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  1. #1
    ripcuda's Avatar
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    MSX110/150 - PPU aftermarket discussion

    The Pedal Position Unit (PPU) on the Weber engine in the MSX 110 and MSX 150 is the item that takes the movements from the handlebar throttle lever (ie the "pedal") and converts them to an electrical signal for use by the ECU. The ECU then uses this pedal position signal to drive the Electronic Throttle Body (ETB). It is a "drive-by-wire" system.



    Over time, by their design, these PPUs will wear out and start to give the ECU wonky pedal position signals. This can manifest itself as fluctuating rpms across the rev range from idle to wide-open throttle (WOT). In this pic of a disassembled PPU, you can see the rotating center part contains the contact paws that rub across a conductive strip on the ribbon... bridging across the conductive strip pairs (top/bottom). As this paw sweeps it changes the voltage readings the ECU sees from this sensor which corresponds to a pedal position. As is pretty obvious, over long-term use... these sweeper paws will wear through the conductive strip.



    New OEM units are no longer available (used are still available from parter-outers). Thankfully, due to some good sleuthing of Throttle Position Sensors (TPS) for European cars (Weber is a German manufacture), a direct-fit TPS was found that matches the unit mounted on the MSX PPU. Credit to GH user 'Landon' who found that a TPS from a Renault Clio 2 was a direct replacement and shared that info with us in this thread here.

    An issue that arises from use of this Clio TPS modified PPU (aka "Clio PPU")... is that the engine is not able to idle at it's normal 1500-1550 rpms. With the Clio PPU, the idle is higher at around 2300-2400 rpms. This increased idle can make it tricky to handle the MSX in tight spaces, around docks, other vessels... for inexperienced riders, etc.

    In an effort to explore why this high idle happens, I used Digi-Wrench and recorded the voltage readings at 0% throttle and 100% throttle. I also recorded the ETB's TPS readings to see what the actual throttle was doing. TPS units have dual "sensors" in them... as does the PPU (which has a TPS on it)... thus you see S1/S2 readings.

    The curious reading to me is the significantly higher Clio PPU 0% reading (0.82 v) compared to the OEM PPU reading (0.37 v). And we can see with the Clio PPU that the ETB is possibly cracking the throttle open a tad(??) with a TPS reading of (0.86 v) for Clio PPU compared to the (0.76 v) for the OEM PPU. This might explain the high idle with the Clio PPU.



    So what could we do about it? I have a 2nd Clio TPS unit coming and I want to measure that one too. Perhaps it's just a matter of getting a "good" one with slightly lower readings... due to the cheap manufacturing standards. But if not, perhaps I can shift the ribbon (that has the conductive strips on it) inside the TPS housing just a hair in the right direction to bring down that 0% throttle voltage. This too would also bring down the 100% voltage... but maybe that would still be within ECU workability.

    Hopefully more to come.

    Cheers!
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    Last edited by ripcuda; 07-30-2021 at 04:04 PM. Reason: terminology correction

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  3. #2
    radio-active's Avatar
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    Great info. Not quite right, the PPU does not convert those signals to digital. The ECU reads the analog signal and does the necessary interpretation to drive the throttle body and make other calibration decisions.

    One goes low -> high as the other goes high -> low. These sensors are fashioned in this way so that the engine controller can detect and fault out if the 2 sensors are "out of sync" so to speak. Still done this way today in most all throttle by wire systems.

    It occurs to me that a simple op amp circuit could be tailored to plug in between the Clio unit and the main harness and adjust those voltages thru some scaling of both the sensor 1 & sensor 2 outputs. In fact both signals could be created from just one of the wipers.

    Another method might be to feed the replacement sensor into a digital black box, might contain an Arduino or the like, that would read and translate those signals as necessary, even remapping them for a more or less aggressive throttle map. Many motorcycles and cars that have "sport", touring, comfort, rain" modes, etc. do exactly that, throttle remapping.


  4. #3
    ripcuda's Avatar
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    Quote Originally Posted by radio-active View Post
    Great info. Not quite right, the PPU does not convert those signals to digital. The ECU reads the analog signal and does the necessary interpretation to drive the throttle body and make other calibration decisions.
    Your right. Still an analog signal... I think I meant was mechanical to electrical... not analog to digital. (post updated)

    I like your ideas about a throttle signal translator/tweaker!

    Cheers!

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