Anyone seen a bad ECM cause a speedo problem?

Discussion in 'Electrical' started by ncpoppie, Nov 17, 2009.

  1. ncpoppie

    ncpoppie Member

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    I'm posting in Electrical rather than Sportster as I think this may be more of a generic ECM/Speedo problem. From what I've read in various model forums it looks like recent speedo systems are all pretty much the same.

    Here's the problem.

    Have an '07 XL 1200L. It's a working bike with nearly 20K miles on it. Runs great with no serious problems in three years - except for this one nuisance that is making me crazy!

    Last summer I had a first instance of the speedo just dropping to zero. You know the gig - no turn signal auto-off, rough/fast idle, check-engine light and stored low VSS output code. I read up on this and it seemed right on schedule. With that many miles, the VSS had a moderate amount of gunk on it, so cleaned it off and no worries. Hah!

    A week later I'm out riding with friends and cracked the throttle pretty good off a stop light and right up around 5K revs the speedo just drops again. Okay. Pull over, turn it off, count to ten, start it back up and everything is fine again. Until I hit the throttle wide open and bang, it will drop to zero every time. BTW, there will be NO stored diag codes until I ride it long enough for the ECM to figure out something is wrong and turns on the "check engine" light, e.g. everything reads like the bike is going 100 MPH but the VSS system says it's standing still.

    So, I read up some more on this and find a lot of folks with newer machines are reporting voltage regulator problems related to speedo dumps as the voltage output is too high at full revs. Okay, this makes sense so in goes a new voltage regulator (this is after having it to my local dealer - good guys - and they can't find a thing wrong after putting their break-out on it and riding it for a while). Now the problem changes but is just as repeatable. I can run the bike all day at 6K and the speedo will never dump. BUT, come to stop light and give it a little too much juice from a dead stop and bang, now the speedo dumps at maybe 20 MPH. Having changed the voltage regulator and cleaned the VSS, I decided to go for another $50 and replaced the VSS. No change at all.

    I can make it dump this way all day long and never see a diagnostics code get recorded so long as I reset it quickly. For instance, if I make the mistake of starting a little too quickly (it seems to never dump if I baby it off the light but can crack the whip once I'm in second gear all I want) and see the speedo dump, I just give it a bit more speed, pull in the clutch, flick off the run switch, count to three, flick the run switch back on and the speedo reads correctly again and I restart the engine all in a distance of 100 yards or so.

    Weather's been rainy here the past week or two, so I took some time to pull off the seat and tank and traced the wires for this circuit. It looks pretty straightforward. Three wires between the VSS and ECM - looks to be power/ground/output. I'm assuming that the VSS output signal is interpreted by the ECM and sends the appropriate signal to the speedo.

    End of story or am I missing something? I rang out the lines disconnected for possible ground/line-to-line shorts and they all checked okay as did end-to-end continuity. My dealer wrenches say replace the speedo next but I'm thinking it's the ECM or something else I'm missing. Has anyone else had this experience? I've even taken it out on some really bad roads to bounce the (EDITED) out of the machine (painful on a low) to see if it would cause the speedo to drop out. That is, the dealer guys think that perhaps the first gear jolt of too much throttle indicates a mechanical problem internal to the speedo which resets when the ignition is cut off and back on. But the speedo held steady after many rides down the bumpy road which is making me think it an ECM problem. I'm just hoping there aren't a dozen other inputs the ECM uses in addition to the VSS input to generate a speedo signal.

    It would be great if someone else has had a similar experience that could yea or nea on this or offer some other explanation.

    Thanks in advance and sorry for the length of the story but its an odd one.

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    Last edited by a moderator: Nov 17, 2009
  2. Hoople

    Hoople Account Removed

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    I have never seen this problem but,

    Would you happen to have a SERT. With it you would be able to monitor the raw VSS input to the ECM (or at least how the ECM sees it) while you ride the bike.
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    How it applies to my '09 Dyna, the raw VSS signal is a Hall Effect signal
    (+5v-0) and not an inductive sine wave. The info From the ECM to the speedo is Serial data and not just a passed on square wave. That makes it a little harder to troubleshoot. It could be corrupt data going to the Speedo.

    I have a gut feeling that the VSS signal is not clean and at the high RPM the ECM chokes.. The fact that the ECM will see a missing VSS signal tells me the signal coming into the ECM may be iffy.

