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Issues testing a tachometer with a function generator

  1. Dec 9, 2008 #1
    I am on an assignment to test the domain of input values of a tachometer, and what output (deflection) different values correspond to. In other words, what's the minimum and maximum signals and what does the curve look like?

    The trouble is I can't get ANY useful information. The tach has three inputs: Ground, +12V, and frequency. I have the output ground of the function generator connected with the ground of the tach (and the ground of the power supply), and the frequency input with the non-ground output of the FG.

    No matter what frequency I use, the needle on the tach does not move. What in the world am I doing wrong here? Can anyone describe or picture in detail what they would use as their test setup?
  2. jcsd
  3. Dec 9, 2008 #2


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    What's the output amplitude of your function generator? Is it sinusoid or square wave? I'm not really familiar with automotive electronics, but just from some quick Googling, there appears to be a wide variety of tachometer schemes (some based off of Hall Effect senders, some off the alternator, ignition, etc.)
  4. Dec 9, 2008 #3


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    Staff: Mentor

    I didn't see where you connected 12V -- I may have missed it. What current is being drawn from the 12V supply?
  5. Dec 9, 2008 #4


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    As someone who has worked for a company which designs and manufactures tachs and speedometers I'll see if I can help. What you describe I would assume to accept a square wave input. Most of the time even if the tach is expecting a sine wave from an inductive sensor it should accept a square wave. At the very least the needle should move. If this is a modern tach it could be expecting a TTL level signal from and engine control module. Older tachs were hooked directly to the negative side of the ignition coil which is switched to ground by a set of breaker points or in the case of solid state ignition, a transistor. When hooked to the negative side of the ignition coil you could expect VERY large voltage spikes which would have to be dealt with by the input circuitry of the tach. What did you have the function generator set to do? Do you have the output offset to go between 0 and 12 volts?
  6. Dec 9, 2008 #5
    heh, you want to reverse engineer someone else's product? or are you working on something like a defense repair contract? if the later, it helps to know where the signal is coming from.
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