[DeTomaso] THE TACH AND THE MSD AGAIN / READS LOW
SOBill at aol.com
SOBill at aol.com
Mon Jun 11 13:23:38 EDT 2007
Ned,
Were the tach readings correct before the MSD implant?
If "Yes" my GUESS is that the tach adapter output is marginal in amplitude.
Here is a way to test an 8920 adapter in the car (other # adapters should
test the same at the wire that goes to the Pantera tach):
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 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
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