Here's some back issues of the digest when Todd Day was first figuring out how to talk to the computer.... just incase it helps you.
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The ENG circuitry is almost impossible to decode. I get lost in a
massive tangle of SMDs with two character labels and laser-cut
resistors. However, I was able to decipher about a third of the
circuitry. Surprise! Except for the protection diode, the third
I could decipher looks almost like the DATA circuitry.
This is curious. Just what is output circuitry doing with input
circuitry? I think that DATA is a mode switch and ENG functions
as both an imput line and output line. You leave it
floating when you want the computer to talk to you. You ground
it and then talk to the computer on the ENG line. Just a wild
hunch, but it's the best I could come up with.
Now the bad news. Since I don't have the slightest clue what the
computer requires (i.e., it's language), I have no idea of what to
try to send to it. Besides, it's just a wild guess.
I'll do some reading of the main manual when I get home, and update
you if I figure anything else out.
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Okay, I went home and looked thru the big manual, and found an interesting
reference to pin 10 in, of all places, the automatic transmission control
unit! So this blows the idea that pin 10 is some kind of data input. But,
it is called "Diagnosis control" in the manual. Strangely enough, although
it is labeled, it is not mentioned at all in the tranny part of the service
manual.
But, the plot thickens! In the schematic for the TCU, it shows the pin
that's supposed to be for reading out the error codes as having the
facilities for reading a signal on that pin!
So, here is what I suspect. Grounding pin 10 puts both the MPI and the TCU
into a special diagnostic mode where you can actually *talk* to them! This
means direct control over the engine computer *and* the automatic transmission
computer! Basically, pin 10 is a direction switch. When grounded, it accepts
data on what used to be the diagnostic output. When floating or high, it
outputs data on that pin.
Here's the bad news. I have absolutely no idea how to talk to the computers.
I only know how to put them into this mode.
But wait! What about the part of the manual where it tells you to ground
pin 10? [This can be found in the Basic Idle Speed Adjustment section of
the manual under Fuel System]. Well, here, I think what happens when you
put the computer into this special diagnostic mode, is that it stops trying
to automatically compensate for changes in its inputs. This allows you to set
the idle speed without interference from the computer.
BTW, I just went out and tried grounding that pin with a semi-warm engine,
and it did nothing. Perhaps the results would be different with a cold
engine.
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Interesting tidbits I found inside engine computer...
Processor is Mistubishi MH6111 (does not appear to have
pinout of any Motorola 6811 part (damn)). It has an
ASIC, which I have verified, part number 60011. It
has another IC which I've not identified, but I forgot
part number (sorry, I should have written it down).
It does have an EEPROM with a window (27128).
Neato findings: (keep in mind I have a '90 Talon AWD)
Missing resistor R110, which is connected to the turbo
boost gauge output. Perhaps this allows use of different
boost gauge?
Missing resistor R102, which is connected to the air flow
sensor. Perhaps this allows use of different air flow
sensor?
Missing jumper J104, which is connected to the only empty
pin (does not connect on any other '90 models, either).
This is related in someway to air flow section.
Jumpers J102 and J103 apparently select whether Pin 13
acts as fuel cutoff or motor position sensor input (1.8L).
Pin 14 goes to some mysterious part of the air flow sensor,
which does not exist on non-turbo models.
Pin 17 is GND for 2.0L, motor position sensor input for 1.8L.
Missing jumper J106 and resistor R111 have something to do
with purge solenoid.
Pin 104 is +5V for automatic tranny, GND for manual tranny.
If you do open up your computer, you will find what appear
to be five tab regulators. Actually, the one that looks
different from the rest is the only tab regualtor, and it
outputs +5V. The other four transistors are the fuel injector
controllers.
Well, that's about all I could get from looking at the circuit
and studying the schematic. Perhaps all of you with manuals
could spend some time this evening and report back with any
info that may have been added in newer models.
Also, please please please, someone who knows a service tech
very well, pump them for info on the engine computer and how
you talk to it!
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Once again, I opened up the engine computer to see what I could see.
This time, I was a bit more intelligent about it, though.
All this talk about controlling the car via a handheld PC has got
me hooked into figuring out how it works. Well, I did a bunch of
tracing on the circuit board, but got caught in SMD hell and was
unable to figure some of it out.
The key appears to be the line marked "DATA". I don't have the main
part of the manual with me here at work, but I could have sworn I
saw something to the effect that grounding this line allows you to
make some adjustment to the car (either idle speed or timing). After
reading the electrical manual, this strikes me as odd, since there are
check plugs for doing either of these tasks. The DATA line is not
mentioned in the electrical manual except for the schematic.
Let's back up a bit here. The line marked "ENG" provides the error
codes for sensor failure, or the "heartbeat" pulse if everything is
okay. So what I'm thinking is that DATA is input and ENG is output.
However, this is not true. The second you ground DATA, the heartbeat
from ENG stops. What is it waiting for? I decided to do some tracing
on the pc board to find out.
The DATA circuitry looks pretty much like what is shown for the reed
switch input. Basically, the line is pulled to 5 volts through a
protection diode and then drives a transistor to give a clean logic
level waveform to the CPU.
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Mike J-Quadcylla-92 Laser RS-T, FWD
Vortex Racing http://www3.sympatico.ca/robin.jackson2/HomePage.htm Just call me Runner-Up