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Adverse effects from code 25?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by KRAZIE87K5, Sep 9, 2003.

  1. KRAZIE87K5

    KRAZIE87K5 1/2 ton status

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    I'm throwing a code 25 on my ECM, which refers to low VSS readings. I don't have the buffer, so I can't get it rigged up proper like.

    SO, the question is... what adverse effects will have have (if any) seeing that my VSS isn't plugged in and functional?

    Thanks all! /forums/images/graemlins/thumb.gif

    -Dan
     
  2. MTBLAZER89

    MTBLAZER89 3/4 ton status GMOTM Winner

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    IIRC it does have some effect on timing and converter lock-up. Not %100 positive though. TTT /forums/images/graemlins/thumb.gif
     
  3. zcarczar

    zcarczar 1/2 ton status

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    Your TCC wont lock up, but you have a 465 so you dont need to worry about that.
     
  4. azblazor

    azblazor 1/2 ton status Premium Member

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    [ QUOTE ]
    SO, the question is... what adverse effects will have have (if any) seeing that my VSS isn't plugged in and functional?


    [/ QUOTE ]

    This is from an old post I made, I've included the info on the P/N switch because it is related:

    OK, here is what "Chevrolet TPI & TBI Engine Swapping" says:
    Regarding the P/N switch:
    Some wiring harnesses ground the Park/Neutral wire so that the ECM always thinks the vehicle is in Neutral or Park. The reason is to prevent the SES (service engine soon) or Check engine light from being turned on due to lack of a VSS signal. The service manual states: The ECM uses the P/N signal as one of the inputs to control: Idle Air control; VSS diagnostics; EGR. If the P/N wire is grounded in drive, the EGR would be inoperative , resulting in possible detonation.
    Without the P/N switch conected (grounded) the engine will run ok but engine speed may drop excessively when shifting into gear.

    Regarding the VSS:
    The VSS tells the ECM how fast the vehicle is going. Most people think the VSS is only used for for the lock-up torque convertor. The VSS is also used to control the EGR valve, the charcoal canister purge valve, the electric cooling fans, idle speed, and air/fuel ratio.
    It must be emphasized that the VSS is used to control the idle speed when the vehicle is moving. Without the VSS the vehicle may have stalling problems under certain conditions. The reason an engine not equipped with VSS may stall is because when the ECM has the signals that indicate the engine should be idling (foot off gas, vehicle moving less than 2 MPH), idle speed is closed loop ( which is not the same as the O2 sensor running closed loop) and the ECM will try to maintain a programmed idle speed. If the vehicle is moving, the ECM opens the IAC (idle air control) a programmed amount, regardless of engine speed, which is usually a position that will make the engine idle about 50-100 RPM above the programmed idle speed. Stalling can occur when the vehicle is in the over-run condition (foot off gas- engine speed above the programmed idle speed) because the ECM will try to lower the idle speed to the stationary programmed speed. The IAC may not be able to open rapidly enough to prevent the engine from stalling.
    Raising the minimum idle speed with the adjusting screw can eliminate stalling, but the engine will not run optimally without a VSS. Some Chevrolet engines are programmed to run lean under highway mode. Without VSS the ECM will not get the signals to run the engine for best fuel economy. There are a lot of other programs in the ECM which depend on the VSS.
     
  5. dyeager535

    dyeager535 1 ton status Premium Member

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    EGR, idle, deceleration. In general, driveability.
     
  6. Mudzer

    Mudzer 1/2 ton status Author

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    Dan,

    If I can find it - I have a VSS and Buffer which would fit into your factory guage cluster. Its the whole speedo from an 87 up Suburban. Should work fine. Again, I'll have to dig it up - forgot where I put it! /forums/images/graemlins/crazy.gif

    I'll see if I can find it before the run - but Im running out of time!
     
  7. big pappa b

    big pappa b 3/4 ton status

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    Any chance this could have been the cause of your backfiring?? /forums/images/graemlins/confused.gif
     
  8. KRAZIE87K5

    KRAZIE87K5 1/2 ton status

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    Nah... the code 25 is on the new small block... the 454 had a functional VSS...

    -Dan
     
  9. laketex

    laketex 3/4 ton status

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    isn't that the code I was throwing? maybe it means you're 4 quarts low on oil. /forums/images/graemlins/rotfl.gif
     
  10. 84_Chevy_K10

    84_Chevy_K10 Banned

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    [ QUOTE ]
    isn't that the code I was throwing? maybe it means you're 4 quarts low on oil. /forums/images/graemlins/rotfl.gif

    [/ QUOTE ]

    /forums/images/graemlins/eek.gif /forums/images/graemlins/eek.gif
     

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