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Thread: MVPI Pro and LC-2 log works not matching

  1. #1
    Potential Tuner
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    Sep 2009
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    MVPI Pro and LC-2 log works not matching

    I hooked the pin 1 to the yellow, then pin 5 to the same ground wire that the LC-2 connects uses. Then setup the scanner with input 1 and transform using built in lc-2.
    Logworks shows 14.7
    Scanner shows 15.1

    I thought as long as the grounds were shared it would avoid this issue...what could I be missing?

  2. #2
    Tuner in Training
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    Jun 2015
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    I have shared ground on my AEM, and had to make adjustments as well to get them to match. Havent validated my innovate yet, working install now.

  3. #3
    Advanced Tuner
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    Ground offsets are a bit mode insidious than that, unfortunately. The offset voltage is caused by the difference between the internal ground of the WB controller ( where the analog signal is generated ) and the ground reference of the scanner ( where the signal is measured ).

    Most of this difference is caused by the high current drawn by the WB controller to run the WB sensor's heater element. This can be anywhere from 0 to about 3 or 4 amps. That's why yo don't, generally, see ground offset errors in other types of sensors. They don't have large current draws. Usually, just a few milliamps ( 1/1000ths of amps )

    It is common practice to try to "zero out" the offset in the logger math. But, this only APPEARS to work. This is because the WB heater current draw ( and, therefore, the offset voltage ) varies over the whole range of 0 to 4amps, based on how hard the heater has to work to maintain its target temperature.

    Because of this, the voltage offset will vary with EGT, exhaust flow rate, and exhaust back pressure. So, an offset value, worked out at idle, to get the scanner to read 14.7, will, almost certainly, be way off, under high load conditions, with different EGT flow and pressure conditions. As the WB sensor heater current will differ greatly, from its idle value.

    Because of the significant currents involved, it is CRITICAL that the WB controller NOT share its power ground wire with its analog signal ground wire. As the voltage at opposite ends of the power ground wire will differ, proportionally with the WB heater current. Even at that, of there is a significant voltage difference between the analog grounds of the WB controller and the scanner, current will flow through the ground wire connecting them, and, spoil the measurement.

    For this reason, CAN or serial data is preferred, as they are immune to analog offset errors.

    Failing that, a differential pair type analog connection can be used. This is where one of either the scanner or the WB controller makes 2 measurements instead of 1. The first measurement is the normal signal. And, the second is a measurement of the ground at the other end of the connection. The actual measurement is then calculated by subtracting the second from the first. For this to work, either the scanner end or the WB end can have the differential interface. But, Usually, not both. When the WB side has the differential analog output, it measures the ground line at the scanner and, adds that value to its normal output value.

    That is why the Analog- line on the AEM X-series controllers MUST be connected to the scanner ground. Even if you are just trying to measure the output voltage with a voltmeter. The differential output interface adds whatever voltage it sees on the Analog- wire to whatever it was going to put on the Analog+ wire. The Analog- wire is not, properly, a "ground wire". It is just another analog input to the WB controller.


    This is not the best example of varying heater load, because the heater current doesn't vary linearly with the pwm% but, you get the idea In this case, the actual heater current varies between about 1.2Amps and 2.8Amps. At every level of the yellow trace, the heater current and, therefore the voltage offset, changes. if you "zero it out" at one level, it's off everywhere else.
    heater-load.JPG

  4. #4
    Advanced Tuner
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    I just remembered, for the LC-2, that Innovate used to have an adapter that, IIRC, converted their analog outputs to differential mode: https://innovatemotorsports.com/supp...structions.pdf

    I don't remember the specifics. And, I don't know if it is still available. But, I think, it used a similar circuit to the one used by the AEM X-series WB's, to correct for voltage offset errors.