Bench Test a Tachometer

[amc_oldsarge]

Is there a way to bench test a stock tachometer. I want to verify mine is functioning before I chase down gremlins.

[ArnZ]

One of these would do it.

http://www.msdignition.com/product.aspx?id=4777&terms=tester

 

Or you could use a function generator.

[Trevor]

How to Check Your Tachometer

 

A 120 Vac 60 Hz battery charger puts out an imperfect DC voltage. It is actually a pulsating DC voltage that corresponds to

 

 

1800 rpm for an 8 cylinder,

2400 for a 6 cylinder

3600 for a 4 cylinder

 

When hooked it up to a V-8 tach's input (+ve to tach +ve and -ve to gnd), it shows exactly 1800 rpm.

 

The technical theory goes like this. The battery charger doesn't put out a DC signal, it is a 120 Hz full rectified waveform. When a battery charger is connected to a battery, the battery acts like a huge capacitor and smoothes out the waveform (simple explanation).

Here's the math:

 

120 Hz = 120 cycles/sec

 

120 cycles/sec * 60 = 7200 cycles/min

 

There are 8 cylinders, so 7200 / 8 = 900 cycles/min

 

but it is a 4 stroke engine which only fires on 1 firing stroke (combustion)

 

It fires once every 2 cycles

 

So 900 cycles/min *( 4 strokes/1 firing stroke) * (1 firing stroke/2 cycles) = 1800 cycles/min or 1800 rpm