Reading 1994 Buick Century "Service Engine Soon" c

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hamhock75
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Reading 1994 Buick Century "Service Engine Soon" c

Post by hamhock75 » Thu Jul 13, 2017 2:18 pm

I have a 1994 Buick Century. The emissions sticker says it is OBD1 compliant. It has a 12 pin aldl connector with only three pins.

1. Pin A is black/white ground
2. Pin M is an orange wire - serial data signal
3. Pin K is a purple wire - diagnostic enable signal

The above descriptions of the wires are from the factory service manual. It has nothing connected to Pin B so shorting B to A is not possible as described elsewhere on the Internet.

Will a 12 pin aldl to USB cable and TunerPro software work to find the codes for the Service Engine Soon light on this car?

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Mangus
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Post by Mangus » Fri Jul 14, 2017 11:27 am

try A & K. Sounds like your K pin should actually be B.
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hamhock75
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Post by hamhock75 » Fri Jul 14, 2017 12:41 pm

I left something off. The purple wire is labelled "diagnostic enable signal - SIR" SIR is the supplemental inflatable restraint system ( i.e. airbag ) test enable.

hamhock75
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Post by hamhock75 » Sun Jul 16, 2017 1:29 pm

I discovered something new from Wikipedia regarding OBD1.
General Motors, some 1989–1995 Ford vehicles (DCL), and some 1989–1995 Toyota/Lexus vehicles have a live sensor data stream available, however, many other OBD-I equipped vehicles do not
This 1994 has live sensor data stream according to the service manual. It has a long list of scan data values including

1. Engine speed
2. Desired idle
3. Engine coolant temperature
4. Intake air temperature
5. Throttle position - volts
6. Throttle angle - %

The list contains 56 items with the last two, PROM ID# and Time From Start. Now if only I could read it.

hamhock75
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Post by hamhock75 » Tue Jul 18, 2017 9:07 am

I am making some progress. Found this article about vehicles of this vintage.

http://www.academia.edu/1963748/A_Simpl ... 1982-1995_

I bought an I/O Gear GUC232 adapter from a local store for $10. Installed it on my laptop. Set it to port COM1, used Hyperterminal to to test it. It worked but be sure to connect pins 2 an 3 together so it talks back to itself.

Downloaded Tuner ProRT and installed it with datastream definition A247 for "EBCM FOR 94 A/W/U AND 95 1W". Checked using "Tool", "Preferences", "Data Acq/Emulation", "Data Acquisition Plug-in Configuration", "Test for Valid Interface Using Setting". All working.

Used my digital storage oscilloscope to check the RS232 output voltage levels ( +/- 7.5 volts out of the I/O Gear adapter ). Tested the output voltage from the Buick aldl connector pin M ( 0 to 5 volts ).

The only thing I am not sure about is that the interface circuit described in the article above is connected to +12 volts and might not provide the right voltage levels to connect between the +/- 7.5 and 0/5 volts of the RS232 and aldl connector, respectively.

Will report back when I build it and see.

hamhock75
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Post by hamhock75 » Tue Jul 18, 2017 4:40 pm

I built the little two transistor converter from RS232 to aldl and vice versa from the paper above. It seems to be the same circuit shown here.

http://forums.parallax.com/discussion/1 ... odb1-cable

I should have realized it would not work. The I/O Gear USB to RS232 converter wants to see a real RS232 signal being sent to it from the TTL aldl serial port. Any two transistor circuit running off a single +V supply cannot supply the +/-V input needed.

I just bought a Maxim MAX232 IC for $0.95 locally. This chip actually has on board voltage converters to create RS232 signals that go positive and negative even though it operates from a single +5V supply.

Will report back when I have that together.

hamhock75
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Post by hamhock75 » Wed Jul 19, 2017 2:34 pm

It worked. The I/O Gear GUC232 USB to RS232 adapter followed by the Maxim MAX232 RS232 to TTL chip allowed me to connect up. The MAX232 is running off an HP6205C power supply at the moment because it needs +5 VDC.

I loaded the wrong definition file though. I used A247 from the OBD1 list because this car says it is OBD1 complient but A247 is for the EBCM ( ABS system ) which is what I am reading.

I will try the 1FA.ads definition from the OBD2 list for the L82 engine & see if that works.

Now that I realize what this car needs, I ordered a USB to serial adapter with TTL output from eBay for $1. Delivery will be in August but if it works I will report back. It would be much cheaper and a lot less work if it does.

hamhock75
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Post by hamhock75 » Wed Jul 19, 2017 2:57 pm

1FA definition worked! So the OBD1 compliant Buick has an OBD2 data stream. Found my problem too. Error in EGR solenoid 3.

Might have to cough up the $39 for Tuner ProRT. Too bad it takes so much effort to find out if it will work but I guess there is so much confusion with these 94 and 95 cars that its difficult to know. From what I have read, some are only OBD1 compliant but have a OBD2 connector ( some call that OBD 1.5 ) where mine seems to be OBD2 compliant with an OBD1 connector, lol.

A lot of credit to the guy who wrote this for making the full version available to try for free otherwise I probably would have coughed up $70 to have a mechanic scan it. It is just such a pain to leave the car at a shop for something so simple though.

hamhock75
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Post by hamhock75 » Fri Aug 04, 2017 1:14 pm

I spoke too soon. The software runs, the cable connects, everything acts like it is working but there is a problem.

The errors identified do not appear to be correct.

hamhock75
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Post by hamhock75 » Fri Sep 08, 2017 11:27 am

I had to give up trying to use Tuner Pro for now. Found an auto repair shop that read the 1994 Buick code with a Snap On tester for $20.

It said the Throttle Position Sensor was at +4.92 volts all the time. Removed the TPS and found it open circuit. Fortunately I have a spare.

I haven't given up on this yet. I ordered a FTDI USB to Serial converter off eBay. I couldn't get a driver to work with it until I took a close look with a magnifier at what the part number on the chip is. It is a Prolific PL2303HX part not FTDI at all. I found the Prolific web site and was able to download the driver. It works but I am having a physical connection issue. The USB plug itself seems so intermittent that i haven't been able to connect to the car yet except momentarily. When it does connect, it looks promising.

robertisaar
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Post by robertisaar » Fri Sep 15, 2017 5:14 am

the PL2303 doesn't work at 8192 baud, which can certainly cause your issues. the only USB to UART bridges I've used that work reliably have been FT232 units, though I have a CP2102(I think?) converter laying around here that I've never tested.

hamhock75
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Post by hamhock75 » Fri Sep 29, 2017 9:06 am

Thank you for that information. I ordered a FTDI part off eBay. What came was the Prolific part. I might look into trying to get another bridge that really is FTDI to see if I can get this to work.

In the meantime the Snap On tester said I had a bad TPS ( throttle position sensor ), bad park/neutral switch, and bad EGR.

It was right. The TPS was open circuit. two screws holding the park/neutral switch had come loose allowing it to rotate, and the EGR valve was bad.

Took care of all three. No more "Service Engine Soon" light.

It took me lots of phone calls before I could even find anyone that could test a pre-OBD2 car. Many had Snap On testers but did not invest in the parts needed to test a pre-OBD2 car.

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