This is puzzling Tachometer

[sigkadetm]

My tachometer bounces while driving but when I turn on the headlight it returns to normal. Any thoughts????

[TX82FIAT]

Yes, please read article below about capacitor fix.

https://www.mirafiori.com/faq/content/t ... achfix.htm

[sigkadetm]

I have one that says 684J400V will that work?

[TX82FIAT]

Not familiar with the number and would encourage some research. if it is a 0.68uF 400V capacitor added in series between the negative side of the coil and the electronic tach is should work.

[18Fiatsandcounting]

I have one that says 684J400V will that work?

Yes, that's it exactly. In the short form for capacitor notation, the first 3 digits relate to the capacitance in picoFarads. One microFarad is 1 million picoFarads. The first two numbers are the value in picoFarads, and the third digit is the multiplier, in this case 10 to the 4th power.

I know that's complicated, but your capacitor is 68 times 10 to the fourth power, or 680,000 picoFarads. Or, 0.68 microFarads.

The letter "J" is the tolerance, and in this case it's plus or minus 5%. 400V means what it sounds like: 400 volts.

Be sure not to use an electrolytic capacitor, which have defined plus and minus terminals. Electrolytics usually come in much higher values like 10 microFarads and above, so chances are very low that your cap is an electrolytic. If it doesn't show a "+" or "-" designation, it's not an electrolytic.

More than you wanted to know, I'm sure!

-Bryan

[sigkadetm]

thanks for the info........it is working