Originally Posted by Old Ben

It indicates a lot of things: Oxygen sensor; Coolant temp sensor; Throttle position sensor; Quad-driver module; EGR; MAP; Electronic spark timing circuit; A/C Pressure sensors and others. There are 34 different codes hidden in that "service engine soon" light. See page 6E3-A-1 of the FSM for a complete listing.

You will need a reader with the older 12 pin ALDL connector, or jump ports A&B on the ALDL port to read them.

The ALDL is above your right knee on drivers side hush panel.

Here's a pretty detailed explanation..

C4 Corvette Diagnostic Code Recovery Techniques

ECM (Engine Control Module) technique for recovering codes on the 1984 through 1993 model Corvette is explained first then the 1994 through 1996 16 pin ALDL connector OBD-II (On Board Diagnostics - Version 2) system explained below that.

Code Recovery: 1984 through 1993:

To recover ECM codes from the 1984 through 1993 Corvette, place a short as shown below between pins "A" and "B" on the ALDL (Assembly Line Diagnostic Link) connector. This connector is located under the dash just to the right of the steering column and has space for 12 pins although not all of the cavities will be populated.


Turn the ignition switch to "On" but do not start the engine.

Depending on the model year, either the "Check Engine Light" or the "Service Engine Soon" will begin to flash.

As the code display sequence begins, you will see a flash followed by a pause and then two flashes. This is the indication for the number "12". The number 12 is a delimiter intended to show where the ECM code display starts and stops.

Code 12 will flash three times and then any stored codes will flash. When all codes have been displayed or if there are no stored codes, the number 12 will again flash three times.

For example, a problem with the EGR system (code 32) will be shown as 12, 12, 12, 32, 32, 32, 12, 12, 12.

This would be displayed on the Check Engine/Service Engine Soon light like this:

12: flash (pause), flash, flash, (long pause)
12: flash (pause), flash, flash, (long pause)
12: flash (pause), flash, flash, (long pause)
32: flash, flash, flash (pause) flash, flash (long pause)
32: flash, flash, flash (pause), flash, flash (long pause)
32: flash, flash, flash (pause), flash, flash (long pause)
12: flash (pause), flash, flash, (long pause)
12: flash (pause), flash, flash, (long pause)
12: flash (pause), flash, flash (long pause)

The ECM code display will repeat until you turn off the ignition switch and remove the short.
Additional Codes available on the 1984 through 1993 Corvettes:

Selective Ride Codes:

On the 1989 Corvette, if you have the Selective Ride Option, any codes associated with the SRC will cause the Service Selective Ride System light to flash when you short ALDL pins "A" to "B". The sequence uses the same 12, 12, 12 delimiter as the ECM code display uses.

Some later Corvettes require you to short pins "C" to "A" to flash codes on the SSR light. It varies with the model and you will need to consult the service manual for your particular model year to find out if these are the proper pins or if there is another technique requried.

ABS Codes:

On the 1990 and 1991 model shorting pin "H" to pin "A" will cause the ABS light to flash ABS codes. (Beginning in 1992, there is another technique which is explained below).



Not all codes are displayed using this technique. There are four codes that will not display without a scan tool. Also, note that the 1986 through 1989 Corvettes will not display any ABS codes. A special test fixture is required for those models.


Sequenced Code Recovery - 1990 through 1993:

With the interior redesign in 1990, Chevrolet added additional computer capability. The CCM (Central Control Module) was added and a method was provided to display codes on the speedometer of the Corvette.

From 1990 through 1993, you shorted ALDL pin "G" to "A" and turned the ignition to "On" without starting the engine.



This action will cause the speedometer and trip odometer to become a troubleshooting aid:



The codes will display on the speedometer. If there are no codes, the speedometer will show three dashes (---) and if the CCM cannot communicate with the other computers, the letters "ERR" will show up on the display.

The module number associated with the codes will be displayed on the odometer and will show which computer's codes is being displayed.

For the 1990 and 1991 Corvette, the modules are Module "1", the CCM computer and module "4", the ECM module.

For 1992 and 1993, an additional module, module "7", the ABS (Anti-Lock Brake System) and ASR (Automatic Slip Regulation System) codes will be displayed.

The codes will show up for three seconds each and once all codes have been displayed, the speedometer will show three dashes. You can either turn the ignition off for five seconds and then back on to repeat the process or turn the ignition off and remove the short to restore normal operation.

Sequenced Code Recovery - 1994 through 1996:

In 1994, Chevrolet went to a 16 pin ALDL connector in preparation for the federally mandated change to the much more capable OBD-II (On Board Diagnostics - Version 2) system.

As with the 1990-1993 model, you can display codes on the speedometer but the pins to be shorted are different:



Beginning in 1994, you short pin "12" to pin "4" and once again turn the ignition switch to "On" without starting the engine.

The speedometer will display any codes using the same protocol as the 1990-1993 model but there are some changes in the designation for the modules plus additional modules are added:

Module "1" is still the CCM module.

Module "4" is now called the PCM module (Powertrain Control Module) because automatic transmission computer control was added to Engine Control Module in 1994.

Module 7 (on the 1996 model only) is the RTD module. (This is the Real Time Dampening module which replaced the Selective Ride Control module in 1996).

Module 9 is the ABS/ASR module number from 1994 through 1996.

Finally, Module "A", the DERM (Dynamic Energy Control Module --- the air bag control module) will be requested to show any codes.

Just like the 1990-1993 display, you read the codes on the speedometer and read the module number on the trip odometer.

Again, you can cause the codes to repeat by turning the ignition off for five seconds and then back on.

Turn the ignition off and remove the short to restore normal operation.

Originally Posted by mustang.vette

True to your thread sir. Yes I did those procedure since I was able to get an ALDL short connectors and it showed H23 and H73 eroor code on the ASR and on the manual it says to earse the fault code on module 9 where the ABS / ASR is to clear on 9.7. My question is how is this done and where will I do the clearing and resetting?

By the way, I am from the Philippines and the C4 is restored fully, we also have a project car which is already running a C3 and a C5 so far these are all in good running condition no christmas lights yet on the dash.

Can anyone help me out on this?

Thanks again guys and have a pleasant day!