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RPM and speed in different gears 04 Sportster 883XL

Discussion in 'Motorcycle Tech Talk' started by NeilP, Apr 9, 2016.

  1. NeilP

    NeilP Active Member

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    Do these figures sound correct?

    I fitted a used from e-bay tach to my 04 Sportster 883

    These are the RPM figures at exactly 40 mph
    1st gear 2400rpm
    2nd 1800rpm
    3rd 1400 rpm.

    does that sound about right tor did I buy a dud tach..

    I was at the only HD store today, and while discussing a misfire problem at 3000rpm..one of the techs suggested that maybe the tacho is mis reading.

    At very slow idle it is reading about 400-500rpm
  2. Lucifer

    Lucifer Well-Known Member

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    I don't think it's a dud tach, I think you have a dual fire tach installed on a single fire ignition system. On the older Harley ignition systems both plugs fired at the same time so that tach would read correctly,on the newer HD single fire systems which I'm pretty sure your '04 Sporty has, only the plug on compression stroke fires, so your tach readings would be 50% of RPMs. There are adapters you can get to run a dual fire tach on a single fire ignition...
    Here's an example of one.

    http://www.cruisercustomizing.com/d...-models-with-single-fire-ignition/part/DT-101
  3. NeilP

    NeilP Active Member

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    OK< thanks

    I just checked the p/n

    68941-99

    Looks like the adapter is very simple
    http://www.dflund.se/~jokke/tach/

    a couple of diodes and resistors...dare I risk it ? or jsut cough up the money and buy an OEM tach $300USD and wiring harnes ($280 USD) !
  4. Lucifer

    Lucifer Well-Known Member

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    I'd cough up the money for the adapter before I'd go buy another tach;) Yeah ,not much to the adapters,if you're handy at electronics, you could make one for a couple of bucks or buy one for 1/10th of the price of a tach.
  5. NeilP

    NeilP Active Member

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    More news...looks OK< it does seem to be a Single-Fire tach.

    I got a reply from a mate who I only just found out is still working for an online Harley parts dealer
    He says
    That one is for
    Model (8):
    DYNA SUPER GLIDE T-SPORT - (FXDXT) - 2001
    DYNA SUPER GLIDE T-SPORT - (FXDXT) - 2002
    DYNA SUPER GLIDE T-SPORT - (FXDXT) - 2003
    SUPER GLIDE SPORT - (FXDX) - 1999
    SUPER GLIDE SPORT - (FXDX) - 2000
    SUPER GLIDE SPORT - (FXDX) - 2001
    SUPER GLIDE SPORT - (FXDX) - 2002
    SUPER GLIDE SPORT - (FXDX) - 2003
    and reading the spec on a few, they do show a Single -Fire ignition..

    http://www.totalmotorcycle.com/phot...arley-Davidson-FXDXTDynaSuperGlideT-Sport.htm
  6. NeilP

    NeilP Active Member

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    Looks like I do not need to, ..we cross posted... but I have loads of electronics stuff here, and yes handy enough with that sort of thing. I'd not even need to buy the parts..have boxes of IN4007's or similar, plus resistors that woudl ahve doen the job.

    But looks like I do now know i have accurate RPM readings..one less issue.
  7. NeilP

    NeilP Active Member

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    I am y confused as F**k now.

    Some times I start the bike and it reads one set of RPM's . other times it reads twice that. Is the ICM switching to some kind of dual fire limp home mode?
    If so ..when it is in Dual Fire Mode...it is reading correctly. which is now back to the supposition that the Tach is a dual fire tach..which is not what I had come to believe, from the part number and the bikes it is fitted to , which are supposed to be Single Fire.

    Here are two sets of RPM figures. I can shut down the bike and re start and the tach then doubles or halves its readings

    Sometimes when I start the bike I get
    Idle 900-1000
    40 mph:
    1st 5100rpm
    2nd 3600rpm
    3rd 2800rpm
    4th 2400rpm
    5th 2000rpm

    cut out cough splutter at 6000rpm.... rev limiter?

    I can stop and start the bike and then:
    Idle 450-500
    40 mph:
    1st 2600rpm
    2nd 1800rpm
    3rd 1500rpm
    4th 1200rpm
    5th 1000rpm

    cut out cough splutter at 3000rpm.... rev limiter?


