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Questions and Answers

Roadmaster Codes

Q. I have a stock 1994 Buick Roadmaster 5.7 liter LT1 with 4L60E transmission. OBD-I system. This car has never displayed any engine/transmission codes on the "Check Engine Soon" idiot light on the dash when pin B (Test) of the 12 pin DLC is jumped to Pin A (Ground), not even code 12, with the engine "Off" and the IgnSw "On".

The "Check Engine Soon" light remains off (however both radiator fans are on).

The "Check Engine Soon" light comes on with no DLC A-B jumper with the engine "Off" and the IgnSw "On". (Normal)

The "Check Engine Soon" light displays correctly for "field test" with the engine "On" and the IgnSw "On". (Normal)

The "Brake" light displays code 12 for the ABS system with the engine "Off" and the IgnSw "On" and the DLC jumped for ABS testing. (Normal)

Apparently, the OBD codes CAN be accessed with a scan tool on the DLC serial data stream since an independent shop and a "GoodWrench" shop both accessed codes recently to diagnose a shifting problem.

The PCM (PN 16188051) was replaced to correct the transmission shift problem but the problem with the OBD codes remains unchanged. Since the "Check Engine Soon" idiot light is controlled by the PCM, I assume that the engine/transmission code feature is not "programmed" into the PCM. The GM 1994 Roadmaster/Caprice service manual (Book 2-2; Driveability and Emissions-Fuel Injection pg.6E7, par. Diagnostic Mode) seems to indicate that the "Diagnostic Mode" will function "on certain vehicles".

Is it possible to have my PCM "re-programmed" so the engine/transmission codes can be displayed with the "Check Engine Soon" idiot light? (or do I need to spring for an expensive scan tool?)


A. To extract a trouble code from the computer for diagnostic purposes, the twelve pin data link connector (DLC) is used.

The DLC is located in the passenger compartment. Terminal B of the connector is the test terminal and terminal A is the ground used for diagnostic display.

If the test terminal is grounded with the ignition on and engine not running, the system will enter the diagnostic mode. In this mode, the computer will display a Code 12, three consecutive times, by flashing the "CHECK ENGINE" or "SERVICE ENGINE SOON" light. A Code 12 consists of one flash, a short pause, then two flashes in rapid succession. After Code 12 is displayed, any stored trouble codes will be displayed by flashing the "CHECK ENGINE" or "SERVICE ENGINE SOON" light. All codes are displayed 3 times each then a code 12 will repeat. In the diagnostic mode, the computer will also energize all computer controlled relays and solenoids.

Whenever a PCM is replaced it should be reprogrammed. If it was replaced anywhere else other than a GM dealer, it is almost certain it was not reprogrammed. So a trip to the GM dealer is required to insure the PCM is programmed correctly.

Additional Information provided courtesy of ALLDATA

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