Old Ben

Got a short duration "Service Engine Soon" light on the way home from work last Monday. Drove it again today and got another short duration light. Stopped at a store and shortly after I pulled out of the parking lot the light came on and stayed on until I got home (10 miles/ 15 minutes). The outside temp was around 95 and engine temp was in the 195 to 208 range at all three times.

Used a paper clip and got codes 26,28 & 32. (QDM #1 & #3 and EGR)

Checked QDM #3 and the fans and A/C work fine.I have not checked the other two yet. Guess it will be the weekend job.

The question that I have is will a duplicate code show up twice when pulling the codes or will a fault only record once irregardless of how many times it occurs?

93Rubie

I believe it will show up once regardless of how many times it occurs (in general). The codes will be stored for so many key cycles.

Nissan's I know for a fact will show how many key cycles since the codes showed up. I don't know specifically for the GM ODBI system without looking it up. Should be in FSM?

IAFF alumni

I've had the "service engine soon" light come on in my 92 a couple of times as well.
The occurrence was random during a weekend trip and has not re-occurred since.

Looked all through the manual and could not find what that light actually means.

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.

IAFF alumni

Wow. Thanks for the detailed response, and where to look in my FSM.

While we were caravanning with our club, my wife went through the owners manual more than once looking for an explanation, and found nothing.
The light has not returned, but I need to check for stored codes.

Thanks!

Old Ben

I found the detailed instructions in a thread from 2005. All that is needed to get it going is a bent paperclip between the A and B ports on the ALDL.

93Rubie

ATTN: Mods, this needs to be a sticky if not already. People ask for this all the time. I hate to type it out again and again.

AirForceOne!

Wow, didn't know you could use a paperclip to get codes, do tell!

Old Ben

You can learn a lot of things on this forum.

Cliff Harris

The process for reading the error codes (for all years) is shown here:

http://corvettephotographs.com/c4vettes/codes.htm

What they mean (for all years) is shown here:

http://forums.corvetteforum.com/1576116036-post35.html

IAFF alumni

Thank you!

mfi

My light has been going on and off for over 8 years...these cars are attention ******...

AirForceOne!

Ya think?

Old Ben

Checked out the code 26 & 32 that I got and could not find anything wrong. I took it for a long drive yesterday afternoon under the same conditions that set the code and nothing.

Drove it to work this morning and will see if it throws a code on the way home.

Is it possible for these things to remedy the fault by themselves?

sierra 43

Mine has done the same thing on two occasions. I remembered that I had a 92 Caprice patrol car with an LT1 engine and it would do the same thing. The cure on that car was to loosen the gas cap. There would be a "woosh" and the vacum would be released and when the cap was put back on and the car restarted the "Service engine soon" would be off.

I tried the same thing on my 92 Vette with the same results the other day. It was a very hot day on both occasions.

Cliff Harris

The ECM has RAM in it that stores the error codes. This RAM is powered directly from the battery through a fusible link and is powered as long as the battery is connected (and the ECM reset connector is closed).

There is only one location in RAM for each error code (it's bitmapped -- for the technical types -- one bit for each error code). The error could have happened once or many times -- there is no way to tell. If the error is transient, the ECM will clear it if you start the car 50 times without any errors. If you have even one current error, it will remember all the historic error codes forever.

You have the classic symptoms of a bad EGR system (SES light comes on while cruising and is out after restarting). I just typed all this in another thread...

The ECM commands EGR when certain conditions are met. They include minimum speed for a time interval, total engine running time, coolant temperature, MAT temperature and others.

The ECM applies a ground to the EGR solenoid, which applies vacuum to the EGR valve. When the EGR valve opens the ECM detects that opening by either seeing a ground from the EGR temperature switch (early L98s) or by a change in the plenum pressure (MAP systems). If the ECM does NOT see that indication that the EGR system is working, it sets error code 32. The whole system (electrical, mechanical and vacuum) needs to be working to keep from setting that error code.

midweekwarrior

Hey, I just pulled Code 13 from my car. Suggestions how to fix it?

Symptoms:

Car shut completely off, then would not crank for about 15mins. During that time the starter engages but no turn over.

History:

The opti spark was replaced last year, and two weeks ago the water pump was replaced.


Please don't tell me it's an opti again *facepalm and maybe a nice conversation with my mechanic...

Old Ben

Code 13 for a 92 model is the Left Oxygen Sensor Circuit. Do you have a copy of the FSM?

chrldo

your temps are a little high. check to see if you need to bleed some air out of your cooling system.

tahoemedic

If none of all this cures your problem, then more than likely you have an ECM that is on its way out.