Courtesy Of 97EldoCoupe

I've been going through some old service manuals I have. This; due to my '97 STS having an over voltage condition that blew out the PCM....

Section 8C1-8 of the 1996 Service Manual Book 1.


Coolant temperature is broadcast from the PCM via Class 2 serial data to the IPC. The IPC compares the temperature data with a sixteen-point calibration table to determine needle deflection. A smoothing filter is used to secure a continuous response.
This says it. Think about what the smoothing filter would do provided there was quick temp spikes or changes. Think about the fact that it uses a 16 point calibration table. One degree of temperature change would be the difference between the IPC commanding the needle to move to the next 16th position or not. This gives a lot of room for variances between cars. Very easily. So after a head gasket job is complete; if the vehicle ran one degree hotter or cooler; this one degree could be the difference between the gauge needle moving past 12:00 or not.

This is out of the 1996 manual; but keep in mind Cadillac retained a lot of the same electronic engineering through many years and models.

I don't know if this has ever been found by another forum member or not; but if possible; Ranger; please Sticky this somewhere for future reference. This has been of topic for a while; and short of talking to the original engineers; this may be the most in-depth information on the subject we will ever come up with.

Also the smoothing filter; since we now know one is used; may be programmed different through the years and models. Doubtful but easily possible.