DIS Tach signal

[Shannen Durphey]

Shawn R. Lin wrote:
> 
> 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.
Speedometer driver box has a switch for 30hz/60hz signal, and another
labeled 0 and 54 mph (aka on/off).  Lately, GM gauges are more of a
"feel good" item.  They provide relative information.
Shannen
> 
> --
> Shawn Lin
> http://www2.cybercities.com/g/gmwbodycars/
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