Bringing TBI and Multi Port Fuel Injection to a New Level.     EFI Conversions and Tuning! Seattle to Portland! E-mail Tuning Consultant!
Results 1 to 7 of 7

Thread: VSS Input/Output Limit?

  1. #1
    Fuel Injected!
    Join Date
    Jun 2012
    Posts
    48

    VSS Input/Output Limit?

    I recently dumped the 16184738 out of my truck. Most things are working OK, but the speedo is off a ridiculous amount. Letting the clutch out and idling in 1st gear puts the speedo over 100. If there was a mechanical connection, then the needle would be bent or broken off by now.

    I've adjusted the ppm and can get the speed close to reality reading the data through Tunerpro, but it has zero effect on the gauge. Upping the correction to 220 or so gets about a 3x reading, going above that will kill the output and it stays at zero. Calculating gas mileage by the odometer reading makes me feel good, who wouldn't like 500 mpg, but it isn't realistic and the gauge needs to work.

    My question is, can this be corrected in anyway, using a 1227727 and 8D, or am I in Dakota Digital territory here? The circuit may just be saturated.

  2. #2
    Super Moderator
    Join Date
    Mar 2011
    Location
    Camden, MI
    Age
    29
    Posts
    3,026
    so both the speedo and odometer are off?

    get the speedo correct in tunerpro and the datastream at least. then figure out the speedo circuit afterwards.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  3. #3
    Administrator
    Join Date
    May 2011
    Location
    Lakes Region, NH
    Age
    49
    Posts
    2,872
    Which ecm/pcm did you install?

    Did that vehicle use a DRAC? If not, you'll probably need one. But if it did use a DRAC and you haven't removed it you'll probably need to adjust the speedometer divisor in the new pcm. There are specific bits which work with the divisor so you can't just go changing the number higher or lower and get the results you'd normally expect.
    Last edited by 1project2many; 06-28-2012 at 06:21 PM.

  4. #4
    Fuel Injected!
    Join Date
    Jun 2012
    Posts
    48
    The speed is in the range it should be in tunerpro. Around 1,500 in 4th is around 35 mph.

    Adjusting the values seems to give a step change. It either hits 100 by the time the clutch is out or reads around 3x what it should or nothing. There isn't a DRAC shown in the factory repair manual. The manual is a little hard to believe at this point, it claims a 2,000 ppm output from the sensor. That is way off.

    The vcm controlled the abs as well as the engine, so it may have been internal to the vcm.

    Is there a way to figure out what the divisor needs to be set at? The bin I started with had a zero and I don't have much experience with this stuff.

  5. #5
    Administrator
    Join Date
    May 2011
    Location
    Lakes Region, NH
    Age
    49
    Posts
    2,872
    The speed is in the range it should be in tunerpro. Around 1,500 in 4th is around 35 mph.
    You've got one of the numbers correct but unfortunately it has nothing to do with the speedometer.

    Depending on where you look in the manual you'll get conflicting information. Fore example, if you're looking at the page with the trouble code for VSS you're likely to see a generic schematic and description. BTDT. The actual wiring diagram is the best starting point ime. Also checking the instrument cluster section can give really good information as to what the speedometer needs to see.

    I just looked up the wiring diagrams for your truck. You'll probably want to get a DRAC as the VCM you removed had that function built in.

    If you want to play around with the divisor you need to know that only the three "highest" bits produce output. The following decimal values work:
    0 = divide by 1
    128 = divide by 2
    64 = divide by 3
    192 divide by 4
    32 = divide by 5
    96 = divide by 6
    224 = divide by 7

    Other values can cause no change or the speedometer to stop functioning. Feel free to try these values but chances are high that you'll need the DRAC.

  6. #6
    Super Moderator
    Join Date
    Mar 2011
    Location
    Camden, MI
    Age
    29
    Posts
    3,026
    i have values that are quite a bit different for 7727/similar.... but i could be wrong.

    divide by = decimal value
    1 = 0
    6 = 128
    7 = 64
    8 = 192
    9 = 32
    10 = 160
    11 = 96



    you'll probably be able to get it pretty close without a DRAC, but for dead accuracy, you'll probably need it or something like it.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  7. #7
    Administrator
    Join Date
    May 2011
    Location
    Lakes Region, NH
    Age
    49
    Posts
    2,872
    The typical speed sensor found in these trucks produces 40 pulses per revolution, and the stock tires (if 4X4) turn 72 revs/mile. With a very common gear ratio of 3.42 you're looking at 5472 pulses per mile. You'll need a "divide by 2.7" to get the speedo close. The DRAC has a very wide range of divide values which can be programmed.

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •