A fix for the throttle tip-in stumble/hesitation | Victory Motorcycles: Motorcycle Forums

A fix for the throttle tip-in stumble/hesitation

Discussion in 'Tech Q&A' started by TwoManyXS1Bs, May 11, 2020.

  1. TwoManyXS1Bs

    TwoManyXS1Bs Member

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    Cruising my 2016 Vegas at the lower rpms, closing throttle to slow down, then reapplying throttle, the engine gives a momentary hesitation, followed by a jerk as it comes alive. Turn out that the injectors are turned-off at full-closed deceleration, then are turned back on when the throttle is twisted. Extremely annoying, drives me nutz.

    A forum search on "Stumble", "Hesitation", "Tip-in", "Bucking", "Jerk", and "jerkiness" produces numerous postings on this issue. For example:

    Stumble on tip-in
    Slow cruisin
    Correct rpms for low speed
    Engine surging or momentarily cutting out? 2013 XCTour

    I've been using the TechnoResearch Maximus and TRO2 wideband for tuning my 2016 Vegas.
    Using the DirectLink display, I've found a way to disable the injector turn-off while decelerating.

    My Maximus struggles are in this thread:

    Texican gets a Vegas

    I've been running with this mod for over a week now, and am pleasantly satisfied that it works fine, no more off-idle or tip-in stumble. With the other Maximus tweaks, tuning, and a complete overhaul of the ignition timing map, the bike behaves very nicely at the lower rpms from 1000-1200 and up. Now time for the reveal.

    Please bear with me as I tend to over-explain things, my how-to threads tend to get quite long...
     
    Last edited: May 12, 2020
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  2. TwoManyXS1Bs

    TwoManyXS1Bs Member

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    On full-throttle-closure deceleration, the Throttle Position Sensor (TPS) signal is at its lowest voltage (around 0.89 volts), and the ECU responds to this signal by turning off the injectors. But only at rpms above about 1300-1500 rpms. A couple of TPS excerpts from the 2012-2017 service manuals:

    TPS01.jpg
    TPS02.jpg

    It would seem that they forgot to include the "injector cut-off" feature.

    I found that if I held the throttle just a smidge above fully closed, the injectors wouldn't turn off. Using the DirectLink Gauge display, I found that the TPS threshold is 1%. As soon as the throttle position drops below 1%, the injectors turn off.

    This mod (or hack) is a way to fool the ECU into thinking that the throttle isn't fully closed...
     
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  3. TwoManyXS1Bs

    TwoManyXS1Bs Member

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    So far, I believe that this mod will work on the 2012-2017 closed-loop bikes. Unsure of previous models.

    This mod simply adds a tiny current (about 50 microamps) to the TPS signal, raising its voltage by about 50 millivolts, so that the ECU thinks that the throttle is being held above 1%, but below 2%. But, this should only be done when the bike is in gear. During startup, in neutral, the TPS needs to report a 0% throttle, else the ECU will flash a diagnostic code.

    The Throttle Position Sensor is located on the left side of the throttle body. The left coilcover and coil mounting bracket must be removed to gain access.

    TPS08.jpg
     
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  4. TwoManyXS1Bs

    TwoManyXS1Bs Member

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    The TPS is simply a precision 1/4 turn potentiometer. Its wiper moves from a near-grounded position (at idle) to almost +5v at WOT.

    TPS_Sensor01.jpg

    Adding a pull-up resistor between 77k to 58k, and a tiny 1N4148 (or 1N914) safety diode, the TPS signal is raised about 50 millivolts, putting the reported throttle position somewhere between 1% and 2%.

    TPS_Sensor02.jpg

    But, this additional voltage must not exist when the bike is first started, or the ECU will complain. So, an additional diode is added to drain that additional current (about 50 microamps) to the neutral switch.

    TPS_Sensor03.jpg

    The diode going to neutral prevents any backflow of voltage from the neutral circuit. When the bike is put in gear, the neutral circuit stops draining the pull-up resistor, and the current passes to the TPS output, raising its voltage by about 50 millivolts.

    So, that's it. A simple way to trick the ECU into thinking that the throttle is never fully closed, except when the bike is in neutral. I've experienced no downsides to this, throttle response is crisp, and no hucka-bucka during deceleration/tip-in.

    Next, how to do this mod...
     
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  5. TwoManyXS1Bs

    TwoManyXS1Bs Member

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    The parts for this mod are very cheap and available. To get the resistance value between 77k and 58k, single resistor of 68K (Blue-Grey-Orange) ???can be used, or a pair of more-common values can be used in parallel. Best would be to use a 100k (Brown-Black-Yellow) and a 270K (Red-Violet-Yellow) as a pair in parallel, giving a resistance of 73k ohms. The resistors and diodes (1N4148 or 1N914) are easily found on eBay or electronics shops.

    Victory_TPS-Mod-01.jpg

    For these kinds of projects, I like to use the JST series mini-connectors. Also cheap and available on eBay. Here's a size comparison to a typical 3.5mm Honda bullet connector.