    The part number on the tach and the bikes it is fitted to, all indicate that it is a Single Fire tach..yet these results???? aarrggggg
  8. hotroadking

    hotroadking Super Moderator Staff Member

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    Might be something inside the tach that is screwed up, I doubt the ignition is sensing anything and sending or changing signals.
  9. NeilP

    NeilP Active Member

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    I'd read on the Hammer Performance web pages that the ICM, when unable to determine compression stroke reverted to 'dual fire' wasted spark firing of the coil.

    That would explain the exact doubling of the RPM. -and also mean that despite part numbers, the tach I have does need 'two ignition triggers per 360 degrees to operate correctly.


    But still, no one can tell me which set of RPM / Speed / Gear figures are correct ?
  10. hotroadking

    hotroadking Super Moderator Staff Member

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    99-03 Tachometer, Part #68941-99 I believe it fits Dyna and Sportster Models 1999-2003 Models 4" Tachometers.

    Since you have an 04 that's why it's not working properly, a change was made in 04

    5/8 mini tach mounting bracket fits 04-later xl customs--Part# 69426-04
    2 5/8 mini tach-black face 68953-04, or 2 5/8 silver face-- Part3 68986-04

    2 5/8 mini tach gauge housing-- Part# 67524-04.
  11. NeilP

    NeilP Active Member

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    Yes, this tach is not original fit..but it is a Single fire tach and the bike is single fire too. so a single pulse per rotation of the crank per cylinder..so taking a trigger feed from one cylindeer shoudl feed it the correct signal..at least that is waht I was told on here...or another forum. The real tach for thsi bike woudl take its tach signal from the databus line from the speedo. whereas the tach I have is a three wire (ground/12// trigger unit)


    Here is a discovery...if I get the bike running and the tach is reading 'half readings', then I quickly hit the kill swithc off/on while the motor is still running, when the tach comes back on line --it reads the correct RPM again.


    Tomorrow I will try feeding the tach its 12 volt feed from an external source, the the power feed to it does not drop when cranking. May be something that causes the tach to mis behave when it gets its trigger signals , whiel battery voltage is a bit low due cranking. May also try taking the power feed/ ground to the tach from a different place..maybe also try starting the bike with a booster battery connected to keep the voltage up whiel cranking.
  12. Lucifer

    Lucifer Well-Known Member

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    These would be the correct figures;

    Sometimes when I start the bike I get
    Idle 900-1000
    40 mph:
    1st 5100rpm
    2nd 3600rpm
    3rd 2800rpm
    4th 2400rpm
    5th 2000rpm

    cut out cough splutter at 6000rpm.... rev limiter?
    Yes, at 6000RPMs you'd be hitting the limiter.
    Just guessing,but maybe you have a loose ground or maybe a couple of broken strands in the signal wire inside the insulation...:confused:
  13. NeilP

    NeilP Active Member

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    I'd pretty much discount the iffy wiring as an issue, unless it is within the crimped plug inside the tach.
    I am very particular about wiring. Each joint, twisted, sleeved, crimped, soldered and heat shrunk.

    Ground, positive and trigger have all been spliced into the loom by:
    First removing the individual terminal from original Deutsch connector.
    Removing section of insulation WITHOUT cutting original copper.
    Twisting new wire to the tach around the bare section.
    Wrapping 5mm wide thin coper strip around joint and crimping.
    Soldering the joint
    Heat Shrinking.

    The jump between half reading RPM and full ' correct' rpm is just too consudtant to be a loose or vibrating wire issue.


    I think this is a 'systems' / understanding /definitions 'error'.
    I have been reading so much about this 'Single Fire /dual fire in the last fee dsys. When you even get comments from the likes of Hammer Performance' stating that even some manufacturers have got the terminology wrong, and called the same devices different things at different times,there is little hope.


    I'd say iit is just not possible to run a tach reliably from this style ICM coil trigger wire

    I have seen stated in several places, that when, with this type of ICM, TDC compression can not be correctly determined, the module goes into 'Dual Fire ( wasted soark) mode. My understanding of that Dual Fire is that a spark is fired ( per cylinder) EVERY 360 degrees at or near TDC, give or take the advance curve.
    It is when it is in this state of firing two sparks PER cylinder per 4 stroke cycle (720 degrees) that this RPM gauge reads correctly.

