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What voltage / amperage / frequencies can I expect from the tach line of a 2001 eclipse v6? Answered

The question is all in the title, but any way.  What numbers can I expect to see from the tachometer signal line of my 2001 mitsubishi eclipse v6?  I've read that some times it can be like 3 cycles for every 1 RPM.  I would assume it was running at 12 volts, but is this false?  What kind of amperage could I expect to be pulled on that line as well?  I plan on connecting an arduino to this to get the frequency as an input (new new new newbie to arduinos!).  Can I expect to need to step down the voltage, if so, how?  Is there some basic principle or standard for the tach line that I am unaware of?  Any help with the tach line info or how I should connect it to the atmega chip would be AWESOMELY helpful!!!
-James

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sephiroth67

7 years ago

Confirmed to run at 12-14 volts. My cheap radioshack multimeter blew its fuse once I checked amperage. The Frequency was a bit crazy, but it did rise as I pushed the throttle. It would need to be calmed down for this application I guess (hysteresis?). Looked like it was just floating in a 300hz range that would go up as I open the throttle and down as I let off.

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FrozenFiresephiroth67

Answer 7 years ago

You don't measure current the same way as voltage - you'll just short it out.
Of course the fuse blows.

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NachoMahma

7 years ago

.  If you are getting 3 pulses per revolution and you have a six cylinder engine, then it sounds a lot like the "old school" tachs that used the spikes from the ignition coil. I doubt that same method is used today (the spikes can be quite large), so I'd investigate the TDC (AKA crankshaft position) sensor that steveastrouk mentioned.
.  To get the right answer, your best bet may be to call a Mitsubishi dealer/service center. Or maybe Googling "2001 Mitsubishi Eclipse tachometer specs"

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sephiroth67NachoMahma

Answer 7 years ago

Turns out it was sitting in front of me the whole time, I just needed to set my multimeter to AC...should've thought about that >.<

Anyway, now all I need to know is, how to get an AC line that can range from 5.5V - I'm gonna say maybe up to 70 volts even, into an arduino!?!?

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NachoMahmasephiroth67

Answer 7 years ago

. BTW... your meter is probably assuming a sine wave when measuring RMS AC and I think your signal is a pulse. You may not be getting accurate readings.

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frollard

7 years ago

What lots of people do is run signals like that through a Zener diode to block it down to a safe voltage, then run THAT through an opto-isolator to ensure there is no direct connection from a potentially high voltage source and your tasty Arduino.
As for the specifics of what frequency to expect, you'd need to look in your car's repair manual.

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steveastroukfrollard

Answer 7 years ago

Or is it picked off the TDC sensor, which is usually inductive ?

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frollardsteveastrouk

Answer 7 years ago

Right, if it's an inductive signal, at high rpms you might see pretty high voltages...

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steveastroukfrollard

Answer 7 years ago

Depends where you're picking it up - the ones I've got condition it in the sensor.

Or its a Hall effect sensor ?

Steve