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Thread: 4000 ppm to speedo lt1 94

  1. #1
    Electronic Ignition!
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    4000 ppm to speedo lt1 94

    Trying to understand the 4000 ppm output.
    Does the ecm send 4000 ppm to speedo when you change values after tire change or change output ppm to example 4030 ppm ?

  2. #2
    Fuel Injected!
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    The PCM outputs 4000ppm once you do the speed calibration correctly.


    Actually, when I think about it, the PCM might output 2000ppm for the speedometer. I can't recall anymore, I set my Dakota Digital to the 2000 or 4000 pulses during it's calibration and then setup the PCM speed calibration and the PCM internal speed and speedometer both match GPS.

  3. #3
    Electronic Ignition!
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    The pcm pinout show 4000 ppm on 2 chart i looked at.
    So pcm strecthes or shorts the pulse to match speedo ?

  4. #4
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    Can you can post the year/make/model of the vehicle(s) that you're working on or that donated parts? Over the years GM has used a few different strategies to drive the speedometer.

    Around 94 it was common for the pcm to drive the speedometer. PPM stands for pulses per mile so the slower you go the lower the number of miles in a minute and the lower the number of pulses in a minute.

  5. #5
    Electronic Ignition!
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    Its a 350 lt1 from corvette c4 1994 and pcm 16181333.
    I installed it in my el camino ss 1970 for 10 years ago.
    Bought aftermarket tacho and speedo (china) , the old was from a van .
    Trying to program the speedo from pcm output.

    Tire rev/gear ratio= tail shaft revs.
    4000 ppm/tail shaft revs= pulses per revolution (ppr).
    Tire revs*ppr/60=???.
    speedo frequecy/???= the value for speedo (on my speedo),
    Last edited by honken; 03-01-2021 at 02:28 PM. Reason: ens

  6. #6
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    Got it.

    Looks like input to OE speedometer is 4000 pulse per mile. Your math should look like this:

    4000 pulse per mile / (tire rev per mile * gear ratio ) = speed signal output pulses per driveshaft revolution.

    Shouldn't need to divide by 60.

    Does the computer currently know the correct vehicle speed?

  7. #7
    Electronic Ignition!
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    The eehack show the speed.
    But not speedo gauge.
    I dont think its going to work tryed different calculation.
    Need to wired me to vvs signal from vss sensor its yellow on pin A32 ??

  8. #8
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    If the speedometer uses an A/C signal you should connect to both VSS wires. In order to use one you need to connect the other to ground. The pcm will lose 1/2 of the speed signal if this happens.

  9. #9
    Electronic Ignition!
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    Speedo works.
    Wired in to pin A32 eehack shows right
    Got error code 97 because there no speedo on wire.
    uncheck Vss Signal Diagnostic (Error 97) i tuner pro and flash ccm.

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