[DeTomaso] Tach signal MSD vs Mallory

[B. SEIB]

Larry,
Just sending you some saved excerpts from sage advice of SOBill:
Barry


From: detomaso-bounces at realbig.com on behalf of SOBill at aol.com
Sent: Thursday, May 05, 2005 11:08 AM
Subject: [DeTomaso] MSD 8920 Installation and In Car Checkout
 
The White wire of the 8920 must be connected to the Tach  Signal connection 
on the 6A. The Tach Signal connection on the  6A will have a 20% duty cycle
12v 
square wave on it. One pulse for each  spark occurrence. The presence of
this 
signal can be checked with a multimeter.  When the engine is running, the 
Tach Signal Terminal will read some positive DC  voltage. It will be much
less 
than 12V but will rise as the engine speed  increases.
 
The Black wire of the 8920 must go to a good chassis  ground.
 
The Violet wire of the 8920 must be connected to the Blue/Black  wire which 
goes to the Pantera Tachometer input terminal. There must be no other  wire 
connected to the Blue/Black wire. My car was built with the Blue/Black
(Pantera 
Tach wire) crimped into a common terminal with the Blue wire which  goes to 
the breaker points and condenser in the OEM distributor. To be  safe, be
sure 
the Violet wire is connected ONLY to the blue/Black  wire.
 
The Red wire of the 8920 should go to the same place as  the small Red wire 
of the 6A. This should be 12V when the ignition is in  Start or Run. Fuse
#12 
is a good place. Using the Pink ballast resistance wire  which previously
went 
to the plus side of the OEM coil may work, but I would  wire it to Fuse #12.

You can do this easily thru the passenger side rocker area  using the seat
belt 
attachment opening.
 
The 8920 takes the 12V Tach signal of the 6A and converts it to a  signal 
equivalent to what would be found on the Negative (breaker point)  terminal
of a 
points/coil ignition.> In a message dated 10/1/2010

SOBill at aol.com wrote:


> The OEM Pantera tach is driven by the signal on the side of the   
> ignition
> coil which is connected to the ignition points.
>
> This signal has an approximately 200V transient signal each time  the
> points open and spark is generated at one of the 8 cylinders.
>
> If an after market ignition box is installed in a Pantera with an  OEM
> tach, a tach adapter module will be required to convert the ignition  
> box  normal
> 12v 20% duty cycle square wave signal into an equivalent of the OEM
> ignition points/coil signal.
>
> Have fun  today,
>
> sobill at aol.com

The OEM Pantera tach is voltage triggered.
 
The tach requires the 200 volt transients that occur at the coil  negative 
terminal when the points open. The various aftermarket ignition
manufacturers 
build tach adapters for voltage triggered tachs which accept a 12V  square
wave 
from the ignition box and using a power transistor and a coil  generate the 
required 200 volt transient required by the Pantera  tach.
 
The Pantera tach contains the electronic circuitry required to  filter and 
detect the 200 volt pulses and convert them to standard  width pulses for 
driving the meter  movement.
 
Have fun,
 
SOBill  Taylor
sobill at aol.com