No sumcheck code in a '95 MEMCAL

[WILMAN]

----------
> From: Ludis Langens <ludis at cruzers.com>
> To: Diy_efi <diy_efi at esl.eng.ohio-state.edu>
> Subject: No sumcheck code in a '95 MEMCAL
> Date: Thursday, November 05, 1998 1:33 PM
> 
> I've been disassembling a MEMCAL from 1995.  It's BCC BKLL1692, used in
> the 16196401 ECM.  It's for a W-body with the VIN X 3.4 SFI DOHC
> engine.  The 32K byte EPROM (27C256) has the standard 9 byte P4 header,
> but the sumcheck field doesn't match the computed sumcheck.  Further,
> the program code does not contain a sumchecking subroutine!
> 
> Has anyone else found a MEMCAL like this?  Could this be in preperation
> for OBD II?  (This MEMCAL&ECM are just OBD I.)
> 
> -- 
> Ludis Langens                               ludis (at) cruzers (dot) com
> Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/
> 

This is not directly related to the above.
I am working on a Marelli 8P20 ECU and it has a self sumcheck feature.
If I alter any single byte without compensating it elsewhere and thereby
making
the checksum value unchanged, the "check engine" light on the dashboard
will
come up.
I have been told that this checksum value is stored in the eprom.   
Can anyone tell me how I can find out where exactly it is stored?
The original checksum  is E63E.


Wilman