No comms on a "new" ECM

[Bob Wooten]

So it is likely then that if the ECM appears to be running fine (not in LHM)
but the comms don't work that the RS232 chip is in fact bad & that changing
it MIGHT make it work again.  I suppose as the spare in the trunk ECM it is
not the end of the world if the RS232 does not work, as long as I can get
home.

thanks amigo.
BW

-----Original Message-----
From: owner-gmecm at diy-efi.org [mailto:owner-gmecm at diy-efi.org]On Behalf
Of Ludis Langens
Sent: Friday, June 22, 2001 2:52 PM
To: gmecm at diy-efi.org
Subject: Re: No comms on a "new" ECM


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/

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