[DeTomaso] Tach Problem

[David & Marilyn Bell]

I should also note that SOBill correctly deduced that I have a MSD 8920 tach
adaptor and MSD 6-AL, which I somehow neglected to mention.

I'll have more info to report this afternoon.


Dave Bell
  -----Original Message-----
  From: SOBill at aol.com [mailto:SOBill at aol.com]
  Sent: Monday, October 26, 2009 12:54 AM
  To: davidabell at worldnet.att.net; detomaso at realbig.com
  Subject: Re: [DeTomaso] Tach Problem


  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