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

[Lloyd Ferguson]

>At 11:10 AM 11/26/95 EST, you wrote:
>>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
>>
>
>How about pin M?  Do you have a wire on it?
>
>                   -------------------------
>                   | F | E | D | C | B | A |
>                   -------------------------
>                   | G | H | J | K | L | M |
>                   -------------------------
>
>  (most are as you've listed above)
>
>      A - Ground
>      B - Diagnostics ("Test") terminal
>      C - Air Select/EFE
>      D - MIL ("Check Engine Light")
>      E - Serial Data (on older ECM's)
>      F - TCC (Torque Converter Clutch)
>      G - Fuel Pump
>      H - Brake
>      J - ???
>      K - ???
>      L - ???
>      M - Serial Data (on newer ECMs/PCMs)
>
>The newer ecms/pcms (at least 93 vehicles) have a serial data wire on pin M...
>
>Donald Whisnant
>dewhisna at ix.netcom.com

No, there is no wire on pin M (or any other pin on the bottom row), another
reader emailed me privately suggesting that my friend with the scan tool
might be misleading me but I helped him and even plugged the scan tool into
the ALDL myself!

Just thinking out loud, is it possible that the serial data is mulitiplexed
on the "Check Engine" light pin? If the signal was only a a couple of volts
it might not make the lamp glow in the normal way and could be set to
either + 12 or ground to turn the light on.  What I also noticed when the
scan tool was displaying the TPS voltage was that the system seemed to be
in the "test" mode (ISC going in and out and the mixture solenoid buzzing).


Lloyd Ferguson
CBC Engineering
7925 Cote St. Luc Road
Montreal, Quebec
H4W-1R5
Tel:    (514)485-5340
Fax:    (514)485-5364
E-mail: ferguson at srcing.login.qc.ca