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Thread: AEM 30-4110 issue

  1. #1

    AEM 30-4110 issue

    Hey guys.

    When I unplug the wideband sensor to test the gauge/controller it reads 14.8 as it should. In the scanner, both with the engine off and with the engine running it reads 14.8.

    When I plug the sensor in without changing the calibration equation in the software, the gauge reads as it should but the data on VCM Scanner is now reading way lean.

    I fudge the calibration equation down to make it read stoich at idle with the sensor plugged in, and then when I unplug the sensor and check the gauge I'm reading ~14:1 with the gauge outputting 14.8.

    To make the idle AFR display correctly I'm using the equation Input/.5 + 9.2, whereas the default equation is +10. With the default equation when the narrowbands are switching the scanner reads ~16.25:1. The gauge is displaying 14.7 on average.

    I have MPVI Pro and I'm using the analog input and a dedicated chassis ground. 12V is coming straight from a fuse box attached to the battery.

    Anyone ever seen this behavior before? I tried searching and couldn't find this exact issue.

  2. #2
    Advanced Tuner
    Join Date
    Dec 2005
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    599
    This looks, likely, to be the effect of a ground offset voltage. The gauge draws MUCH more current, when the sensor is connected. So, the ground offset voltage is different. The offset voltage changes with the current draw of the gauge. And, the current draw of the gauge changes with the state of the sensor. Which... changes with EGT, exhaust gas flow, and AFR. So, you can't, really, "zero it out" with conversion math

    You should be able to verify this with the serial data, which should match the gauge display. But, is really slow and low resolution

    This is why the newer X-series gauges have the ground-offset compensation, built in. They are worth the extra $ for that bit, alone.

    The best you can really do with an old style gauge, like that, is to use the shortest possible run of the thickest possible wire to run ground to both the gauge and the scanner, from a common point.

    Or, build a ground offset compensation circuit, yourself; with differential input and output. It's, really, just an OPAMP