    What I would really like to know is how the Hall effect switch is triggered. Is there a small magnet mounted on a shaft or gear or is the magnet self contained in the switch. It can be either way and I don't have enough HD experience to know.
     
  3. glider

    glider Veteran Member

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    The VSS triggers from the teeth on the 4th gear cog to the magnetic switch of the VSS. The teeth are used as the trigger for the unit.
     
  4. Hoople

    Hoople Account Removed

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    The Teeth. Wow, that kills my theory.

    It's got a new VSS switch and the ECM does see the missing VSS signal. If the cable from the VSS to the ECM is good & sound, it sure points to the ECM.
    But before I changed it I would have to make a few more tests. especially using a SERT.

    It's a good Bug for sure!
     
  5. ncpoppie

    ncpoppie Member

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    Thanks for the tip Hoopie.

    I know all about the mechanisms at work in the signal generation but it did not occur to me to monitor the Hall effect (minute difference in potential and induced EMF voltages in a conductor) on the signal lines. I'm pretty sure now that I think about it that this is what the H-D mechanics at my dealer did with the break-out box but I suspect that they only monitored the VSS signal into the ECM. They may never have checked the ECM signal out to the speedo as they never saw a VSS problem and could not duplicate the speedo problem. (It did of course drop out on the way home that day just about as I pulled into my street a good 40 miles from the dealer!)

    I had given some thought to putting a small scope on the tank and watching the speedo signal tapped at the speedo head but this is a bit dicey and I'd need an inverter. At my age (my first official H-D shop job was in '65 before my all-expense-paid trip to lovely SE Asia), I'd probably drop the thing trying to watch more than one thing at a time.

    I've got a recording Fluke around here somewhere that will accept a Hall effect loop (if I can find one). Seeing that it is easy to reproduce the problem I'm thinking that I could first put the loop on the signal line out of the VSS to see if it drops or holds when the speedo dumps. If not, then I can move the loop to the signal out wire off the ECM and check again. If the signal drops out when the speedo dumps then I think it would be safe to say that the ECM is bad (errr!). Finally, if the ECM signal holds steady I could then loop the signal wire at the speedo head. If there is still a comparably strong signal from the ECM when I whip it, then I think it's a safe bet that the speedo has had a little too much rockin' and rollin'.

    Hope I can find the bits and pieces I'll need otherwise I'll have to take this idea back to the dealer and see if they can test for this. If they were just checking for straight voltages off the VSS, then they might not see an ECM signal-out problem or it may be so transient that they could miss it if not using a recorder (unlikely). Here's an interesting point though that might point to the speedo. The speedo never recovers on its own. I can drive another five or ten minutes with the speedo at zero MPH and everything works okay except for the turn-auto-off and a bit rough at idle and eventually the check-engine will come on and then there will be a stored diagnostic code (always low VSS). But immediately flick the run switch off for a couple of three seconds and back on and the speedo comes right back at the appropriate speed reading. I suppose I should check to see what the effect of turning off the run switch (ignition key still on) has on the speedo (e.g. does it de-energize it?). I don't have an ECM breakout box so can't check the pin taps for voltages but suspect that turning off the run switch does NOT de-energize the ECM. I suppose I could pin through the wire insulation but don't like to do this as it invariably leads to wire corrosion even after applying a little scotch kote.

    Any one know about the effect of turning off the run switch on the ECM other than killing the ignition circuits? I'm thinkin' that it does a lot more as fuel pump comes on, speedo lights up, etc. if you turn the ignition switch on with the run switch off and flick the run switch on.

    Any other thoughts/possibilities?
     
  6. NEWHD74FAN

    NEWHD74FAN Experienced Member Retired Moderators

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    NCP...sounds like your assessment and plan are sound and well thought out. Being as the ECM probably has a simple microprocessor to combine all the sensor data and adjust the duty cycle of the ignition, the speedometer part is probably just a simple counter, divide by logic and such (Hoop could probably dissect that one you have), it is probably a safe bet that is not just an "analog" guage.
     
  7. paulmack

    paulmack Active Member

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    I'm having a similar problem. My speedo is at 0 and never shows any speed. It will jump when the ignition is turned on so I know it's getting voltage. I don't get any diagnostic codes or check engine light. The turn signals, auto cancel and idle all are ok but no speed indicated on the speedo. Let us know what you find out this is driving me crazy too.
     
