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Question about speed sensor and it's effect on shifting/tcc lock up

Discussion in 'The Garage' started by foxman, Dec 8, 2003.

  1. foxman

    foxman 1/2 ton status

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    Question about speed sensor and it\'s effect on shifting/tcc lock up

    I have an 88 k5 with 1ton front and rear (off a 74 p/u) They are geared at 4.11 and I have a stock 700r4. I'm running 35"ssr's so the speedometer is quite a bit off.
    While using the "SCAN TOOL" the other day I noticed that It reads the same value on the speedometer on the scanner when in motion. I initally thought that in 1988 the speedometers were driven mechanically and the computer could not take a reading of the speed. My question is... Does the fact that the speed sensor is reading a false speed affect the way my truck is shifting, and if so, How can I correct it? Tha truck seems a little sluggish, maybe due to lock up happening in the torque converter too soon???... Please help!!!
     
  2. MTBLAZER89

    MTBLAZER89 3/4 ton status Premium Member GMOTM Winner

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    Re: Question about speed sensor and it\'s effect on shifting/tcc lock up

    Mine does this too. Shifts early and locks up early. AS far as I have found out if you change the speedo gear it will correct this problem. IIRC 1989 is the last year of mechanical speedo, but it still has VSS which is what is messing with stuff.
     
  3. HarryH3

    HarryH3 1 ton status Author

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    Re: Question about speed sensor and it\'s effect on shifting/tcc lock up

    The VSS signal doesn't affect the shifting on a 700R4. However, it is used by the ECM to determine when to tell the torque converter to lock. So with the wrong info fed into the ECM it could be making wrong decisions based on invalid input. (It's the old GIGO effect of computing. Garbage In - Garbage Out!). /forums/images/graemlins/cool.gif
     
  4. dyeager535

    dyeager535 1 ton status Premium Member

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    Re: Question about speed sensor and it\'s effect on shifting/tcc lock up

    As Harry sai, it will affect lockup. Lockup ocurs based on a few inputs, but one of the things VSS controls is the low speed lokup threshold...with the right amount of engine vacuum, no brake applied, and the preset (factory) lockup speed met by the VSS, the converter will lock.

    So if the speedo says you are going 35 in 3rd gear (example, thats when mine locked up), when in actuality you are going 25, the converter will lock up. Should be able to easily test this on the road by very lightly applying the brake while keeping constant throttle position at the vehicle speed you think is where the problem exists, and watching (or have an assistant do it) the engine rpm's, which should be shown in the scanner. If the engine RPM's jump up when the brake is tapped, then drop back down when the brake is released, the converter was locked up.
     

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