How does an auto tach work?

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How does an auto tach work?

Postby thanksbastards » Thu Feb 23, 2012 5:52 pm UTC

kind of a dumb question, but i am looking for a difinitive answer without taking one apart. A car tach measures the engine rpm with a wire that is connected to the negative side of the spark coil, which is alternatingly floating and grounded, producing the engine spark. but how, in EE terms, does it do this? there must be some mean V spikes on that signal, duty cycle is constant and pretty high, but how do the guts of the dial translate that pulse to a needledeflection? the internet wont tell me
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Re: How does an auto tach work?

Postby LikwidCirkel » Mon Feb 27, 2012 6:57 pm UTC

As far as I know, each revolution will trigger a pulse of equal duty cycle using some kind of simple timer circuit. The output of this feeds to a very low pass filter - effectively a charging capacitor. This outputs the average voltage as a DC level, which will vary linearly with RPM, and can be used to deflect a needle.

I suspect with modern cars, this is replaced with a simple digital micro-controller circuit which measures RPM directly by comparing rotation rate to a known (much faster) clock source.
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Re: How does an auto tach work?

Postby Great Justice » Wed Feb 29, 2012 4:41 am UTC

I would imagine like any other frequency meter, but scaled

Hertz * 60 = RPM

Maybe this will help you out http://www.electronicecircuits.com/elec ... er-circuit
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Re: How does an auto tach work?

Postby wumpus » Thu Mar 01, 2012 4:19 pm UTC

I strongly suspect the computer counts the number of times it sends pulses to the sparkplugs and and moves a motor to indicate RPM. I'm pretty sure I've seen dashboard mountable tachs that simply read the ODB(II) data and displayed it in LED format. I also recall somebody claiming that during a sportbike scandal where a certain engine was supposed to rev to umpty-up warp rpm, the engineers who calibrated it knew they where lying since they were moving a stepper motor from data in the computer. YMMV.

I'm pretty sure whatever method is used is exactly the same as the speedometer. One reads before the transmission, one reads after.
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Re: How does an auto tach work?

Postby Endless Mike » Thu Mar 01, 2012 5:10 pm UTC

wumpus wrote:I strongly suspect the computer counts the number of times it sends pulses to the sparkplugs and and moves a motor to indicate RPM. I'm pretty sure I've seen dashboard mountable tachs that simply read the ODB(II) data and displayed it in LED format. I also recall somebody claiming that during a sportbike scandal where a certain engine was supposed to rev to umpty-up warp rpm, the engineers who calibrated it knew they where lying since they were moving a stepper motor from data in the computer. YMMV.

I'm pretty sure whatever method is used is exactly the same as the speedometer. One reads before the transmission, one reads after.

I don't *think* this is entirely correct when you consider that a car's computer, in addition to running the car and displaying the necessary information, have to monitor the functions. If it was simply indicating how many times it was pulsing the plugs, it could get things off, especially in cases where there's a non-computer reason for things to be changing, such as downshifting a manual car without rev-matching. There's almost certainly still a way for the computer to have the information and ensuring things match.
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Re: How does an auto tach work?

Postby thanksbastards » Thu Mar 01, 2012 5:16 pm UTC

I guess I should have been clear when I said tach I meant a dial. the indash type, that you can go buy at the store, hook 1 wire to teh coil, one to ground and one to 12v and enjoy a nice analog reading of your RPMs.

I think i need to take one apart.
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Re: How does an auto tach work?

Postby Seraph » Thu Mar 01, 2012 7:10 pm UTC

I believe that LikwidCirkel was correct, it's just a low pass filtering of voltage swings of the low side of the coil.

In particular I suspect your assumption that the "duty cycle is constant" isn't correct. Rather I would suspect that the spark takes a specific amount of time, so if it's firing at 100Hz the duty cycle will be double what it is at 50Hz.
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Re: How does an auto tach work?

Postby LikwidCirkel » Fri Mar 02, 2012 3:53 pm UTC

The schematic linked to by Great Justice is pretty much what I described. The 555 takes a waveform or pulse input and generates an output at the same frequency, but guarantees that the high time of the output pulses are constant. Duty cycle as a percent will increase with frequency, but duty cycle as absolute time will remain constant. The average DC level will then increase as input frequency increases. The Rs and Cs around the current meter are a low pass filter of sorts. You could accomplish the same thing with a voltage-deflected needle with a bit of re-arranging of the components at the output. The schematic uses a current-deflected needle.

With modern cars, I suspect that wumpus is correct. Almost any microcontroller in the digital systems relating to spark control, transmission, fuel injection or similar could measure the frequency directly with a single input pin and some voltage-protection stuff at the input. The interface to the deflection needle could be as simple as DC output from a DAC or a PWM-driven servo. I doubt it's a stepper, as I think that would introduce complexity like the need to reset to zero when the car stalls.
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