[DeTomaso] 8920 Tach Adapter Checout In The Car

[SOBill at aol.com]

EA,
 
 
Installation 
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 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. 
Checkout 
You can verify that  the 8920 is working in the car using a multimeter. In 
the following, +12V  DC means the voltage of the battery. 
With the engine  running the White wire between the 6AL and the 8920 should 
read around +2.8 V  DC. This indicates that the 6AL is providing a 20% duty 
cycle +12 volt square  wave to the 8920. This voltage will not change with 
the speed of the engine  because the signal duty cycle % does not change. If 
the White wire reads 0 V DC,  you may have a bad connection. If the White 
wire reads +12V DC, the 6AL tach  output signal is bad. 
With the engine  running, the Violet wire tach signal out of the 8920 
should read as  follow: 
1,000 RPM = 1.27 VAC,  2,000 RPM = 2.62 VAC, 4,000 RPM = 4.75 VAC 
This indicates that  the 8920 is putting out the normal point/coil type 
signal to trigger the Pantera  Tach. Note that we are reading AC voltage here 
because we want to see the  inductive ringing signal which occurs with each 
spark occurrence. The AC voltage  reading on the Violet wire increases with 
engine speed. 
With the engine not  running, if you disconnect the White wire from the 
8920 (i.e. remove the 6AL  tach trigger signal), the Violet wire of the 8920 
will read +12V DC. With the  White trigger wire then reconnected to the 8920, 
the Violet wire will read  slightly less than +12V DC. Although the 
difference between readings is only on  the order of 0.5V DC, this is a second 
indication that the 8920 is working  correctly.
 
Have fun,
 
SOBill  Taylor
sobill at aol.com