Smiths digital tachometer

[spoonerdtg]

Hello has any one else had problems with their retro fit smiths rev counter

Mine works fine on tickover but is wrong when driving?

My old VDO Rev counter at 3500rpm in 4th gear with a 5rib box was 63mph ( gps ) now with my amazing looking smiths gauge to match the smiths speedo is showing 3500rpm at 55mph (gps) and 70mph is 4000rpm?

 

[starbiscuit]

That percentage difference can't be explained away by a 4/6/8 cylinder selection switch.

How old is the VDO tacho? The new Smiths one is digital, so it counts ignition pulses and uses that number to drive the needle to a predetermined position.

If the VDO is analog, then the ignition pulses are used to charge a capacitor to create a voltage to drive the needle proportionally and will therefore have (and require) some facility for calibration, due to lots of voltage tolerances involved. The higher the RPM, then the more important the calibration accuracy (percentage error)

A good way to check is with an oscilloscope or frequency meter to count the ignition pulses independently at the coil, to see if 3500 rpm on either tacho is correct.

3600 is an easier number because if my mental maths is right, 2 sparks per revolution at 3600 RPM is 120Hz on oscilloscope / meter.
Or if you have an automotive test meter, it should show RPM directly.

HTH

 

[ozziedog]

I was just about to say exactly that ^^^ but you just beat me to it :roll: :roll:

Ozziedog,,,,,,,,it’s just numbers,,,relax :mrgreen: