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Vss Question

Discussion in 'The Garage' started by dheavychevy38, Mar 15, 2006.

  1. dheavychevy38

    dheavychevy38 1/2 ton status Premium Member

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    Ok I have searched and have only found part of the answer i need. Ok I havea 91 jimmy fullsize and am swapping in a th350 and 205 yes i have found the part for the vss but what do i do about my speedo cause it is not cable driven any advice would be great cause the emissons out here suck thats why i need everything to work. Thanks Denny
     
  2. Blazer1970

    Blazer1970 1/2 ton status

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    Swap to an '87 or '88 instrument cluster. They have cable driven speedo with the VSS built into the cluster.
     
  3. 1985_K5_Silverado

    1985_K5_Silverado 1/2 ton status

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  4. dheavychevy38

    dheavychevy38 1/2 ton status Premium Member

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    Thanks for the info guys
     
  5. CHEVY 4WD

    CHEVY 4WD 1/2 ton status

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    ide like to buy your old 241 if you do the swap
     
  6. DPI

    DPI 1/2 ton status

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  7. dyeager535

    dyeager535 1 ton status Premium Member

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    '90-91 R/V VSS is 40 pulse. There is a reluctor ring in the transfer case that drives it.

    The DRAC module is what translates that to the correct pulse for the ECM, speedo, cruise, ABS.

    As far as I can tell the '747 ECM uses a 2 pulse, so you could run the '81+ 2 pulse VSS stuff with that ECM, eliminating the DRAC. But if you are dealing with a '90, then your speedo would need changed, then any problems with cruise and ABS.
     
  8. DPI

    DPI 1/2 ton status

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    Thanks for the information. Using some keywords from your response I found some more helpful information.

    http://www.tbichips.com/drac/

    Now I just need to figure out how to convert a mechanical speedometer to 40 PPR signal so I can keep the DRAC static. The most I found so far is 16 ppr...
     
  9. dyeager535

    dyeager535 1 ton status Premium Member

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    I think jagsthatrun makes a 40 tooth external reluctor, but for an off-roader I didn't like the setup. If it can be mounted to the transfer case end, that would be better and might be an option.

    You are basically looking for a 40 pulse generator that can be driven by the speedo gears, (and feed the DRAC) is that correct?

    What t-case are you dealing with? I believe the 205 mech to VSS conversion has been done on this board.
     
  10. DPI

    DPI 1/2 ton status

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    I agree about the external reluctor. I don't like that option.

    Yes, I am needing a 40 ppr generator. I have found them from 2-16, but nothing close to 40.

    The tcase is an Atlas 4.3:1
     
  11. DPI

    DPI 1/2 ton status

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    Just had a brain fart. Do you think a DRAC from a 2 wheel drive would work with the 2 pulse generator?
     
  12. dyeager535

    dyeager535 1 ton status Premium Member

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    I doubt it.

    As far as I know there are no differences in the DRAC's. (40 pulse anyways)

    There is some different camaro stuff, but I've not looked into that much. I think JTR talks about those as well.

    Are you going to be running ABS and cruise? AFAIk those are the only two things the DRAC runs that are NOT 2000PPM. If you aren't running those, I don't see why you couldn't run the speedo and ECM off of a 2 pulse transfer case mounted generator. The DRAC article/pinout that is pretty widely available (it's a PDF and in the archives at diy-efi.org/gmecm) has the output for each DRAC terminal in PPM. I'm almost certain the speedo uses the same 2000PPM signal as the ECM, it just comes off a different pin.

    Not quite sure why the 40 tooth setup isn't more popular, at some point I think everything including cars was using it, and might still be.
     
  13. DPI

    DPI 1/2 ton status

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    Just spoke to the guy at JTR. He said to bypass the DRAC as you did. But said the engine uses the 2ppr while the speedometer uses the 4ppr.

    He said the solution would be to run two converters...

    What are your thoughts?
     
  14. dyeager535

    dyeager535 1 ton status Premium Member

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    I think that will generate them some money. :) (commenting about having to purchase two signal generators)

    JTR seem to be great people, so don't get me wrong, but the R/V trucks are an entirely different animal, and I suspect they are incorrect.

    ftp://ftp.diy-efi.org/uploads/DRAC2.pdf

    The pinout there clearly shows 2000PPM for either speedo output, and that's what I used to wire mine, (ECM/Elec. speedo) which works perfect, so I'd say their 4000PPM statement is incorrect. (either that, or the DRAC pinout is wrong)

    Last I checked their site, they didn't deal much with DRAC stuff anyways, mainly the 2 and 4 pulse stuff that GM used in the Camaro's and the like.
     
