducer and 14 pin connector. I documented the process in a thread which I linked into the ultimate mfi resource thread.

I will try to find you the link.

[Flieger]

Here is the Jeff Higgins thread where I documented my repair:
For MFI Gurus. . .and friends of Warren . . .



Here is the "Ultimate MFI Resources Thread":
Ultimate MFI resources thread

[Biff]

Below 2000 RPM's the car shuts down completely.

[304065]

First of all, Warren Hall posted a ton of information on this, I assume you have read every thread with the words "speed switch."

The thing is, the speed switch is a Schmitt trigger-- it triggers when rpms fall below a certain point and then stays off until they fall below another point, where it switches back on.

If yours is sensing when the RPM falls below 1500 and it turns ON and cuts the fuel, then RPM falls below 1300 RPM it's supposed to turn OFF, no power to the solenoid, which releases the rack and the car continues to run. Either your EE buddy buggered the speed switch when it was repaired (a trigger level of 2000 suggests the wrong components were used, these are VERY critical to the timing) or the rack is sticking in the "FUEL OFF" position.

Easy enough to check with a test light on the solenoid. Alligator clip on either wire to the solenoid, test light to ground. Have a helper operate the engine while you note the exact RPM reading where the light kicks in and out.

Good luck!

[Flieger]

If the resistor value is wrong, the bottom RPM limit can be too low and the engine will not "catch" the idle on the downswing. The fuel cutoff must be too high, too, by about 500 RPM.

You should be able to bring the revs to 2000 RPM, holding with the throttle pedal. Then have a helper depress the microswitch. This should trigger the solenoid to shut off fuel (with the switch depressed) until the RPMs reach about 1200. Then the RPM Transducer takes the microswitch signal out of the loop and the idle fuel returns to keep the engine from stalling, whether the swith is pressed or not.

Make sure your microswitch trigger screw is properly adjusted on the throttle arm in the engine bay (only closing the switch when you are off throttle)

If you want, you can tune the resistor value in the RPM Transducer to make the fuel shut off at most any RPM value you choose and raise the lower limit to prett much any value below that.

Take a look at the threads.

[Biff]

Thanks for all your help so far! The microswitch on throttle is working-that is a simple on/off and functions properly (adjusted properly). I obviously have more issues with the circuitry of the transducer. I'll try to sort through the methods on how to test the transducer, my EE buddy has all the testing equipment available, just have to set up a testing board? We'll see what happens, while I'm waiting, I contacted a friend whom has 4 spare mfi rpm transducers - so I'll try those and confirm that the wiring in the car is good.

[Grady Clay]

Warren posted a circuit diagram of the RPM transducer. Your EE friend will want that.

Best,
Grady

I miss my friend Warren Hall. We had lots of 1/2 done projects.
Hope to see him again someday.

[sithot]

http://www.systemsc.com/

[304065]

Originally Posted by Grady Clay

Warren posted a circuit diagram of the RPM transducer. Your EE friend will want that.

Best,
Grady

I miss my friend Warren Hall. We had lots of 1/2 done projects.
Hope to see him again someday.

Hopefully no day anytime soon! Hope you are well and enjoying.

[Flieger]

Here is said diagram. I owe Mr. Hall a lot of thanks. His diagram helped me a lot.

[JohnOllen]

I just wanted to add a note of appreciation for the info provided above, especially to Flieger. I decided to tackle my RPM Transducer which has not been working on my 72 911T for years. I had tried years ago to repair it when it suffered the typical problem of all transistors falling off, but had no success. After reading the above posts I decided to give it another go. This time I replaced all the transistors and moost diodes with the ones suggested and replaced D1 with a zener diode 8.2v 1w, also as suggested. Still no luck. Last night I decided to try pulling C6 as Flieger had found worked for him. It worked! I can't say definitely that it was pulling C6 alone that did it as I also cleaned up some suspicious solder joints and replaced D6, although once off the board the old one tested fine, so I suspect it was C6 that made the difference. Thank you all so much.

[72sdriver]

Hello Folks

Old thread, but still as importent

I want to repair the RPM Transducer relay myself. It has all the problems everyone else has posted, With loose/rusted/broken off transistors and so forth.

What komponents should I buy and replace?
Only the ones that look bad? Or all the diodes and transistors?

The last post talks about pulling C6(Capacitor), you mean removing it or replacing it.
All the capacitors has rusty or similiar legs.

What do you use to fasten the New Components, so they dont stand in free air and shake loose againg?

Thanks, Marius K. Norway

[304065]

Marius-- read all posts by early_S_man containing the words "speed switch"

such as this one

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

[72sdriver]

Hello folks

Yes, i think i`ve read everything, so much I almost cant see the forrest for all the Trees
Or maybe the Circuitboard for all the components

I want to Place an order for all the Components I might need.
What do you guys use? The old style Transistors TO-18 Cantype for authentity or modern for less trouble.

The Transducer is not functioning at all and all the wiring is checked and Works.


Picture of my transducer.

Thanks, Marius K.

[72sdriver]

[tharbert]

Marius, you need to remove all that brown gunk with a dental pick, the stuff that's used to secure the components? And, be sure to inspect the tall diodes closely right up under bodies. I had two with broken leads up high. That gunk may hide more damage as it did on mine.

Being just a novice, I simply followed Warren's suggestions and my transducer works fine. I got all my replacement bits from Jameco. It was like less than $10.00. One suggestion my electrical guru made was to put an adjustable pot at R5 and R6 to set your own parameters.

[dicklague]

I know this is about fixing this component.

I have been running my 2.7 MFI engine for over a dozen year without an RPM transducer.

I should take it off the shelf and see if I can make it work, so I at least have the original part around in working condition.

[RSTarga]

I also ran without mine for a couple of years with no ill effects. I think it reduces backfires and fuel consumption a bit. Nice to have but not essential.

[72sdriver]

Hello folks.

I bought all the components to fix my old relay. But life has been very busy and i haven`t got around to fix it.
SO, I bought a new one. New production from Bosch. Part nb. F 026 T00 002. 98£ on UK Ebay. Included shipping.
I installed it a couple of weeks ago and had a few trips already.
I AM VERY HAPPY WITH IT
My experience. Engine rpm drops rapidly when i let of the throttle. It is very noticable.
It then stabilize at idle quickly. You can feel the fuel comes on at about 1200rpm again.
Engine feels a lot more responsive.
Last year i drove for some time with a lamba in the exhaust and did some adjustement.
This told my that when i let off the gas and was coasting. The engine was running ritch.
As soon as i applied throttle is was then to ritch and not as responsive initially.
I always did get the plugs fouled a little. We have a lot hills and therefore a quite a bit of coasting. I am shure this will be better now. It feels like that, now at least. Time will show.