SNAIL

phoskam

I've got a nascar style tachometer, wich can be set to 4,6 and 8 cilinder engines, but not at 2 cilinder.

Is there a way to modify the instrument itself or it's signal so it gives the right values ?

Neil

Olli · Posted: Sat Dec 08, 2007 1:32 pm Post subject:

One change is to use sensor on flywheel. Example BX have sensor on
crack what shows position of it. This sensor could give same information
for tacho than coil, but you can adjust it to show 4 pulses in one engine
round, like 4 cyl engine. Jasu knows much more about this.

-Olli
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Jasu · Dropped Joined: 30 Sep 2007 Posts: 211 Location: Finland

You can modify Your tachometer quite easily. Open the tacho, find resistors from switch where You can choose 4-6-8 cylinder -mode, find a resistor what is connected in switch for 6-cylinders and put adjustable resistor (trim pot, trimmer potentiometer) to place where that resistor was.

Then connect tacho to 2CV, put the switch to 4-cylinder, try to get idle for example 1000rpm, move switch to "modified 6-cylinder" and adjust trimmer to get tacho reading to 2000rpm, check the reading by changing the switch between 4- and 6-cylinders, and thats it.

You can also get "tacho-signal" from cheap battery charger, it gives not-so-good-dc, full bridge rectified dc in 100Hz. Put this "signal" to tacho's coil input, and try to adjust reading to 6000rpm. This wors with most tachometers, but not with all...
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Jasu · Dropped Joined: 30 Sep 2007 Posts: 211 Location: Finland

Modification was quite easy to old Yazaki tacho, just becausen there was already trimmers for adjusting... It needed only to cut one line and put extra resistor with trimmer to get right scaling for 2-cylinders...

Like this.

More photos there.
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phoskam · Posted: Sat Dec 08, 2007 9:20 pm Post subject:

here's an inside picture of my tacho, there are 3 resistors just near the switch, each of them connected to one of the switch positions, also there are 3 trimmers, maybe one for each setting (4,6 and8 cilinder adjustment) ?

I'm still working on my body and my chassis and engine are waiting for there treatment, so I can't test it in short time, but any information is more than welcome, so I know what to do when it's time to mount the tacho.

Jasu · Dropped Joined: 30 Sep 2007 Posts: 211 Location: Finland

dyanut · Posted: Thu Sep 25, 2008 10:39 pm Post subject:

Hi Jasu,
is there any chance of advising me what the value is of that extra resistor which you used to recalibrate your Yazaki tacho for 2 cylinders?
We've got two of these to play with, but it's very difficult to figure out the colours on the 6 band resistor which is shown on your picture.
I'm wary of fitting the wrong value, when it could be 'goodnight old tacho'... Sad

Thanks, Ken.

Jasu · Dropped Joined: 30 Sep 2007 Posts: 211 Location: Finland

ami8i · Posted: Sat Oct 04, 2008 4:57 pm Post subject:

Hello phoskam,
I'd try to make some reverse engineering, which means to draw the part of the circuit/schematics beginning at the 3 position selector (each direction).

I guess there are just a few resitors (R1, R2, R3, R9(?), R10, R11?) and caps on each track until all tracks meet at one point/potential.

Then make a table with lines called cylinder 8 6 and 4 in 1st column and each component in the next colomns. I estimate that you could find out the correct number with a little bit of math (proprotional, indirect proprtional).

The IC LM358N seems to expect a frequency. Check http://www.datasheetcatalog.org/datasheets/166/49945_DS.pdf and study the circuits at page 6/9. I guess circuit of Fig13 could suit to you.

If you have an idea which components should be changed remove them off the 8cylinder track and solder the 'calculated' components for 2 cylinders.

I do not recomend to change one of the marked trimmers before you don't know what to do (potentimeter of capacitor).

@BX sensors:
Diesel and BX14 sensors are not typical inductive sensor, just those of the 60-2 tooth wheel engines (BX16 BDY, BX19 ???)

I guess your speedometer is made for triggering by coil (minus) --> would not work with 'tiny' 5V squrae waves otherwise google for frequency doubler.

»Horst
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