<VV> tach testing / calibration

[Craig Nicol]

Charles asked:
Our battery charger is bad, so I'll have a new one this evening to test the
tach. Thank for all ya'll's help. Could someone indulge my unbeknownst
nature and explain to me just how a tach works and how the electricity runs
through it?

Charles   4/30/08 

Craig replies:
Here's the condensed version:
Each time the points open, the wire from the distributor changes from
0-volts to +12-volts. The voltage goes back to zero when the points close.
This creates a little electric "chain" of 0-12-0-12-0-12. The faster the
engine runs, the more often the 0-12-0 signal changes state. This signal is
carried by the "tach" wire to the "D" terminal on the back of the tach.

A diode and filter in the tach "clean up" the signal, which then gos to a
single transistor amplifier.  This amplifier runs on 12V from the ignition
key, and of course also requires a ground back to the body.

The transistor amplifies the signal 'cause we need power to run the
instrument and we don't want to draw any from the distributor. The signal
coming out of the transistor is massaged by some diodes (voltage is capped,
dc offset blocked, and it's rectified to square-wave DC). 

The signal from the amp, after massaging is just about like the signal from
the distributor.  It's effective power changes with rpm and this operates
the DC tach movement to indicate rpm.  The calibration resistor determines
how much of the DC signal goes to the movement and how much is bled to
ground.

Only a tiny amount of electricity from the distributor "runs through" the
tach.  The majority of the electricity (which in itself is very small) runs
through the + terminal on the back of the tach.  However, when the tach
isn't indicating rpm, there should be no consumption at all.

The only way I know of to test a tach is to connect power and ground (from a
battery) and an input signal to the D terminal. (actual dist signal, signal
generator, or (according to others) a battery charger.)
Hope that helps
Craig

No virus found in this outgoing message.
Checked by AVG. 
Version: 7.5.524 / Virus Database: 269.23.6/1407 - Release Date: 4/30/2008
11:35 AM