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VSS required in carb to TBI swap?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by tys, Mar 21, 2003.

  1. tys

    tys Registered Member

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    Swapping in an 87 OEM TBI truck setup. I have a pinout of the ECM connectors, and see that it has an input called VSS. It orig had a TH400 transmission, which I will not be using (SM465 instead). So, do I need the VSS? What relevance does it play on the operation of the ECM?

    Thanks,
    Chris
     
  2. azblazor

    azblazor 1/2 ton status Premium Member

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    Here is some info regarding the park neutral swith and the VSS which are a little interelated:

    Regarding the P/N switch:
    Some wiring harnesses ground the Park/Neutral wire so that the ECM always thinks the vehicle is in Neutral or Park. The reason is to prevent the SES (service engine soon) or Check engine light from being turned on due to lack of a VSS signal. The service manual states: The ECM uses the P/N signal as one of the inputs to control: Idle Air control; VSS diagnostics; EGR. If the P/N wire is grounded in drive, the EGR would be inoperative , resulting in possible detonation.
    Without the P/N switch conected (grounded) the engine will run ok but engine speed may drop excessively when shifting into gear.

    Regarding the VSS:
    The VSS tells the ECM how fast the vehicle is going. Most people think the VSS is only used for for the lock-up torque convertor. The VSS is also used to control the EGR valve, the charcoal canister purge valve, the electric cooling fans, idle speed, and air/fuel ratio.
    It must be emphasized that the VSS is used to control the idle speed when the vehicle is moving. Without the VSS the vehicle may have stalling problems under certain conditions. The reason an engine not equipped with VSS may stall is because when the ECM has the signals that indicate the engine should be idling (foot off gas, vehicle moving less than 2 MPH), idle speed is closed loop ( which is not the same as the O2 sensor running closed loop) and the ECM will try to maintain a programmed idle speed. If the vehicle is moving, the ECM opens the IAC (idle air control) a programmed amount, regardless of engine speed, which is usually a position that will make the engine idle about 50-100 RPM above the programmed idle speed. Stalling can occur when the vehicle is in the over-run condition (foot off gas- engine speed above the programmed idle speed) because the ECM will try to lower the idle speed to the stationary programmed speed. The IAC may not be able to open rapidly enough to prevent the engine from stalling.
    Raising the minimum idle speed with the adjusting screw can eliminate stalling, but the engine will not run optimally without a VSS. Some Chevrolet engines are programmed to run lean under highway mode. Without VSS the ECM will not get the signals to run the engine for best fuel economy. There are a lot of other programs in the ECM which depend on the VSS.

    Mark
     
  3. dyeager535

    dyeager535 1 ton status Premium Member

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    I kind of doubt the TBI setup you have is setup for VSS. One of the reasons VSS was used (in the TBI trucks) was for the lockup of the torque converter, and since the TH400 didn't have lockup, one less reason to have VSS.

    Still seems strange to me that they didn't require VSS but I've looked at the wiring manual myself, and there is no reference to VSS where a car would have.

    VSS typically is a big deal for driveability, so when it's present, you can't just delete it and expect everything to work fine. In your case, if the ECM is for the TH400 setup, and there is no VSS wire to the ECM, then you don't need to worry.

    If the VSS wire is there, yes you need to run it. You don't need to have VSS for the truck to work, but you need it to get the most out of the swap.
     
  4. tys

    tys Registered Member

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    I am heading home from work now, and will look over my harness to make sure I have a VSS. I know there is a wire coming from the ECM which pins out to the the VSS signal, but am unsure if a connector was provided. What can I do to get a VSS on my 74 SM465/NP205? Also, the 350/TH400 came from an 87 or 88 (don't remember) G30.

    Thanks,
    Chris
     
  5. dyeager535

    dyeager535 1 ton status Premium Member

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    All (that I'm aware of) 81-87 truck speedometers (and of course later R/V trucks) are set up for VSS, but it's not actually present on all of them.

    If the cluster change doesn't matter to you, you could go with an entire '81 plus cluster, just the '81+ speedometer/cable, or go to someplace like www.jagsthatrun.com for a VSS signal generator.

    I'm basing most of my information from others experiences on here regarding VSS and TBI. The consensus seems to be that 465 equipped trucks didn't have VSS (at least to the ECM) but the 700R4 trucks did. Since lockup is really the only difference between the two transmissions as far as electronic controls are concerned, and based on the fact that no TH400's had lockup, I believe that the ECM for a TH400 C/K/G/P vehicle of the same vintage wouldn't "need" VSS.

    The weird thing is that cars used VSS since 1981, and it wasn't just to control lockup. Thats a long time before the '87 trucks, and thats why I'm still just a hair skeptical. Again, as others have found, no VSS on 465 equipped trucks, and from the manual I looked at, there was no VSS "required" either. Should be in the wiring manual if it was there, and I just didn't see it.
     
  6. Thunder

    Thunder 3/4 ton status

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    I would hook it all up and drive it around. If you get driveability problems or VSS code you can always get a chip programed to ignore the VSS.
    As said in previous post no VSS can cause idle/ stalling problems. But on the early TBIs this may not be a problem. I think After 1988 the VSS becomes more of an issue.
     
