There are several reasons the CCM will enable the "SYS" indication.

But, as you are aware, in your case, it is highly probable that there is a lack of communications between the ECM and CCM.
The CCM needs information from the ECM for security and to pass on the vehicle speed to other modules, like the ASR and C68 climate control.
The ASR needs the speed reading (along with with input from the lateral acceleration sensor, etc) to make a determination on how to respond to an ASR event. The ASR unit can use the Adjuster on the throttle, or individual rear wheel braking to respond.

So, since you don't have a standard ECM, the security handshake will never work. If you could disable that part only, it would not affect you anymore.
But, I don't see how your ASR is going to function properly.

And, even if you don't care about any of these, I don't know how to tell the CCM to ignore the ECM.

It is my understanding that the CCM "polls" the ECM for data. The CCM makes several attempts to retrieve that data from the ECM. If the CCM cannot get the data, it sets the "SYS" indication and sets a "communications failure" code.

If you intend to keep the CCM, the only possible way I can see around it is to "hack" the software in the CCM to tell it to ignore the ECM.

Tom Piper

 

Hi Tom, no I am talking about closed loop with the o2 sensors in control. I have replaced a couple CCMs but I never tried a later model CCM in a earlier car. It would be interesting to see if the protocol is actually different. I would be surprised if it was. I think the main reason I never tried it is because the CCM is such a RPITA to change out! I have a known good 95 CCM in my shop, if I ever get a chance to work on a 93 where I have to replace a CCM I would give it a try and see what happens. I think the pin outs are the same but I would have to verify it before trying the swap.

 

I don't know if anyone has hacked the CCM Code. I do know if you hook up the serial lines from the 1993 ECM to the CCM and power the ECM it should resolve the SYS light.

If I could figure out a way to change the data stream from the 91 ECm to make it readable to the 1992 CCM, then I could use the digital gauges display on the LCD..

I just don't know where to start to look for that.

If there was a way to 'sniff' the ALDL port to see the comm between the CCM and ECM then I could possibly get somewhere

 

Using the standard software supplied by Vetronix to the dealership for use with the Vetronix Tech-1A and MasterTech, I have programmed the CCM.
But, the only options in this area I had were to inform the CCM whether the ASR/ABS and/or C68 climate control options were in the car.
That way, the CCM knows if it should attempt to communicate with them or not.
The CCM is the master computer and it controls the communications on the serial link. The CCM polls the ECM/PCM for data; and, if the CCM is aware the ASR/ABS and/or C68 are in the vehicle, it passes this information on to them on the serial link.
The ECM/PCM has no direct knowledge of the ABS/ASR or C68.
I wouldn't be suprised if the code for the CCM to ECM was in ROM instead of being flash programmable.

Tom Piper

 

As an added note, I find it interesting that the vehicle speed is passed from the ECM/PCM to the CCM over the serial link, and then the CCM passes that information on to the ASR/ABS and C68 over the same serial link.
However, the throttle position information is passed directly to the ASR/ABS computer from the TPS sensor through a buffer/amplifier.
My guess is the ASR/ABS needs "real time" information on the throttle position that could possibly get delayed through the serial link communications.

Tom Piper

 

As another note, the ASR controls an ASR event in three possible ways:
1) It adjusts the throttle with the ASR adjuster.
2) It controls the rear brakes, possibly individually.
3) It can control spark retarding.
It can use any one of these or any combination of them.

The only way I can figure the ASR can control spark timing/retarding, is to pass that information over the serial-link to the CCM, and the CCM would have to pass that to the ECM for the EST (Electronic Spark Timing) signal to the ignition control module.

So, this implies there are two way communications between the ECM/PCM and the ASR/ABS with the CCM passing that information back and forth -- because the ECM/PCM does not have knowledge of the ASR/ABS or C68 climate control.
The CCM is the only one to have knowledge of everything on the serial-link.
And, as DieL can testify to, the CCM always assumes the ECM/PCM is there.

