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Thread: Need a signal source for the MFI RMP Speed Transducer

  1. #1

    Need a signal source for the MFI RMP Speed Transducer


    I think I've finally tracked down the issue with my deceleration backfire. After successfully verifying the micro switch works, I discovered that the RPM transducer was not working. I sent it out and had the circuit board rebuilt but that did not solve my issue. I ran the test where you bring the rpm up to 2500 and the depress the micro switch. Nothing happened. So then I went about running continuity tests throughout the system, only to discover that the signal wire to the RPM Transducer is not connected. You can see in the below picture that there is no wire in the plug that leads to the transducer which would carry the tach signal. Can anyone tell me where I can pick up this signal and send it safely to the RPM Transducer?

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    Thanks in advance,

  2. #2
    Looks like you have a 69/70 911….but I don’t have a rectangular plug like you ?
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  3. #3
    Hello, It’s a 1974 and I do have the plug going into the bottom of the transducer. It’s circled in the top picture but I have the transducer removed so you can see it. I need to know what is the source wire for the purple and black wire that goes to it.


  4. #4
    Nice diagnostic work Miked.
    On my '72 the black/purple wire is plugged into an external spade on the distributor then up to rear the multi-pin connector.

    What's odd looking to me on your setup is you have the round RPM sensor installed which was in place on '69-71 models that had the earlier version of tachometers. I'm Not sure that (or the fact you have a '74) would change any of the wiring I mentioned above. Do you have a black/purple or another colored wire plugged into your distributor?
    Last edited by nickb4; 08-20-2023 at 06:41 AM.

  5. #5
    Thanks Nick! I do actually have the rectangular RPM transducer (pictured below). What is the cylindrical relay? Are you saying that I can get the tach signal directly from the coil? I only have a black and red wire coming from the distributor.

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  6. #6
    Senior Member
    Join Date
    May 2006
    The black/purple from the distributor feeds into the RPM transducer and to the tach. Here are some photos of the RPM transducer connections from my '70E. You need to connect the black/purple wire that feeds your tach to your RPM transducer.
    Attached Images Attached Images    
    Tom Butler
    1973 RSR Clone
    1970 911E
    914-6 GT Clone in Progress

  7. #7
    Thank you both! I only have a black and a red wire coming from my distributor which is what’s causing my confusion. Since the car was restored from boxes of parts, I believe that the electrical panel was sourced from another car, which could be why there is a cylindrical relay still on the board.

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    It looks like the black wire transitions to black/purple so now I’ll trace that.

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  8. #8

    That did the trick! The car now shuts off fuel on lift off throttle. Thanks one and all for the help.

  9. #9
    Could you tell me more about your wiring ?
    My transducer is also working but at idle the RPM signal seems to be bad and the engine is running very badly at idle (and everything is ok if I disconnect the transducer).

    On my car there is a black/purple wire from the distributor and going directly to the tachometer.
    And there is a black wire coming also from the distributor that transitions to a black/purple wire going to the CDI.

  10. #10
    If that's the only variable you are changing then I would send the switch out to be certain that it isn't triggering the fuel cut off below 1500rpm. When the engine is running at that rpm the transducer should know not to interfere, even when the micro switch is engaged.

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