Quote:
Originally Posted by Trapper
I knew that was impossible because I was following traffic and the speed limit was 65.
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That's a little like getting an overdraft notice from your bank, but protesting that you can't be out of money because you still have checks.
On some of the roads where I lived in Georgia the posted limit was 65, but if you followed traffic, you were frequently doing 85 or higher.
Quote:
Originally Posted by Trapper
This electronic stuff is great until it breaks and then you need a computer whiz to figure out what is wrong.
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Actually, this problem probably has nothing to do with the computer at all. I know this is small consolation, but the trouble is likely inside the dial itself.
Many modern speedometers are driven by stepper motors. Unlike mechanical gauges, the pointer does not return itself to zero in the absence of input, but will merely sit in one place until it is told to move to the next location. The engine control module directs the motor to move the pointer clockwise so many steps as you increase speed, then counterclockwise so many steps as you slow down. Otherwise, it will sit in the last position it was in until it receives a new signal to move. Modern vehicles have a routine at both shutdown and startup to re-zero the pointer, to ensure that it's starting afresh from the same point every time.
In this case, one of two things happened: either the signal to step down was blocked temporarily by an intermittently bad connection to the gauge, or the pointer itself was temporarily stuck due to a failing bearing in the stepper or some other obstruction. Because it was unable to follow the instruction to step counterclockwise for a while, the displayed speed accumulated an error. Until it got the word to re-zero, it thought zero was at 35 on the scale.
Stepper driven displays are very reliable, but nothing is infallible.
Linear Mode
