[DeTomaso] Tach Problem

[SOBill at aol.com]

Dave,
 
 
 
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.
 
If you have access to an oscilloscope, the 8920 Violet wire ringing  signal 
is around 8,300 cycles and has a voltage peak of around +20 volts. Since  
the 8920 has been used on many Panteras, the +20 volt peak and the 8,300  
cycles is adequate for the Veglia tach.
 
Hope this helps.
 
 


SOBill  Taylor
sobill at aol.com