    Victory_TPS-Mod-02.jpg JST-Connector.jpg

    The mod involves tapping into and splicing onto existing wiring. I just slice into a wire and solder in a pigtail lead. For those uncomfortable with this kind of surgery, there's a new series of Posi-Taps, much better than the old Scotchlocks.

    Posi-tap01.jpg Posi-tap02.jpg
     
    Last edited by a moderator: May 31, 2020 at 10:28 AM
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  6. TwoManyXS1Bs

    TwoManyXS1Bs Member

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    We'll start with the Neutral wiring, BK/PK (Black/Pink). It's accessible in 2 places. In the headlight shell, as part of the speedometer harness. And at the 20-pin body harness connector beneath the seat. I chose to tap in at the harness near the body connector.

    Here's the neutral wire (BK/PK) at the top of the connector, accessible from above.

    Victory_TPS-Mod-05.jpg

    But, that's too short for my surgery. So I chose to remove the fusebox, push the connector down, and gain access to a longer portion of the neutral wire, where the harness emerges around the rear shock.

    Victory_TPS-Mod-06.jpg

    Cut the insulation to expose a tiny section of the wire.

    Victory_TPS-Mod-07.jpg

    Wrap the end of a 3-foot piece of wire around the exposed wire.

    Victory_TPS-Mod-08.jpg

    Solder the thing.

    Victory_TPS-Mod-09.jpg

    And wrap it up.

    Victory_TPS-Mod-10.jpg

    This pigtail (neutral wire extension) will be routed alongside the top backbone, below the tank, following an existing harness there, and dropped down near the left side of the throttle body...
     
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  7. TwoManyXS1Bs

    TwoManyXS1Bs Member

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    Next is to remove the left coilcover and mount, to gain access to the TPS wiring.

    Victory_TPS-Mod-03.jpg

    You have to push in at the upper part of the retention tab to release the TPS connector, you may be able to see my scratch marks on it from pushing with a small screwdriver. I had already tapped-in to the TPS wiring before taking these pics, so you'll see the pigtail extension already attached.

    Victory_TPS-Mod-04.jpg

    This annotated excerpt from the 2012-2017 manual shows how the circuit taps into the TPS wiring.

    TPS_Sensor04.jpg
     
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  8. TwoManyXS1Bs

    TwoManyXS1Bs Member

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    Now, the gadget.

    I soldered the parallel resistor pair together, one end going to a connector. The two diodes are soldered to their connectors. Then some heat-shrink is slipped over the diodes, leaving their other ends exposed for the final soldering at the other end of the resistor pair.

    Victory_TPS-Mod-11.jpg

    The unused wire is for a future project.

    Heat-shrink to protect the thing.

    Victory_TPS-Mod-12.jpg

    The neutral pigtail from the previous task is routed beneath the tank, alongside the backbone with an existing harness, and dropped down near the throttle body. The wire is trimmed to length, and its connector is soldered, then heat-shrinked...
     
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  9. TwoManyXS1Bs

    TwoManyXS1Bs Member

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    Hook it up.

    The gadget is plugged into the TPS pigtail connector, and the neutral pigtail is plugged into the gadget.

    Victory_TPS-Mod-13.jpg

    Fold it up and tie-wrap it to the TPS wiring. Plug the TPS connector back into the TPS. Tuck the wiring away.

    Victory_TPS-Mod-14.jpg

    Put everything back together,

    And pray to the great motorcycle gods that things will work...
     
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  10. TwoManyXS1Bs

    TwoManyXS1Bs Member

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    Display of DirectLink's gauge panel to show how it works.

    Switch on, ignition on, the Neutral light is lit, and the gauge shows the throttle position at 0%.

    Victory_TPS-Mod-15.jpg

    A better look.

    Victory_TPS-Mod-15b.jpg

    Now, put it in gear.

    The Neutral light goes out. The throttle position shows as 1.3%.

    Victory_TPS-Mod-16.jpg

    Back into Neutral, and the neutral light comes on, throttle position returns to 0%.

    It works...!
     
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  11. TwoManyXS1Bs

    TwoManyXS1Bs Member

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    An alternative method, stealthy, and much cleaner, would be to attach the circuitry to the ECM connector.

    The 2012-2017 ECM connector map.

    2012-2017_ECM-Connector-Map.jpg

    An annotated pic of the ECM connector, showing the gadget attach points.

    Victory_TPS-Mod-17.jpg

    Personally, I'm very happy with this mod.

    I think I'll keep it...

    ******** End of Report ********
     
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  12. mande

    mande Member

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    Great writeup, Steve! :clap:
     
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  13. slickvic

    slickvic Well-Known Member

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    Yes....... very informative. Thanks for taking the time.
     
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  14. jedi-mcfly

    jedi-mcfly Well-Known Member

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    Awesome job - very comprehensive :0)
    :clap::clap:
     
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  15. Half_Crazy

    Half_Crazy Well-Known Member

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    Just put a bigger pilot jet in it. :angel4:
     
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