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Old 06-19-2005, 01:01   #1 (permalink)
ross via CarKB.com
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Re: New PCM? And will it need the calibration code?

Happy Traveler wrote:
>Thanks, Ross. Great advice.
>Just a follow-up question that came to mind:
>If it comes pre-programmed for the engine, how about the axle ratio (mine is
>3.73)? Does this thing want to know how fast I am moving, or just how fast
>the transmission output shaft is turning... And what about 4WD vs RWD? (mine
>is AWD, actually). And while we are at it, how about tire size? That's going
>too far, for sure. Sounds like I need a crash course in engine management
>that goes a bit beyond the Probst book that I read 10 years ago...
>
>And by the way, I happen to know something about non-volatile memories - Got
>an EE degree and quite a few years of experience in that field ... Wouldn't
>mind putting my hands on a PCM schematic. Any clue where to find it?
>


Late answer - just discovered your reply.

You are going to exceed my knowledge level of PCM's pretty quickly. Others
here will probably know more......

I believe the PCM relies on the speed sensor on the transmission, there is no
compensation for tire size. If you do a radical tire size change my
understanding is you have to use a different vehicle speed sensor, or buy an
aftermarket electronic device that corrects the output. So, changing axle
ratios would result in the same answer.

There is software available that will let you monitor your PCM in real time
and select the various sensors you wish to monitor, such as oxygen sensor
voltages, etc. The pricier the better the software (in general), and
manufacturer specific, unless you cough up the big bucks like a repair shop.

There are also some programming tools available for changing variables in the
PCM tables. Don't know anything about them, but I've seen some messages from
people with turbos / superchargers that revise EGR output levels to control
ping, etc. Kind of like programming your own speed chip. Expensive I suppose.


On the late 90's vehicles there are transfer case sensors as well as vehicle
deceleration sensors for ABS, and many others, but in general none of those
feeds back into the PCM to modify vehicle behavior, they are there for the
functioning of the particular system they are part of. The auto transmission
is the only one that is really integrated I think.

However, the new 2006 Explorers now have an antirollover system. You can see
that the sensor inputs are now being incorporated into vehicle behavior in a
more integrated way - if you are cornering too quickly the rollover system
starts applying brakes using programmed logic, reducing engine throttle, etc.
Won't be long and you will drive by a roadside sensor that will poll your car
for its speed, issue an electronic speeding ticket, and bill your mastercard.


I have not seen a reference to a circuit diagram anywhere. Proprietary stuff
I suppose. There must be something available however. Would not think you
could create the software / programming tools without it.

I noticed you had gotten pretty far in your electical diagnostics and thought
that was a little unusual........

Sorry for the late reply.
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