Tom Piper

 

thanks for the info. What would happen if i removed the ccm? Im guessing all the guages get info from somewhere but im sure the tach, speedo and all guages recieve info from sources that might have nothing to do with the ccm? If thats the case i would be interested in removing it if it can stop the sys. It used to drive me nuts long ago when the car had bolt ons but it was only flashing because i installed an aftermarket stereo and went away when i installed LEDs in the place of the stock bulbs. Im not sure if it will annoy me now.. But from what i remember driving at night it always catches my eye

 

The CCM is the source of the odometer mileage.

It also controls the "Delayed Accessory bus" that allows you to use your radio, power windows, etc for about 20 minutes after the key is turned off.
It also sets the time of the interior lights being on.
Without it, depending on how the relays are defaulted, you may have no courtesy lights or power window functioning.
Not to mention, it controls the entire digital readout on the dash.

Tom Piper

 

dam that sucks. Ill just tell people my sys light is the dfi unit cycling to check systems Or i'll just pretend it is I think that delaye thing is gone already though i put an aftermarket stereo in and it hasn't worked since.

 

Tom, if you really want a 94/95 PCM in the car, There is a way to do this. You can leave the stock ECM in place, then piggy back a 94/5 PCM to take over engine management. That way the CCM and the old ECM can talk all they want to. The late model PCM would take over all fuel management. However this would delete the VATs system entirely.

 

Thanks, but if I can't make it like the '94 was original, I really don't want to do it.

I still think it may be doable, but I would have to know for a fact the '92 CCM would communicate with the '94 PCM.

Tom Piper

 

I just got a '96 shop manual.
If they are all like the '96, the statement I made above is incorrect.
The ABS/ASR has a dedicated output directly to the ECM/PCM to signal it to retard the timing for an ASR event.

I'm going to try to see if the '92 has the same connections on the same pin in the harness.

Tom Piper

 

I always figured that batch fire was a lousy system compared to sequential but based on what I have learned here I am not sure that there is that big of a difference in performance.

I didn't know the 92-93 LT1's were batch fire until I read this thread. I assumed that it was sequential. Once I started reading the posts I was interested in trying the mod to sequential but now I am convinced to stick with what I have. I am one of the ones that prefers speed density anyway. Thanks for all the good info.

 

I'm going to have to agree with you. I didnt realize how much went into this. I was under the impression it was as simple as swapping the wiring harness, computer, through a maf on it and call it a day. I think Ill just stick with what I have. I only wanted to do it because at the moment I'm swapping my 1994 Camaro motor into my 93 vette. I never wouldve thought it would be this much of a headache. The injectors still plug in the same correct?

 

the advantage isn't so much performance, in the sense of power, but improved efficiency.

A SFI LT1 will get upwards of 30mpg, highway, versus 26 or so for the older batch systems.

 

I know this thread was many years ago,, But Just curious if you or anyone ever tried this out...
I have a 92 and a complete 94 Donor car and was thinking of attempting it!

 

You may not need to go through all of that to get the benefits of SEFI.

On my Gen1 SBC with Minram and batch fire (using a 90-91 $8D system), I had really good results by putting ~10% bigger injectors on cylinders 1-4. I run 30 lb injectors on cylinders 5-8 and 32 lb injectors on cylinders 1-4. I then set my injector constant to 31 in the calibration. Splitting the difference like that resulted in essentially no change to the VE tables either.

If you look at the fuel trimming in the calibration for 94-96 ($EE), that's about how GM trimmed the fueling from front to rear... about ~10% richer at the front. It varies from car to car in terms how much engine load they go up to with this trimming... for example on the Impalas, they did this all the way up to 40% throttle... on Camaros/Corvettes, I think it was like only 15% throttle.

It's obviously not an exact duplication of how sequential injection works, but in terms of low/mid RPM air/fuel distribution issues that plague these style manifolds, it's a pretty good (albeit crude) approximation of how GM tuned the SEFI systems. But SEFI is mainly beneficial over batch fire at lower engine speeds so it largely accomplishes the same goal.

Of course the front cylinders will run richer at higher rpms, but I'm ok with that... it's a good trade for how much better the car runs at lower speeds. So for example on my wide band, the car seems to make more power at about 12:1 vs 12.5:1... at least by seat of the pants feel... (due to the front cylinders running richer).