  8. glider

    glider Veteran Member

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    Yours sounds like the speedo itself or the wiring to it being the signals work properly and no running problems.
     
  9. Hoople

    Hoople Account Removed

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    Poppie,

    I am basing the following as it applies to my '09 Dyna.

    The ECM acquires the road speed from the VSS in the form of a square wave. Three wires are going to the VSS pickup. Ground, Reference +5 volts and the output. You wont need an inverter to monitor the raw output signal. The reference +5 is supplied by the ECM. Look at the output of the VSS while you turn the wheel on a bike jack. You should see a swing of say 4.8v to near Zero. Check the Reference voltage to make SURE it is regulated to +5 volts.
    The ECM sends out that speed value in SERIAL DATA form to the speedo and turn signal module. It does not just pass the Vss signal on to the next cpu. It is communicating that info on a twisted pair buss The fact that BOTH your speedo and turn signal logic goes "out" at the same time, tells me the problem your having is not in the Speedo Head. If it was, your turn signals most likely would continue to work fine and they are not.
    You can view that VSS signal with just about any scope. The frequency is going to be very low.

    If your bike is chain drive, I would pull the master link, drop the chain and run it under power while looking at the amplitude of the VSS. If it falls out or attenuates at Hi speed, you will easily see it. Once you KNOW that the Vss is Good & Solid, you can go from there.

    Another thing you could do is take a frequency generator and substitute you own validated Vss signal. Inject the signal directly into the ECM right at the Vss leads/pickup. A +5 to zero will do it easily.
    You wont hurt a thing. As you change the generator frequency the speedo should follow. This would be a Great test to see exactly what was going on.
    With the bike at idle, you could "goose" the Generator and watch the speedO.
    I know it would work.:p

    Hoop!
     
  10. ncpoppie

    ncpoppie Member

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    Thanks once again Hoop for making me think about this problem. It's a pleasure to hear from folks that give serious thought to these kinds of complex and often costly issues.

    Well, here's the latest. No SERT on this machine so no help there. You did make me think about the logic that's likely involved (would love to get my hands on the Delphi microcode for this one) as in fact there are a lot of inputs and outputs involved here. It had actually been a while since I had tested the related functions to loss of VSS signal input to the ECM so on the way home last night I pushed the machine into a speedo fail mode and double checked the signal auto-off function and it had quit. I also ran it for about ten miles w/o resetting the speedo and the check engine light eventually came on too. Also the light came back on after the next startup indicating stored codes which I read (same as before) and cleared.

    So knowing this, I located a powered transducer that I could clip on the signal lines and use as an input for the recording Fluke. Actually for info (this might be handy for others trying to run down a signal problem) I used a tach/injector transducer that I had lying around that is real easy to clip over the signal wire with an adjustable gap and will detect extremely low EMF induction. (They are great devices - and not expensive - for putting aftermarket tachs on bikes without having to mess around with any of the oem tach wiring so you end up with just making a DC connection plus the clip-on.) It was just a matter of setting the setting the scale right on the Fluke. I could detect the square wave signal off the VSS using an A/C input although I'm not sure that would work okay on an analog meter but maybe. I don't know what the speedo signal was like but I did detect a DC signal.

    The Fluke is pretty small so I was able to just strap it onto my rack and run the leads up under the seat. I tested the VSS first and did not see any loss of signal when I forced the speedo to dump. So I reset the system and clipped the transducer on one of the signal wires into the speedo. Trial and error there as there were a bunch of them in the harness going into the back. I just clipped one after another until I found one that gave me an output from the transducer only when moving. I figure that was one of the speedo signal lines as when I forced the speedo dump I lost the signal.

    So I think this points pretty clearly to the ECM being buggy. I called my buddies at the dealer and they couldn't think of any other input to the ECM that might be causing this. I made an appointment to run up there next week as they have a couple of newer sportster basket cases in for rebuild and are willing to swap out an ECM and test it to see if it makes the problem go away.

    If that's the problem then there goes another $400. Arghh! I'm getting old and all sorts of things are busting. I suppose that if I can adjust to losing much of my hearing I should be able to live with this stinkin' silly speedo problem. Kinda' have the business of a rolling reset down to a couple of seconds but with my luck I'm probably gonna' forget to pull the clutch in one time before flicking off the run switch.

    That might hurt.

    Thanks for your help.