  7. the professor

    the professor 1/2 ton status

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    The VSS is use in virtual ALL CCC and TBI's
    --
    Lock-up or no lock-up
    -
    It in the speedo, and plugs into a green interface box... /forums/images/graemlins/cool.gif
    --
    The ECM will have no speed sensor and will set a code..and go into "limp-home mode", limiting timing and mixture contro /forums/images/graemlins/truck.gif
     
  8. tys

    tys Registered Member

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    Ok, so if it was needed/required, where would the VSS be located on an 87/88 TH400?

    Chris
     
  9. BILLY RAY

    BILLY RAY 1/2 ton status

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    I grounded mine so the comp. won't look for the signal and I have had any problems as of yet. /forums/images/graemlins/thumb.gif
     
  10. azblazor

    azblazor 1/2 ton status Premium Member

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    The actual Variable Speed Signal was generated in the Instrument Panel. On some trucks there was a Speed sensor on the transmissions (87-88) that generated ONLY a speed sensor hi - lo. The Instrument Panel/ Speed sensor buffer generated the actual VSS signal (437) which the ECM uses. This arrangement changes depending on if the truck is 4WD and with the introduction of Electronic transmissions. The easiest way to get a VSS signal is to get a VSS generator from JTR or Howell. They take a signal from the mechanical speedometer cable and generate a pulse signal that the ECM can use. They used to cost less than $100. Once I get the schematics scanned I'll put them up on my webshots.
     
  11. tys

    tys Registered Member

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    This makes more sense, at least in my case. The VSS signal that goes to my ECM WAS generated within the cab as the wire did not pass through the firewall grommet. The engine/trans combo came from an 87 or 88 G30 van. It was 2wd, and had the TH400. I've since ditched the TH400 in favor of the SM465/NP205 combo in my 4x4. Since I have forever before my project is legal, I will attempt to start it without VSS to see how it runs etc (engine may need to be rebuilt). As I progress to getting street legal, if the drivability suffers, and it's determined to be b/c of a missing VSS signal, then I'll procure the appropriate sig. generator. Still need those darned schematics though /forums/images/graemlins/thumb.gif

    Chris
     
  12. KrebsATM02

    KrebsATM02 1/2 ton status

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    I swapped in a SM-465 about 3 weeks ago inplace of my 700R4. I drove 1500 miles round trip to Las Cruces and back over spring break with a lap top hooked up to the aldl connector. It never told me any codes or anything weird, the only i did notice is the cruise control didn't work, which really sucked.
     
  13. tys

    tys Registered Member

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    What year is your vehicle, etc? I suspect in your case, it's a newer model and did infact require the VSS signal for the cruise control to operate. I do not have cruise on my rig.

    Chris
     
  14. dyeager535

    dyeager535 1 ton status Premium Member

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    Do a search. It's been posted before, 465 TBI trucks didn't have VSS. (according to people that have actually taken them apart) Since the only difference between a 465 and 700R4 truck IS lockup as far as the ECM is concerned, (and a Th400 and 700R4 for that matter) you don't need it.

    If someone believes otherwise, show me in MJ's wiring diagram where the VSS is for the ECM. Not all trucks were even CCC by 1986...saying that cars had something so trucks must have, doesn't work unfortunately. Dunno how or why GM could get away with not running VSS on 465 trucks, (seems really dumb) but they ARE trucks, and don't have to meet the same emissions standards as cars.

    The early VSS buffers (all cable driven speedo's) have two "ports" for connections on them. One for the cruise control box (above the gas pedal) and one for the ECM.

    For those of you that don't think the ECM needs to know what speed the vehicle is going (in originally VSS equipped applications), ignorance is bliss eh?
     
  15. Blue85

    Blue85 Troll Premium Member

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    My truck is carbed, but it does generate a vehicle speed signal in the speedometer. It's used for the cruise control. There is an optical sensor screwed into the back of the speedo and it counts the rotations.

    Here's how the speedo works: there is a spring that returns the needle to 0mph. Then there is a round magnet attached to the back that rotates with the needle. There is a steel "U"-shaped piece that is spun by the speedometer cable. It is just slightly larger than the round magnet and spins around it. Every time is passes the magnetic poles, it creates a force that tends to turn the needle clockwise. The more often the steel piece passes, the stronger the overall force becomes. The optics are counting the rotations of this steel piece.

    So if you already have a cruise control, you may already have VSS. If not, you probably just need to the components from a TBI speedo.
     
  16. dyeager535

    dyeager535 1 ton status Premium Member

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    Up until the electric speedco's, the TBI speedo was the same, it's just that either they used the ECM VSS port on the VSS buffer or they didn't. Still had the "eye" on the back of the speedo, and the same buffer itself.

    Edit: Except of course in '87 or '88 a trip odometer was added to the speedometer.

    I've seen plenty of the truck VSS assemblies that were setup only for cruise, and if you look at the buffer, you'll see the "empty" port I'm talking about.
     

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