# tachometer ppr conversion?



## dladd (Jun 1, 2011)

I would like to get my stock tachometer functioning. From my reading here, it appears that all I need is to supply the white wire to the tach itself with a 6ppr signal (it's a 1995 Saturn). I currently have a speed sensor on my motor that sends a 4ppr signal to my controller.

Is there any device that can take the 4ppr signal and convert it to a 6ppr signal? 

If that is not avaliable, the options I see are to either to install a second pulse sender on my motor end shaft, or wire up the tach with the 4ppr signal, and re-label the tachometer face to match the lower readings.

Can anyone think of other solutions I could use?


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## DIYguy (Sep 18, 2008)

dladd said:


> I would like to get my stock tachometer functioning. From my reading here, it appears that all I need is to supply the white wire to the tach itself with a 6ppr signal (it's a 1995 Saturn). I currently have a speed sensor on my motor that sends a 4ppr signal to my controller.
> 
> Is there any device that can take the 4ppr signal and convert it to a 6ppr signal?
> 
> ...


First thing to do is make sure you know what the OEM tach needs to be accurate.. by that I mean, how many ppt. I'm not positive, but I would say that it is unlikely that your OEM tach requires 6 ppt. In my case, I was convinced mine needed 2 ppt only to find out later that it was off by 1/3. Luckily, the Soliton will output a programmable # of ppt, so, within a few minutes, I had it working fine. A general rule (which can be wrong) is that the tach drive needs a pulse frequency equal to the ice # of cylinders divided by 2. Example 3ppt for a 6 cylinder and 4 ppt for an 8 cyl.
Perhaps the easiest way may be to change the one you have to what your tach needs as long as your controller can be configured to accept this. . . then u only have one device to worry about.
Also, do some searching, there is more info here. . as well as a few Saturn drivers.
If you have some bits a pieces laying around, you could rig up a crude tacho-generator with a drill and a prox or hall effect sensor and connect to your tach. Of course you would need a hand held rpm meter also. Even connecting your current 4 ppt signal to the tach would tell you how close it is. A bit of math and you would know what you needed. If it was off by 25%, it would indicate need for a 3 ppt for example.


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## dladd (Jun 1, 2011)

I tested it this morning by hooking up the signal direct from my motor sensor (putting out 4 pulses per revolution) to the back of the tachometer.

In the photo below you can see that at ~2000 rpm (read by the controller software) my tach reads ~1300 rpm. Which verifies I need 6ppr (2000/6*4=1333). Which is what I suspected (many thanks to the other David's with Saturns, david85 and Dave Koller for the archived wisdom on Saturn tach's!).










I think I'll probably just live with it reading low, and maybe get around to making a cool overlay at some point, but that's more work cause I'd have to pull the front of the dash apart to get behind the clear plastic lens in front of all the gauges. 

Unless anyone has any cool ideas of how to convert a 4ppr signal to 6ppr. 

fwiw, my controller needs 4ppr, I don't think it's selectable. and it does not output a signal for the tach as far as I know (Synkromotive). I have an email in to them to verify.


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## piotrsko (Dec 9, 2007)

my standard reply is getting to be: on turning on the ignition, prior to starting the motor, if it swings from 0 to max then back again, it is a PCM driven gauge with a stepping motor drive. I haven't figured out how to trick these yet because I don't have one. these count pulses.

The air core ones can be changed with either an op amp set up as a differential amplifier, or a simple resistor network. these accept an analog voltage and move the needle appropriately. to make it read more, make the sense voltage bigger. Many Fords max out at 5 v. Start by adding a AA battery in series with the neg battery term going to the tach wire and the pos lead going to the tach. add resistance as required to calibrate. measure voltage going to tach and design resistance ladder to suit.

another way to go is if saturn had a 4 cyl option. get the tach out of a junk yard, take apart the dash and substitute. good luck


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## dladd (Jun 1, 2011)

piotrsko said:


> my standard reply is getting to be: on turning on the ignition, prior to starting the motor, if it swings from 0 to max then back again, it is a PCM driven gauge with a stepping motor drive. I haven't figured out how to trick these yet because I don't have one. these count pulses.


it does not swing when the ignition comes on, but from the factory it is definitely fed through the PCM.



> The air core ones can be changed with either an op amp set up as a differential amplifier, or a simple resistor network. these accept an analog voltage and move the needle appropriately. to make it read more, make the sense voltage bigger. Many Fords max out at 5 v. Start by adding a AA battery in series with the neg battery term going to the tach wire and the pos lead going to the tach. add resistance as required to calibrate. measure voltage going to tach and design resistance ladder to suit.


but isn't the tach reading either 12v or 0v many times/second? At least that is what my input signal is, direct from the sensor on my motor (i think...). This is the sensor I have. The tach senses the frequency, not the voltage, right? I'm bypassing the PCM and wiring direct to the tach, but I'm sure there's further electronics inside there. I do not plan on diving inside the cluster to modify it, but would rather modify the signal if possible.



> another way to go is if saturn had a 4 cyl option. get the tach out of a junk yard, take apart the dash and substitute. good luck


All S-series Saturns are 4 cylinder.


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## piotrsko (Dec 9, 2007)

Perhaps I'm wrong, But I had thought you wanted a 6 cyl tach driven by a 4 cyl pulse.

You are correct about the drive signal, but as an air core, it actually uses a pulse width signal similair to what your controller does to the motor, where the effective voltage applied to the tach increases as rpm increases. To increase meter swing you need to increase voltage applied, all other things being equal. This is why I suggested the AA battery in series with the tach signal at least for the initial experimentation.

Further reflection requires me to add: since there are all sorts of patent issues, it is possible that you have too much signal going to your tach and you would need to add a series resistor of about 100 Ohms 1/4 watt, because the tach uses the downgoing side of the pulse instead.

Yes there should be more electronics in the tach, possibly a LM1819 or variant driver circuit and discreet components.

Your sensor just detects a "thing" going by the end and turns "on" and "off". It doesn't care about frequency. The rate at which it does this state change is the frequency.


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## coulombKid (Jan 10, 2009)

piotrsko said:


> Perhaps I'm wrong, But I had thought you wanted a 6 cyl tach driven by a 4 cyl pulse.
> 
> You are correct about the drive signal, but as an air core, it actually uses a pulse width signal similair to what your controller does to the motor, where the effective voltage applied to the tach increases as rpm increases. To increase meter swing you need to increase voltage applied, all other things being equal. This is why I suggested the AA battery in series with the tach signal at least for the initial experimentation.
> 
> ...


A micro controller, such as the Parallax BS2 can easily make the conversion. You would need a voltage divider set of resisters to drop your incoming pulse train to 5 volts. One programs this uP with pBasic. The program would calculate the RPM and then generate what ever pulse train that scales correctly to that. The out put generating the new pulse train would need to be amplified back up to 12 volts for your car tach to read right. If your tach signal works on "dwell" (percent signal high) it can handle that too.


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