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Old 06-19-2020, 11:56 AM   #121 (permalink)
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Originally Posted by MeteorGray View Post
As noted before, I also have observed variations in the TPS (throttle position sensor) screen of my Scan Gauge when at WOT using a throttle-stop device. These variations seem to depend on on road grade and wind speed and direction, ie, the engine load.

For instance, at a consistent WOT pedal position, I found that my Scangauge reported TPS values from 26% at 70mph to 28% at 75mph. This was during an uninterrupted 10 minute WOT run over a stretch of approximately level and approximately north-south oriented road with a wind that the weather bureau reported was 10mph from the northwest. I was going approximately south at the time, so it was essentially a tailwind. I attribute the range in speed to imperceptible road grade and wind direction-and-speed variations. The only surprising thing to me was the 2% TPS variation. This is obviously something other than a mechanical throttle position being reported by the gauge on its TPS screen.

As part of the same test, I turned the car around and drove in the opposite direction, ie approximately north, which then put the wind as a headwind. On that run, which was only a minute or two so did not see as many road or wind variations, I recorded a speed of 65mph at a TPS of 27%.

This kind of puzzled me, since I didn't see why the throttle position value would change with these variations in load when I know that the throttle is being held in exactly the same position, mechanically speaking.

So, I must conclude that the throttle position sensor screen is not reporting the actual throttle position per se, i.e. mechanically speaking. I think in my drive-by-wire Mazda3, the TPS value is actually a calculated derivative, probably resulting from a combination of such computer-read sensors as the throttle sensor on the throttle pedal, the mass air flow sensor that measures the volume of air going into the engine which is used to determine the load for the proper engine settings, and whatever else the engineers felt useful. Just a WAG.

As noted before, I quit looking at the TPS value on the gauge because it is not useful in trying to measure the precise mechanical throttle position, which does not change with load. I just take it on faith that when the throttle is being held at WOT by the throttle stop, the actual throttle position does not change with load. The speed does change with load, of course, but not the mechanical position of my throttle pedal. What the computer is doing with that mechanical pedal position obviously is up to the engineers and what their design dictates.

Incidentally, I also looked at the LOD (engine load?) screen during my tests, but it gave values that ranged widely that seemed even more useless to me.

The world of computers has passed me by without even a courteous honk.
That's a very thoughtful 'WAG' !

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