Accessing GM Serial Data (was: Help needed reading Ford

[Donald Whisnant]

At 02:07 PM 11/27/95 EST, you wrote:
>>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
>
>

It is VERY possible that it is multiplexed with the MIL signal ("check engine
light") ... I've got an '85 Grand Prix with shop manuals, but unfortunately
my shop manuals aren't here with me at the moment --- but since I've been doing
a lots of wiring and rewiring to put the '93 Chevy engine in, I seem to recall
seeing mention of the "check engine" pin begin called a "MIL/SERIAL DATA"
pin ...  So it is very possible...  The MIL signal works (at least on the '85)
when the MIL pin is at ground, but the signal from the ECM is active HIGH
instead -- i.e. it first goes through a "lamp driver" circuit. ....  I don't
know if the ECM must be in the "TEST MODE" for it to output data or not, I've
never tried...

Good luck...
Donald
dewhisna at ix.netcom.com