Re: 3.9k Ohm Diagnostic mode on a P4.

["Bruce Plecan" <nacelp@bright.net>]

Two "reasons" that I've heard of know of.

The know is:
Back when they started with ecms, the backup mode gave the tech a way to
read a lousy running engine.  Forced a 1K idle, and fixed timing.  With a
timing light and tach, that way you could at least tell if the ecm was awake
(remember this is 20 years ago).  There were a bunch of mechanics that were
real leery of the new fangled stuff.  This was an intro to get used to the
idea, that they couldn't totally fail (the ecms).

Then for a while this was the mode that when done on the assembly line, that
the EPA demanded that the car be recorded in.  What the policy is now for
the new car recordings is now I haven't a clue.
Bruce




> I was told once that using a 3.9k DIAG resistor would force
> Backup Mode. This is apparently similar to Limp home mode but
> the processor is still operating (normally Limp home mode is
> selected by the analogue processor when it's not initialised
> by the CPU - when the EPROM or CPU has gone to lunch).
> Can anyone shed some light on how this mode is used by either
> the factory, or for field service? Specifically this is for a
> 1227165 and the answer may depend on the actual BIN used (?)
> Peter.

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