How does the tachometer work?

[kobachi]

I disassembled all the bits of the speedometer recently, trying to troubleshoot a zero reading. There's sure a lot going on in there to move that dial up. I notice there seems to be no such complicated system for the tachometer -- by what mechanism does the tachometer needle get moved?

[DMCMW Dave]

I disassembled all the bits of the speedometer recently, trying to troubleshoot a zero reading. There's sure a lot going on in there to move that dial up. I notice there seems to be no such complicated system for the tachometer -- by what mechanism does the tachometer needle get moved?

There is an electronic circuit right in the tach that counts pulses from the ignition module (white/grey wire) and divides by 3, and then points the needle at the right number of RPMs

[kobachi]

There is an electronic circuit right in the tach that counts pulses from the ignition module (white/grey wire) and divides by 3, and then points the needle at the right number of RPMs

Glorious. Does that mean that there are always two cylinders firing together?

[DMCMW Dave]

Glorious. Does that mean that there are always two cylinders firing together?

Not exactly, it means that it's a 4-cycle v6 engine so that every revolution has three plugs fire.

[Rich]

You might try deciphering Post #2 out of this Corvette forum article because it's the same tachometer + circuit except for the cylinder count math, of course.

http://www.thevettebarn.com/forums/c3-tech-performance/34-tachometer.html

Maybe it will help you troubleshoot.


"....If the needle does not move,either the circuit board is bad or 12 volts or ground are missing to the tach. Remove the tach from the dash and apply 12 volts and ground, tach should go to zero if working...."

Delco/GM/s best stuff at the time no doubt, complete with random RPM reading upon shutdown.

[kobachi]

You might try deciphering Post #2 out of this Corvette forum article because it's the same tachometer + circuit except for the cylinder count math, of course.

http://www.thevettebarn.com/forums/c3-tech-performance/34-tachometer.html

Maybe it will help you troubleshoot.


"....If the needle does not move,either the circuit board is bad or 12 volts or ground are missing to the tach. Remove the tach from the dash and apply 12 volts and ground, tach should go to zero if working...."

Delco/GM/s best stuff at the time no doubt, complete with random RPM reading upon shutdown.

Oh, thank you. But there's nothing wrong with my tach :) I just wanted to know how it works.

[jawn101]

Delco/GM/s best stuff at the time no doubt, complete with random RPM reading upon shutdown.

I always wondered about this part in particular. I've heard some wild stuff about why this happens but none of the reasons make much sense to me. Anyone know for sure?

[kobachi]

I thought it was well-established that this behavior is caused by the ghost of JZD.

[jawn101]

I thought it was well-established that this behavior is caused by the ghost of JZD.

That's one of the more plausible excuses I've heard...

One source told me it was from voltage spikes at shutdown of the motor..

The most believable was that there's a spring inside the tach and a powered-up but 0 RPM signal drives it to the zero marker. That just seems like a silly design - but I don't know any better :)

[Ron]

I thought I posted what I had on the Tach in the Resources section long ago... anyway, Everything you wan to know about the D Tach here (http://dmctalk.org/showthread.php?8587-Tachometer-Info).



I always wondered about this part in particular. I've heard some wild stuff about why this happens but none of the reasons make much sense to me. Anyone know for sure?Because it uses a LM1819, which has no "Return to Zero" function. You can use a CS8190, but it is physically different (pinouts), so a new circuit board would be required....

[jawn101]

I thought I posted what I had on the Tach in the Resources section long ago... anyway, Everything you wan to know about the D Tach here (http://dmctalk.org/showthread.php?8587-Tachometer-Info).


Because it uses a LM1819, which has no "Return to Zero" function. You can use a CS8190, but it is physically different (pinouts), so a new circuit board would be required....

Cool info there! Never saw that post, I'm gonna go read those PDFs right now.

[Elvis]

Two weeks ago I posted this part in another thread so that
people can read the datasheet or at least google it to understand
that it's the chip, it's the design that causes it !
Or to get a new chip if the tach is broken.

But nobody gave a shit. Thanks guys !

[Ron]

Two weeks ago I posted this part in another thread so that
people can read the datasheet or at least google it to understand
that it's the chip, it's the design that causes it !
Or to get a new chip if the tach is broken.

But nobody gave a shit. Thanks guys !I went back a month or so and can't find it??????

[NightFlyer]

I went back a month or so and can't find it??????

http://dmctalk.org/showthread.php?8520-Tach-Question&p=124847&viewfull=1#post124847

[Ron]

THX.

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Anyone have a precision resistor in their bones box to trade on?