No comms on a "new" ECM

[Ludis Langens]

Bob Wooten wrote:
> 
> I went to the wrecker a while back & got two new 7730's.  they both appear
> to work fine, but one does not have any comms' through the ALDL. it is not
> setting any codes & seems to run fine, but no comms.
> 
> Which brings me to another question now that I think about it.  when the ECM
> goes into LHM, does it automatically shut off the ALDL, or have I blown that
> up as well on the two that I have that are damaged (not the one above).

On a '730, limp home mode disables a bunch of outputs controlled by the
microprocessor.  From my memories of experiments, as part of this, limp
mode forces the outputs of the I/O expander chip to zero.  Limp mode
doesn't actually clear the output port - leaving limp mode will restore
the original outputs.  Anyway, in the '730, transmit via the ALDL chip
is enabled by an output bit of the expander chip.  Hence, ALDL transmit
will be disabled during limp mode.  This applies for any cause of limp
mode, either a real fault, or the 3.9K test mode.

Other P4 ECMs can act differently.  For example, the 1227165 doesn't
have an I/O expander chip.  The ALDL is controlled by an output bit on
the MPU itself - a bit not masked by limp mode.  The 1227748 ECM doesn't
have an I/O expander nor bidirectional ALDL chip.  It's transmit only
ALDL circuit is always enabled.

-- 
Ludis Langens                               ludis (at) cruzers (dot) com
Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/

----------------------------------------------------------------------------
To unsubscribe from gmecm, send "unsubscribe gmecm" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org