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What does the VSS (Vehicle speed sensor) do?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by PetaKane, May 24, 2002.

  1. PetaKane

    PetaKane 1/2 ton status

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    I'm having a little bit of trouble with the installation of my electronic speedometer. No troubleshooting guide in the instructions, but I get the feeling it has something to do with my VSS. Can anybody explain what exactly the thing does? And does it need to be hooked up for the speedo to function?
     
  2. abusedtrucks

    abusedtrucks Registered Member

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    I haven't dealt with this on my Chev's yet, but I did have to deal with it on my 96 Explorer. On there, that thing controlled damn near everything other than the air pressure in my tires.

    It controlled the speedometer, cruise, and how the vehicle shifted. I think the cruise just didn't work because the truck never knew how fast it was going since the speed sensor was out.

    Not sure what else it controls. I just know that for $40, it sure fixed a lot that was wrong with that thing.
     
  3. PetaKane

    PetaKane 1/2 ton status

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    Thanks for the reply. In case anybody else reads this and can offer some advice, I'll shed some light on the problem I'm having. I installed an Autometer electronic speedo and had to get a sender as well. It has a connection for the VSS and I just cut all my wires for the Vss as I thought they would just be in the way and I didn't want a big mess in my dash. Now I'm in a bigger mess, all of my gauges work except for the speedo, lights up but the pointer will not move no matter how fast I go. I must've been going about 160mph and it still didn't budge. Has anybody ever done an electronic speedo install before?
     
  4. abusedtrucks

    abusedtrucks Registered Member

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    Got any electrical tape? hehe... you'll need them wires you cut out of the way.
     
  5. PetaKane

    PetaKane 1/2 ton status

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    Lol, I knew that was coming. ! I'm going to find those wires right now in the dark and see what I can do.
     
  6. BurbinOR

    BurbinOR 3/4 ton status

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    Keep the VSS. It is a crucial part of any computer controlled vehicle, affects many functions. Many. Lots of TPI swapper guys do not use it and it works OK, by jury rigging the wiring and fooling the compujter, but not properly. Figure out a way to keep it and make it work.
     
  7. PetaKane

    PetaKane 1/2 ton status

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    Okay, now that I know I need them, does anybody know off-hand which wires are the VSs wires, I can't seem to find them now. Do I need to plug them all into my speedo?
     
  8. YtseJam

    YtseJam 1/2 ton status

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    Your vehicle speed sensor is like your governer on your transmission (AT) The VSS senses road speed. It than sends a signal to the vehicle speed sensor buffer or PCM and on to the instrument cluster. I'm a bit confused as to what exactly you have already done. Let me know if I can assist further, add some more info...
     
  9. azblazor

    azblazor 1/2 ton status Premium Member

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    OK, here is what "Chevrolet TPI & TBI Engine Swapping" says:
    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.
     
  10. PetaKane

    PetaKane 1/2 ton status

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    Instructions from Autometer...."This speedometer provides an output terminal that can be used with late model vehicle installations using the existing vehicle speed sensor. In these installations, the VSS signal wire should be connected to the SIG terminal of the speedometer only. The OUT terminal should then be connected to where the VSS output was originally wired. This will provide a buffered VSS signal to the PCM/computer in the vehicle." So basically, if anybody has a clue which wires these are (what color they are, where the come from/lead to, etc) it would help me tremendously. Thanks to everybody who's helped out so far.
     
  11. dyeager535

    dyeager535 1 ton status Premium Member

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    Which late models? : )

    IIRC, early GM VSS ('81-90 or so?) was a 2 pulse system, then went with 4 or something even higher. I'd imagine the retrofit stuff would be 2 pulse, but that is definitely something you'd have to find out first, it's my understanding they are not interchangeable, which makes sense : )
     

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