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[Reply #15 on:]

I highly doubt its 60hz in an automotive wire, you really need to borrow an oscilloscope to see what's going on, what does that wire come from?

[Reply #16 on:]

From the wiring diagram I found, it has something to do with the tachometer. When I read it, I get 12V (battery) when the key is turned to the position right before cranking. After the engine is started and idling the voltage I read is about 14V and if I push the throttle the voltage will increase with RPMs.



Sorry for the stupid image but that's my circuit. If I read across the cap and across the entire circuit here are the readings I get:

input 12V   cap 2.3V
input 14V   cap 2.4V
input 16V   cap 2.3V

When I rev the engine, the voltage reading across the cap is going down! It should be going up like the input voltage is. What is going on? I'm not good with caps. The cap is rated at 25V.

[Reply #17 on:]

My guess is that as the frequency increses the impedance of the cap goes down, affecting the rms voltage you are reading, essentially the cap is acting like part of the voltage divider, if you were to check the top resistor I bet the voltage would increase across that

[Reply #18 on:]

Maybe that's how the tachometer is read. Its output is a frequency. That means the frequency could range anywhere from 0 to who knows what. Hundreds. Thousands. What should I do?

[Reply #19 on:]

You can use a digitalRead then , and just count, but as for the hardware side most of those methods above will work fine for a digital read, though now your input impedance can be higher