Adjustable electronic ignition

[J.W. Harris]

>According to the info I have (for a VN 1988 commodore with 3.8 V6 and GM
>P3 computer), there are 2 hall effect devices near the harmonic balancer
>that read pulses from 2 seperate rings. The outer ring has 18 pulses of
>equal spacing and the inner ring has 3 un-equal length slots & the
>computer counts how many of the 18 pulses occur in the length of the
>un-equal pulses to determine where the engine is. (It doesn't need to
>know if the cylinder's TDC is for the compression or for the exhaust stroke
>as it uses a wasted spark system).

Information on this system, as well as other GM service information that 
would answer many diy efi questions is available from Mascotech Marketing, 
800-393-4831.  You want the book on distributorless ignition systems, 
16020.05-2. $8.00.

I can tell as much as the book says.  The DIS module syncs itself with the 
crank by observing the 18x signal while the 3x signal is low.  3x has three 
unequal length slots.  It then knows which cylinder pair to fire.  18x is 
divided by 6 to produce a 3x symmetrical signal to send to the PCM along 
with 18x.
----------------------------------------------------------------------------
J.W. Harris                           Delco Electronics Corp.
Project Engineer                      One Corporate Center
Powertrain Electronics                M.S. CT40C
                                      Kokomo, IN 46904-9005
jwharris at eng.delcoelect.com
jwharris at holli.com

Of course, all responses are my own and should not be mistaken
for those of Delco Electronics or General Motors.