Trouble Codes/Limp Home Mode

[Ludis Langens]

"Bruce Plecan" <nacelp at bright.net> wrote:
> Is there a list of which trouble codes actually put the ecm into a 
> limp home mode

Not as far as I've found in a limited set of ECMs.  Of course, there
might be some that do.

Umm, just checked something.  A code 51 (PROM sumcheck error) _will_
cause limp home mode, at least in some ECMs.

Limp home mode is controlled by a hardware watchdog module.  If the CPU
stops tickling the watchdog, the watchdog declares the CPU out-to-lunch
and takes over.

There is no way for the CPU to directly cause limp home mode.  In fact,
the ALDL 3.9K limp home mode test mode just turns off the tickling. 
Eventually the watchdog decides the CPU is dead (even though its not)
and causes limp home mode.

Software that causes an action to occur by _not_ doing anything is kind
of wierd.  It is a strange way to program.  I've also found a second
such instance in an ECM: when the engine stalls, the program causes a
hardware reset of the entire ECM.  This took a long time to figure out. 
I guess GM didn't want to handle engine restarts in software.

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