  15. DPI

    DPI 1/2 ton status

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    Look on page 4 of 12. Connection C13 shows vehicle speed to ECM as 40ppr. Is this the signal that feeds the ecm/pcm?

    Also there is a vss to PCM. This shows 2ppr.

    Which connections need to be made to insure the vehicle runs correctly. The speedo is of secondary concern.

    Thanks for all your help on this!

    dp
     
  16. BobK

    BobK 1/2 ton status

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    When I added TPI to my 71 I added a VSS signal generator to my NP205.

    Unscrew the speedo cable from NP205,screw on VSS,screw on speedo cable to VSS then ran the wires to the ECM.

    Got mine from www.jimsperformance.com for $75.00.
    See it 1/2 way down this page.
    http://www.jimsperformance.com/carty.html

    Here it is installed.
    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]



    Hope this info. helps.
     
  17. BobK

    BobK 1/2 ton status

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    Sorry...jumped the gun there...I should read the post fully.:doah:
     
  18. DPI

    DPI 1/2 ton status

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    bump
     
  19. dyeager535

    dyeager535 1 ton status Premium Member

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    DRAC's are used to feed a variety of ECM's so what the ECM/PCM require from the DRAC can vary. For instance, I believe a '730 ECM processes the "raw" VSS signal from the sensor itself, with no buffer or DRAC. (so yes, some applications may have fed a different ECM/PCM with a 40 tooth signal. As you can see by the diagram, there are a bunch of redundant/sometimes useless outputs)

    As far as I know, the '747 NEEDS the 2 pulse VSS signal, the same one used since 1981 in cars/trucks up until each respective line changed over to non-cable speedo's. It was used before electric speedo, and was used after. Stands to reason that since the ECM number is the same, input requirements are the same.

    The whole problem with the DRAC application again is the variety of computers it was used with. The '747 is an ECM, while something like a 1991 R/V3500 gas w/4L80E used a different computer, with DRAC, but was a PCM because it controls the transmission as well as engine. Make sense? So the 40 pulse COULD in some ECM/PCM's be what is required. If you strip out the "non-essential" stuff, cruise and RWAL when looking at an R/V harness, there is not much connected to the DRAC itself, a bunch of empty terminals.

    The only things needed for the DRAC to feed a '747 correctly is a ground, 12V source, two VSS sensor wires, and the 2000PPM output.

    Only difference between this and the previous cable drive VSS from our trucks wiring-wise is the lack of the VSS sensor wires, and a buffer instead of the DRAC. (excluding RWAL and CC)

    Here's the '90 R/V DRAC wiring:

    http://community.webshots.com/photo/181820064/1182190654058177455PQLFjt

    bigger picture, impossible to read DRAC info:

    http://community.webshots.com/photo/181820064/1181823705058177455wErKaq
     
    Last edited: Jun 1, 2006
  20. DPI

    DPI 1/2 ton status

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    I purchased a 30 pulse generator that attaches to the mechanical speedometer. I did this to keep the integrity of the entire system so everything will work, not that I need cruise control... I found the generator at Nevada Speedometer. The 30 pulse generator, along with a low number tooth count on the speedometer gear (to increase the pulses generated per revolution). The Atlas uses a Jeep style speedo gear. I found a 25 tooth gear and replaced the existing 34 tooth that came with the Atlas. The 34 tooth read 15 mph when doing 60 per my GPS. The 25 tooth bumped it up to 30mph. Now if I can just fine tune my DRAC, I will be set...

    However, now I am really confused! So on the way home I bought a 7 pin DIP switch from the local electronics store. I get home ready to install it and I pull my DRAC. Thats where the fun began. My K5 is a 1990. My DRAC is a 1994 with the 14 jumpers. To make things more confusing, these two pages show conflicting data on the 14 jumper DRAC. The main difference are how each number the pins. ftp://ftp.diy-efi.org/uploads/DRAC2.pdf) and http://www.tbichips.com/drac/

    There's more. All 14 jumpers on my DRAC have jumpers in them. There are no open jumpers. I think I need to find a DRAC from a '90-'91 to make things easier...
     
    Last edited: Jun 13, 2006

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