Dwell Control

[G. Scott Ponton]

Back to the problem with using a SY/TY bin with a system designed for DIS.

    All of my notes tell me basically the same things. All of the control
signals between the ECM/PCM and the ignition system are the same between the
HEI and DIS style systems. Although there are 2 different variants of the
DIS system and 2 different HEIs. The differences seem to be the same for
both systems in that each one has a fuel sync generator of some sort in
order to work with SPI. In the HEI system there is an aditional hall effect
IC built into the distributor which generates the fuel sync info. The DIS
systems generate this by way of additional circuitry in the ignition module.
    In both systems the ignition module is responsible for the entire
operation of the ignition system. This includes dwell and some minor amount
of advance(which is actually more of a variable dwell as RPMs change). It
(ignition module) is even responsible for determining which coil is fired in
a DIS sytem. The ECM does two things in order to interface and control the
timing.
    During cranking and until the ECM senses 400+ RPM it sends a signal to
the module to "bypass" the timing control signal from the computer. After
the engine reaches 400+ RPM it releases the "bypass" signal and takes over
timing control. It does this by receiving a reference signal from the
ignition module which it then modifies and sends back to the ignition
system. If you connect the reference signal, to the ignition control signal,
on both the HEI and DIS systems the ignition system will work totally
independant of the rest of the engine control system. Provided that the
bypass signal isn't "active".  The only exception is the opti-spark on the
LT engines. This system is more or less parts of both systems used in such a
way as to integrate the major part of the ignition operation into the PCM.
In this system the module is basically a simple switching device for the
coil.
    OK background info finished. On the the other problem. At this point it
becomes a magor problem too!! After careful though I think the problem that
is occuring has to do with the operational software for the SY/TY ECM. There
is something missing in a HEI system. It is missing a sync pulse. It's not
needed for the HEI system as the timing and position of the HEI determines
which cylinder gets the spark. The DIS system is more or less self contanted
BUT.................. a very large BUT here. The computer ( more conjecture
on my part from here) probably receives a sync pulse in the reference signal
from the ignition module. It then uses this to calculate overall ignition
timing based on the sync pulse. With the HEI it can just calculate ignition
timing per pulse etc. and let the HEI "worry" about which cylinder it is
firing.
    Anyone have more input on this or maybe something I have missed??

Scott

ASE certified Master
With Advanced Engine certification.