Accessing GM Serial Data (was: Help needed reading Ford EEC-IV...)

[Lloyd Ferguson]

My 1984 Riviera has a 12-pin ALCL connector under the steering column.  I
have a GM service manual for a 1983 Grand Prix which says the pinout of this
connector is as follows:
        A       Ground
        B       Test Terminal
        C       Air Select/Early Fuel Evaporation Solenoid
        D       "Check Engine" Light
        E       Unused (no wire)       
        F       Torque Converter Clutch Solenoid
        G-L     Unused (no wires)

The connector in my car seems to match this description.

Grounding the "Test Terminal" with the engine not running will cause the
system to display the trouble codes on the "Check Engine" light, activate
all the solenoids and motors under the hood, and the ECM will not record any
additional codes while troubleshooting.  Grounding the "Test Terminal" with
the engine running causes the system to display the codes on the "Check
Engine" light and prevents the sysetm from storing any new codes.  The
manual also says to set up the TPS for example, a digital voltmeter must be
"tapped" into the leads at the sensor.

I got hold of a Mitchell maunal for the car which says pin E of the ADCL
(which comes from pin 8 of the ECM) is for serial data, but there is
physically no wire on this pin in my car!  My Grand Prix manual says pin 8
of the ECM is the Throttle Control Switch Input.

I took my car to a mechanic friend of mine and mentined that I was having
troublegettin my TPS adjusted properly, he took out his Scan Tool, plugged
it into my ADCL and was able to read the TPS voltage on the display.

One of the readers sent me some interesting information of the GM serial
data communications protocol which seems to run at 160 baud and says that it
is a continuous data stream. Another reader has explained the format of the
data, but my remaining question is WHAT PIN IS THE DATA ON AND/OR HOW DO YOU
GET ACCESS TO IT?

Lloyd Ferguson
ferguson at srcing.login.qc.ca