Dwell Control

[Shannen Durphey]

G. Scott Ponton wrote:
> 
> 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.

Better correct your notes.  For the EFI apps, ECM sends _no_ signal to
module during cranking.  
After bypass rpm level is reached, ECM sends 5V signal on BYPASS line,
and sends EST signal out.

>     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". 

If the bypass signal isn't active, the module does not use input from
the EST line. Jumpering the two wires without applying 5V to bypass is
ineffective.

> 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. 

The synch pulse in the 2.8,3.1 batch fire engines is sent only to the
module.  It's an "extra" pulse in the crank sensor signal.

>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. 

The ignition module handles this chore.  In batch fire PFI, or TBI w/
DIS, the ECM connections to the module are the same as HEI.

>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.
Shannen