When I try to get codes by shorting 4 and 12, I never get info for module 4 ( PCM ). I get 1, 9, and A. My car has no Low Res signal at EST or the ignition module. I'm getting no fire at ign coil. Also I lost the tach signal last time car ran. Can someone tell me if it is common on OBD2 not to get the 4. module when you short 4 and 12 ? I'm thinking the PCM or the chips inside maybe bad or the opti.

 

This is basically opinion of what I think might be the correct answer based on what I have read in the past.

The 96 is OBD2 and even though some sites on the internet give codes for a 96 as if it was a 95 and earlier it is a mistake. I was in a post a few months ago discussing this same related topic. I don’t believe that you can view the PCM codes on a 96 as the earlier vetts on the IP read out. I think you will need a OBD2 code reader for the PCM codes (mod 4).

I also don’t think there is anything wrong with your on board diagnostic system. It will display all modules except module 4, the PCM.

I hope I am close to the truth and this gives you some good direction.

As far as no spark at the coil, I would check the drive signal at the ICM from the PCM. Should be about 1- 4 VAC (when cranked) using a DVM. If that is good, ICMs go bad more frequently than coils.

No low resolution signal will definitely kill the clocking pulse for the PCM hence the drive for the ICM and injector pulses. How did you determine that you have no low resolution pulse?

This problem could definitely be caused by a bad opti but would collect a littlie more data before I changed the opti.

No totally familiar with the 96 so I have to ask what is the EST?

 

I meant the Ignition System Test Connector vice EST. I unplugged the ignition module and checked for AC voltage at the white wire of this connector while someone was cranking engine with the ignition switch. I got no AC volts. A was 12 volts DC, D was 12 volts DC, C was grounded. Something I'm still not sure is why C was showing grounded with ignition in ON position ????

 

My Service Soon light stays constantly lit with ignition switched on should it go off ?? Also what position should the sensor in the air passage ( between the air filter and MAF ) be in, it will rotate 360 degrees. I took the air filter apart and the sensor may not be in proper position.

 

I know you have been playing with this problem for a long time and I have been on many of you posts. I believe in the past you said that you tried a replacement PCM for a test and the engine would still not run and had the same problems. If all you did was correct technically and same problems still exited, then the PCM can be eliminated (assuming it ran in the other car. Did you try your PCM in the other car).

Terminal “C” is a hard ground. Which means its always ground no matter what else, or anything, or the key is doing.

The service engine light will stay on if the engine is not running. If it stays on when running, that’s a problem

I don’t think the position of the air sensor matters in regard to measuring temp of the air coming in. Aside from that, it has nothing to do with your problem of having no spark.

I am going on the assumption you are measuring the AC volts on pin “B” white wire correctly so we are not chasing the wrong symptoms. Meter on A/C volts, black lead grounded, red lead to ICM hanging connector pin B and meter verified that it works on AC.

If the PCM is not putting out anything, then the opti could be the problem if there was no low resolution pulse. But before I went there, I would check some basic inputs to the PMC as far as it getting power when the ignition switch is turned on. This is when you need to use schematics which I think you said you had. Hopefully they are correct if they are not the FSM.

I know you have been working on this problem for months and have asked the same questions many times answered by me and others. The answers are all good and true but you seem to be stuck in the same area going in circles doing the same things. Would like to help more but it seems you need to use this information, take the results and branch in to another related area to find the problem. You going step by step but doing the same thing do not seem to be getting you to the cause of the problem.

Something seems to be causing the PCM not to work at all. Either you have missed something like proper power and ground to PCM or the opti is just bad. And I know its tough to check, and on an expensive part, just to just throw another one at it.

I would check all the wiring back from the opti, the connector and see if you have voltage on the red (pin C) wire. Its either 5 or 12 volts….can’t remember at this time. Either you have missed something very basic to the PCM or the low voltage section of the opti is defective and intermittent. I now say this because I have gone back to your original post, square 1, and in regard to all you have checked.

Quote: “It will not run and I cannot find a Tach filter. IT was running good for about 15 minutes and would shut off and when cooled down would always start backup. This is why I started fooling with the ign coil and module. I replaced both. After replacement I actually got car to run for about 45 minutes and then it shut down. Somewhere I lost the Tach reading.. Now it will not fire at the ign coil. Also my hand scanner a ( Innover 3030 OB 2 ) will read 3 systems but can not read the other systems so now I'm wondering if the PCM is bad or something blown.. “

http://forums.corvetteforum.com/show....php?t=1672776

I can answer questions all day but you might just have to spring for the opti eventually.

 

Following my flow chart on 96 LT-1 for Dist Ignition System Check. 1. Turn ignition OFF. 2. Disconnect the Tachometer.
This is where I do not know where to disconnect the Tachometer. So I did my checks without disconnecting the Tach. After my checks I found no Low Res Voltage at the ign module but I do not know if the Tach not being disconnected could have anything to do with this signal not being at the ign module. Next thing is one of the steps in thi flow chart is: Turn the ign switch ON, use a test light connected to B+, probe terminal C of the ign module connector. If the test light illuminated then repair the ign coil module ground circuit. My test light does illuminate with the switch on so this is why I asked this question. You tell me it is hard wired but this flow chart seems something is wrong.