Quote:
Originally Posted by maxman
Well, I now know that the speedometer is not the problem. I've tested it for hours and it has not failed.
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That's not quite true. What you know is that your speedometer is reliable on a bench in an office with a stable voltage power supply.
1) You refer to the power supply as unregulated, but in your application where the current draw variation is almost zero, the output will be quite well regulated probably to within 50 mv. You want to at least be able to run the speedo from 10V to about 16V for testing purposes. (Hook an oscilloscope up to the supply voltage.)
2) Your speedo is subjected to considerable vibration in the car. If you are looking for an intermittent electrical problem, you need to simulate vibration of some kind. In situation like this I usually tap the device under test with the handle of a screwdriver. This works really well to find lose flaky connections.
3) In the car the speedo is also subjected to a considerable temperature variation. To simulate this a hair dryer works quite well. For cold testing the freezer is my choice.