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1991 fullsize chevy blazer speedometer calibrating

Discussion in 'The Garage' started by scottnj, Jan 5, 2004.

  1. scottnj

    scottnj 1/2 ton status

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    I have larger tires on my blazer and of course the speedometer is off by about 7 mph. Varies depending on speed.
    What is the best way to correct this?
    Thanks
    Scott
    scottnj@cfl.rr.com
     
  2. woody9

    woody9 1/2 ton status

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    this mentions some of the fix.. Superlift Fix
    but this one gets a bit techie: Techie Stuff
    I was able to get the adapter kit (about $35?)from the dealer.. but haven't gotten ambitious yet and got behind the dash to install it. Seems it's a small adapter that you clip the pins to achieve the desired settings that plugs in 'in-line' with the connectors.
     
  3. dyeager535

    dyeager535 1 ton status Premium Member

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    DRAC is what you have. Under dash near the ash tray. Superlift site shows the one you will have, white plastic housing.

    Not available from GM anymore apparently, don't screw yours up. That article on doing it yourself is gold I tell you, Gold! /forums/images/graemlins/smile.gif
     
  4. scottnj

    scottnj 1/2 ton status

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    Could you send me a picture of this. Also do you have the gm part number for the kit?
    Send to scottnj@cfl.rr.com if you would. They two links you said were awesome.
     
  5. scottnj

    scottnj 1/2 ton status

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    Is the drac what I have. I will check it out this weekend. Seems as easy as a jumper on a computer...well kinda of haha. Thank you all for your help!!!!
     
  6. R72K5

    R72K5 Banned

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    search usign google.com for speedometer calibration and related terms and youll come up with all kinds of sites and pages on how to do this,
     
  7. K5 NUTT

    K5 NUTT 1/2 ton status

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    ISS 800-333-7976 DRAC $85...info needed...gear ratio...tire circumfrance...and they will burn you a new one...never did ask if they could reburn the one you have though...
     
  8. scottnj

    scottnj 1/2 ton status

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    I cannot find it. Do you know exactly where it might be.
     
  9. Sidepipes

    Sidepipes 1/2 ton status

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    I just did mine, check under the radio. Mine was mounted there in what looked like a factory mounting clip. MY harness and drivetrain are outof a 90-91 so it should be the same. Otherwise it is said to be near the computer which is just above your glovebox.

    Mine works excellent. just cut two jumpers and soldered in a small piece of paperclip for the one that needed to be added.

    I find my truck is driving a lot nicer too, meaning the tranny is shifting better and at more proper points and better feel.
     
  10. scottnj

    scottnj 1/2 ton status

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    ok I will check it out tonight and look near the computer. I know where that is when I replaced the chip.
    How big is this box I am looking for? and is it white?
     
  11. scottnj

    scottnj 1/2 ton status

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    I found it. It is mounted right below the stereo.
    Clipped to the metal part of the dash. Thank you all for your help. Next step soldering!

    Here are some pictures. Scroll down to the bottom of the site

    Click here for my Blazer site [​IMG]
     
  12. scottnj

    scottnj 1/2 ton status

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    Completed.... tranny shifts smoother, that was a easy thing to notice. Speedometer is changed, should be alot closer then it was. Have not tested that yet. Will do that later, we have a street here that has a radar speed sign.
    Thanks everyone!!!

    [​IMG]
     
  13. scottnj

    scottnj 1/2 ton status

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    Does anyone know if 1996 and 1998 chevy trucks also use the drac system? If so where are they located?
     
  14. dyeager535

    dyeager535 1 ton status Premium Member

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    I would be surprised if DRAC was used much longer than '91. ECM's could easily handle that function, it was just a bandaid until they could add that function I bet.
     
  15. scottnj

    scottnj 1/2 ton status

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    I am thinking now that maybe in 1996 when they switched to obdII systems is when they stopped using them.

    I guess the only way to calibrate the speedometer in 96 and newer trucks would be that add on thing from superlift
     
  16. dyeager535

    dyeager535 1 ton status Premium Member

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    Well, unless GM is only using one tire size on all of their vehicles now, there HAS to be something that can be changed to compensate for the different tires, right? /forums/images/graemlins/smile.gif
     
  17. scottnj

    scottnj 1/2 ton status

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    Thats true. Wonder what it is or do you think it is still the same item. I will look over my friends 98 chevy 4x4 soon and see what I can come up with. Hopefully it is a drac. He has 38's
    1998 Chevy 4x4 website
     
  18. dyeager535

    dyeager535 1 ton status Premium Member

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    I really don't know for sure, but I seriously doubt it's a DRAC. The way anti-tamper stuff is, they probably got rid of it for that reason, plus all that extra wiring and components are additional expense.

    I would think a "newer" full size only forum would have the information you seek. Putting bigger tires on is obviously done quite often with the newer trucks. Of course, most probably don't even bother correecting the speedometer, but thats their problem.
     
  19. scottnj

    scottnj 1/2 ton status

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    Its not a drac on the 96 and newer trucks.
    It is alkl controlled by the computer itself like you said intially. The computer is programmed with the gear ration and tire sizes at the factory.
    superchips has a handhelp computer you can buy to change all kinds of stuff in the obdII systems including the gear ratio and tire sizes.
     
  20. dyeager535

    dyeager535 1 ton status Premium Member

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    It's kind of funny...you expect them to make things almost tamper proof as time goes on (carb mix screws, torx on the canopies) yet here they are with OBD2, and you can still get in and easily change the speedometer calibration.

    Heck, whats to stop someone from going in and making the computer think you are putting less miles on the vehicle than you really are?
     

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