USM Newsletter – Oct/Nov 08

Finding the Code on computerized Corvettes

Check Engine or Service Engine Soon codes can never come at a good time (troubleshooting excluded). Whether on a road trip or heading to work, the ability to access the codes present in your vehicle’s ECM/PCM can be a great aid in determining how to proceed.

I have compiled a list of the computer controlled Corvettes with basic information on where to interface with the computer, how to access the information, & how the information is displayed.

Although automotive computers are great tools, they are not meant to be used to determine whether a part is good or not. The computer is essentially telling you how a problem is being detected. A code 45 does not necessarily mean the oxygen sensor is bad. It means that the sensor is detecting too little oxygen, which is interpreted as a rich condition. This could be caused by a blocked fuel return line, a leaking injector(s), bad ignition system components, or several other conditions. It might even be a bad computer.

Codes can be found online easily.

There are two types of diagnostic displays.

1980-1989

The earlier display (1980-89) involves the check engine or service engine soon light blinking in a specific pattern. When the diagnostic terminal is grounded and the ignition switched on, the CE or SES light will flash code 12, letting you know the ECM is in diagnostic mode, followed by any stored codes, and then ends by flashing code 12 again.

1980

In 1980, Corvettes came with three engine options; L48, L82 and LG4. The first two had M4ME carburetors, while the LG4 was a California variant 305 engine with an E4ME set-up. The E4ME was part of the C-4 or Computer Controlled Catalytic Converter system, marking GM’s entry into Electronic Fuel Injection.

There is a black and white wire coming off the computer from the black connector. I think the computer was near the glove box and the wire has a green connector at the end, which is not too far from the computer. Ground this wire and turn on the ignition to initiate diagnostic mode.

1981

For 1981 the C-4 system became standard on Vettes. Getting to the codes became easier as the ALCL (ALDL) was introduced. Located under the ashtray, this connector has 5 positions. Two outside terminals, D & E, have a recessed divider so you can short the two with a spade terminal and retrieve the codes.

1982

Basically 1982 is the same as 1981 except that the ALDL diagnostic terminals are now A & B.

1983

Finding the ALDL in the 1983 is nearly impossible. If you can get to it, let me know.

1984-1989

The ALDL was moved to its present location near the driver’s right knee in 1984. Like the 82, terminals A & B are shorted together to access codes.

1990-1995

The second display type (1990 & up) is more advanced, providing a digital read out as well as interactive capability. Codes are segregated by which module (ECM/PCM, BCM, etc.) they refer to, facilitating troubleshooting.

For 1990 to 1996 models, when terminal G is grounded (1990-93) or terminals 4 &12 (94-96) are shorted together and the ignition turned on, engine off, codes are displayed via the digital speedometer & odometer readouts through the CCM. The odometer shows the module & the speedometer shows the trouble code. The CCM runs through any codes present for the modules it monitors automatically. Once it cycles through, it then goes to manual mode. To the right of the dash are several buttons that

are used to navigate in manual mode. The button functions are as follows:

INST/AVER or FUEL INFO……..previous value

ENG MET ……………………………..next value

RANGE or GAUGES ……………….previous test

TRIP ODO …………………………….next test

FUEL RESET …………………………previous module   CCM is module #1.

TRIP RESET ………………………….next module       ECM/PCM is module #4.

There are several functions and tests that can be performed through this interface which are outside the scope of this newsletter.

1997 & up

To enter the diagnostic mode in C5’s use the buttons to the right of the dash. The info is displayed just below the speedo/tach assembly. Turn the ignition to on, engine can be on or off, hit the ‘reset’ button, then press and hold the ‘options’ button while pressing the ‘fuel’ button four times within 5 seconds. This will enable the automatic code display mode, which can be converted to manual mode but pressing any button except ‘E/M’. The display will show an abbreviation for the module on the left with the code to the right, with either a ‘C’ meaning the code is current or an ‘H’ for a code that was registered but is no longer being detected.

To navigate in manual mode, use the ‘Options’ button to scroll forward & ‘Trip’ to scroll backward through the modules, and ‘Gages’ for forward and ‘Fuel’ for backward through the codes within the selected module.

To clear codes in a module, go to that module and press and hold ‘Reset’ until the codes are cleared.