What do you think the signal looks like that is fed back to the ECM from the distributor module? Basically, the magnetic pickup ring connects to the distributor module... but what is fed back from there? Is it a 5vdc on/off square waveform or ??

The problem is that the car exhibits spark scatter throughout the rpm range only under part throttle. If I'm into the pedal hard-- it is fine. The cap, rotor, wires, module, reluctor ring and pickup have all been replaced. In the base timing mode with the EST connector "open" the car will also run fine. So, my theory is that the reference signal that is fed back to the ECM is goofed up.

 

FYI the DRP is a 5 v square wave. HTH

For a circuit description, see this thread:

http://www.thirdgen.org/techbb2/show...hreadid=243633

 

thanks for the heads up and the link to the other thread!

The 5vdc square wave is not the problem. More of the problem is the position of the magnets on the distributor shaft. Apparently they were allowed to move when the distributor was disassembled to replace the pickup assy.

What is the theory behind the reference input to the ECM and it's position in the timing sequence? If the timing signal references the trailing edge of that 5vdc input... it would seem to me that the position of the rotating magnet is at least 1 degree or more ahead of the rotor. This would allow the computer (and ignition module) to allow the secondary ignition circuit of the coil to fire... dunno for sure.. just talking out loud...

 

It is not clear to me if there was a question somewhere in there. The HEI module triggers at
the 'Zero Crossing' point of the reluctor wheel; the wheel's position relative to TDC is adjusted
by turning the distrib base.

Do you have a more exact definition of the 'scatter' situation you are trying to address? Are
you concerned about a data logging issue or a performance issue?


If you read the reference papers given in the TGO thread, you have the description of how
the HEI module works WRT the reluctor, DRP, and EST signals. To see where the DRP is
used in the ECM you need to open a copy of the code hack; generally DRP is the main ECM
sync for engine speed, fuel, and spark calcs (and their transients).

Looking at the Nat Semi LM 1815 application notes, you will see:

1. Module operation is sensitive to the correct phasing of the pick-up coil signal. In my
experience you can NOT rely on the marked polarity of new components - you need to
verify correct hook up in the field.

2. DRP signal is sensitive to good connections for the Reference and ECM Ground lines.
In my experience, the connectors at the distrib base seem to get a lot of oil & heat - old,
brittle wiring should be REPLACED with new pieces. The back of the 7-pin HEI module needs
to be well grounded AND have thermal paste applied.

3. I'm unaware of any difference in the circuits or program between part-throttle and WOT
operation. If you are not 'seeing' spark scatter at WOT it might be due to the limited data
transfer rate of the ALDL commo bus. The same scatter can be present, just invisible to the
Scan prog's slow update rate. There is some info on data rates & data acquisition on my CF
page(s).



My own hypothesis on L98 'spark scatter' is that the traditional SBC distrib drive stacks up
too much lash in the timing chain/cam/oil pump/distrib gears. While the system works fine
on a mechanical distrib advance, it has too much slop for an ECM's ability for accurate time
resolution, particularly with the later P4 ECMs (i.e. ~15? usec clock counts). Note that GM
changed to various kinds of crank triggers on the SB after '91.