DIS Tach signal

["Shawn R. Lin" <slin01@mail.orion.org>]

This is not directly ECM relates, but sort of related I think...
What type of output is the tach signal on a '90 Turbo Grand Prix (7727
ECM)?
The signal is generated by the DIS module and not the ECM.
More specifically, is it 3 pulses per revolution?
Also, is the instrument cluster tach supposed to be dead-on accurate, or
is it supposed to read a few RPM higher.  I ask because the service
manual mentions a test tool in which you set its output to read 54mph
(comes out to be 60Hz on a 4000 pulse per mile cluster), but the tach is
supposed to read 1350rpm.  It would sound like if the test jig is
sending out a 60Hz signal, the tach should read 1200rpm if the tach
signal is 3 pulses per rev.  Could be the test jig sends out 2 different
frequencies, but the fact that the speed must be set to a specific
frequency sounds like it just sends one frequency.

-- 
Shawn Lin
http://www2.cybercities.com/g/gmwbodycars/
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