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1990 K-5 speedo calibration

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by shawnboy, Jun 22, 2001.

  1. shawnboy

    shawnboy 1/2 ton status

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    I've just purchased another truck. This one is a 1990. It has a NP-241 transfercase. How do I recalibrate the speedo for 33" tires. I was told you screw up your shift points and will get lowsy performance otherwise.
    Thanks, Shawnboy.
     
  2. GRINCH

    GRINCH 1/2 ton status

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    Yes you must get it recalibrated. I finally went to a speedo shop north of atlanta to get mine done. They replaced some box under the dash. I'm sorry but i knew what it was called but now I forgot. Cost me 130.00 well worth it. I will try to find the number of the shop that did it and post it.

    I tried the dealer route with no luck. 1990 was diferent than any other year so I was told.

    The Few, The Proud, The K5 Blazer
     
  3. 90K5

    90K5 1/2 ton status

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    VSSB...Vehicle Speed Sensor Buffer. Thats probably the box.

    90K5

    See my truck at <A target="_blank" HREF=http://albums.photopoint.com/j/Albumindex?u=1329584&a=9886502>http://albums.photopoint.com/j/Albumindex?u=1329584&a=9886502</A>
     
  4. 90blzr

    90blzr 1/2 ton status

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    So every vehicle taht has oversized tires you must replace that box? Even with a gear change? Where would one go to get this done or buy it from?
     
  5. GRINCH

    GRINCH 1/2 ton status

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    Check your phone book for Speedometer Calibration. I could only find 2 in Ga. Also you may check with the local police department. Yeh I know right!!! Really they have to get theres calibrated. When I was at the speedo shop there was 8 cars there.

    The Few, The Proud, The K5 Blazer
     
  6. 84_Chevy_K10

    84_Chevy_K10 Banned

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    The shift points are regulated by the TV cable on the transmission. It's not electronic, so that has nothing to do with it.

    That said, I'd love for my speedo to be accurate. My truck came from the factory with 2.73 gears and 235/75R15s. Now it's got 4.10s and 35x12.50R15s. My speedo reads about 30% fast. It's a PITA.

    Tim
    '84 Chevy K10, lifted, loud, fast, and 3/4 ton axles
     
  7. 502Burban

    502Burban 1/2 ton status

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    Superlift makes a speedo recalibration unit, but your fix may be even less expensive and more simple. Check out this page on their website for details: http://community.webshots.com/user/robbrj
     
  8. woody9

    woody9 1/2 ton status

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    I was trying to find that product myself for my 91 and didn't have any luck finding it on SuperLift's site! Thanks for the post!
     
  9. 502Burban

    502Burban 1/2 ton status

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    No problemo. Glad I could help. [​IMG]

    502Burban
    <A target="_blank" HREF=http://community.webshots.com/user/robbrj>http://community.webshots.com/user/robbrj</A>
     
  10. 84_Chevy_K10

    84_Chevy_K10 Banned

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    What about my NP 208?

    Tim
    '84 Chevy K10, lifted, loud, fast, and 3/4 ton axles
     
  11. 502Burban

    502Burban 1/2 ton status

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    Not absolutely sure on this, but I believe you can correct your NP208 with a different drive gear in the t-case. If you count the teeth on the drive gear and increase them by 30%, this should give you the correct number of teeth for the replacement, which you should be able to get from GM.

    If anyone has any better info on this, please help out, since my experience is mostly with the electronic sending units. I sorta remember seeing this topic posted before; you may want to try a search to see if you get any better info. [​IMG]

    502Burban
    <A target="_blank" HREF=http://community.webshots.com/user/robbrj>http://community.webshots.com/user/robbrj</A>
     
  12. woody9

    woody9 1/2 ton status

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    502-burban.... 'how' experienced are you with the elctronic ones? I changed my 91 sub to 4:10s and now the speedo seems 20% off.. faster! (read putting more miles on!)...1 shop in Cal (via web/phone call) said there'd be a little white box behind glove box and if a 14 pin, it could be reprogramed. Well, it wasn't there.
    ALso, checked the superlift site, but bythe time i called, seems they'd left for the day (or maybe they do just M-F)...
    Any suggestions?
    Also called the dearler here, and they said they'd have to check and see, and get back to me on it... Don't sound promising, but this is RURAL wisconsin!
    Thanks in advance!
     
  13. 502Burban

    502Burban 1/2 ton status

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    Not terribly experienced, I'm afraid. I had this problem with a Thunderbird SC I used to have. Changed wheel/tire combo and threw off the speedo. Just remembered seeing the Superlift unit back then while looking for a fix for the T-Bird, did a search on Google.com and found the page I posted.

    From Superlift's page:
    "1) 1988-91 full size These models require the installation of a reprogramming clip into the back of the instrument cluster housing. The relatively inexpensive clip and reprogramming tool are sold by your local GM dealer. Reprogramming instructions are included with the Superlift lift system. Labor time is under one hour."

    Sounds like you should be able to get what you need to do this from your local GM parts counter.

    On all my non-computer controlled vehicles, I've always run Autometer gauges, which have a built-in calibration sequence that allows you to recalibrate when you make tire or gear changes. Depending on how much it would cost you to fix your stock speedo, you might just want to swap in an Autometer gauge and eliminate the problem for good. I found a really clean install by a guy named Mike that I will be doing to my Suburban. Here is the link, if you are interested: http://community.webshots.com/user/robbrj
     
  14. DERBST

    DERBST Registered Member

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    The item you need to change is called the "DRAC", it is a small electronics box under the dash just below your radio. It costs about $100. You can go to your local Chevy dealer and ask them to help you order one. The folks will need your tire circumference and the gear ratio in your axles.
     
  15. shawnboy

    shawnboy 1/2 ton status

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    Thanks for the info Rob.
     
  16. Thunder

    Thunder 3/4 ton status

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    1st I'd like to say that the 700r4 in your 90 is not electronically controlled except for the torque convertor lock up. So calibrating the spedo will have no effect on the shift points. The shift points are contoled by the TV cable. This is located and adjusted next to the throttle cable on the TBI. The TV cable is also used to controll the fluid pressure in the tranny. So it must be adjusted properly to keep the tranny healthy. Get a manual to do this properly.
    However,Becaust the computer uses vehicle speed to maintain proper engine performance (and gas milage).You should recalibrate the spedo if you change tire or gear size by 10% or more. The easiest way to do this is to change the spedo drive gear in the transfer case.It is an easy job that simply requires the removal of 1 small screw and hold down clamp then pulling out the gear assembly. You can get the gear from any spedo shop. Just tell them what size tire and gear ratio. This is far easier than pulling the dash to get to the chip on the spedo.
     

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