    I have also read that the ICM determines Compression stroke by monitoring cranking speeed from the CKP output or Cranking slows towards compression, and pulks battery voltage down. What ever the means, the ICM determines Front cylinder compression stroke and goes into Single Fire mode ( one spark oer cylinder per 720 degrees) non wasted spark.

    In this case, the tach is getting one spark trigger per 720 degrees, and therefore reads only half the correct RPM.

    When I start the bike on the battery it virtually always starts with the tach reading half RPM. The bike battery voltage drops to about 11.3volts on cranking

    If I start the bike with a huge battery booster pack connected, I see no voltage drop when cranking , and the bike starts and tach reads correctly ( one soark per 360 degrees, (wasted spark mode.
    ( my booster pack is 1200kilo of electric aircraft tug batteriy, 2 volt Lead acid cells wired as 12 volt and 3600 Ah,( 36 cells in 6 series, 6 parallel config) capable of a CCA of about 2000 amps or more)

    If I start the bike by building up speed, down a hill, and letting the clutch out in 2nd gear, so starting without the starter motor, the tach reads correctly.

    If I am riding along at 30-40 mph after a battery start, and the tach is reading wrong ( half rpm) , I leave the bike in gear and coast under engind braking and flick the Kill Switch Off/On, when the bike comes back on, the tach will now be reading correctly.

    If I start the bike on battery, Tach reads half.. If I then blip the throttle to get rpm up, and quickly cycle the kill switch off/on, then when tach comes back 'online ' it again reads correctly

    Some battery starts, (1 in 10 at a guess) result in correct RPM reading when starting from cold.
    When starting from hot ( shutdown followed in seconds by battery restart) , .. when bike starts on first half rotation, this figure of correct readin rach increase to probablh 1 in 3


    Each time if reads correctly, it has been 'started' in such a wsy that potentially the ICM has not been able to determine compression.


    With all of this, I am tending to believe Hammer Performance and others statements that this ICM switches between wasted ( dual spark) or non-wasted (single spark ) mode depending on if it can determine No. 1 Front TDC compression stroke or not.


    Problem is,
    The tach comes from a bike stated to be Single Fire ( non-wasted spark)
    It is fitted to a Single Fire ignition bike
    Yet it only seems to read correctly if the bike is running in Dual (wasted) spark mode.


    Well ,it is now time, 0442 , I have been unable to sleep, and have typed all this on iPhone while flat on my back on bed swuintying at the small screen.
    So apologies for typos/fat finger typing.

    Interested to hear your views.

    All this is totally new to me, 5 days ago I had not heard of Twin/dual/ single spark differences in relation to Harley.
  14. charlie46

    charlie46 Well-Known Member

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    Uhhh ! I think you should invest in the proper battery. Just my opinion though.
  15. NeilP

    NeilP Active Member

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    I do have the proper battery 5 day old brand new Harley Battery for £120
  16. NeilP

    NeilP Active Member

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    The booster pack was used as test to guve massive overcapacity to stop voltage sag under cranking load
  17. charlie46

    charlie46 Well-Known Member

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    could the starter be causing this ?
  18. NeilP

    NeilP Active Member

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    SOLVED

    It turns out the 'issue' is how the ICM works when it can't determine compression, combined with an analogue tach that requires analogue RPM signal of two pulses per revolution or on Single fire bikes, a 'conditioned' signal from either the ICM or the Speedo.

    It IS a SINGLE FIRE tach..well a tach from Single fire bikes..as listed further up this thread. But for those bikes it requires the 'conditioned ' signal..of two pulses per revolution if fitted to a electronic ignition type system with two trigger feeds to the coil. If fitted to a bike where the coil has has a single trigger wire firing both plugs at once, then it would work correctlyAs also discussed, the OEM ICM goes in to 'two spark per 720 degrees' mode, firing on exhaust and compression, when it can't determine compression...so the tach give the correct reading..since it gets two sparks per rev. But when the bike starts 'correctly' and the ICM runs in single fire mode..the tach reads half.See this really crap video I made:






    So to get around all of this, Harley DO supply a ready made solution, on the 04 Sportster at least. The Speedo has a 12 pin plug. Pin 3 is blank, and Pin 3 IS the analogue Tach output...so the answer to get the tach reading correctly was to feed its trigger wire from that output. It works..I did it, until I screwed up the plug and then went and bought the correct tach and wiring harness from the Harley dealer.

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