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Discussion Starter · #1 ·
Thanks in advance for any help on this. I'm hoping to finally get this figured out and provide future problem solvers some info as well.

I've done numerous searches on this site and others, and applied everything I could learn. I'm getting pretty frustrated!! I'm wondering if it is possible for my computer or the opti to be bad at this point. I have a spare computer I can try. I'm considering getting an oscilloscope if that will help me diagnose further. I won't be replacing the opti if it is bad as I would rather invest in the Torqhead unit but want to make absolutely sure the Opti is the problem before I replace it.

I have a 94 Firebird Firehawk. Quite a bit of work was done to the engine and suspension before I bought the car. It has a 383 stroker with long tube headers and a Vortech supercharger as well as several other goodies. I'm working with Solomon at LT1 PCM tuning and the car was starting to run really good. I've never had a problem with the ignition system or starting the car, and the guy that I bought the car from said the vented MSD Opti was new.

What happened was I testing the latest tune from Solomon and after a few 75-100% pulls and regular driving I was heading back to the house. My plan was to cruise all the way back, but I left from a traffic light and laid my foot into it about 75% and in first gear the rpms dropped slowly but there was no power. I managed to coast the car and park it, but it would only crank and not start, and I had to get it towed.

Using the EEHack software, I have been getting a Code 41. I also use the software to clear the code. I unplugged the battery once as well.

Here is what I have done. Please post any ideas, and I will try them!

1. Went through the shbox test procedure several times. Both the ICM connector and opti connector (not harness) tested fine for voltage, ground, etc. I also tested the opti harness, unplugging it from the opti and is shows the same numbers as the opti connector. 4th Gen LT1 F-body Tech Articles

2. Replaced the ICM with a new AC Delco 1905E. Bought another ICM at Oreillys Friday and did not bolt it to the cooling plate (AC Delco still on it), just unplugged the ICM harness and plugged it into the Oreilly and put it on a flat metal surface by the thottle body links. Will it work like this? Car still did not start.

3. Bought another coil. Tested both coils I have and they both are within spec. One is MSD. I tested the 3 wires that run to the coil and they show 12 volts with the key on. I also disconnected the connectors at the coil and tested them with the key on. With this test, only the 1 wire primary connector showed 12 volts. The two wire connector did not show any volts. It only shows volts when connected to the coil along with the primary. The spark tester tool seemed to be defective as I tested it on my battery and the light wasn't going on, so I am going to get a new one. What I did do was crank the engine with my positive multi meter electric probe taped to the coil spark plug connection, and the other probe to ground. The volts jumped from 12 to 18 when I cranked it. Not sure how accurate this was or if this is the number it should have.

4. Checked my fuel pressure. The fuel pump primes with key in and the pressure drops a bit when I crank it. The code 41 shuts off the injectors.

5. Printed the GM Service manual troubleshooting for DTC Code 41 recommended by Cocobolo95. Went through all the tests on page 2. All the answers were yes For number 3 test, I assumed that the B+ meant 12 volts or batter, so that is what I used. At which point the manual points towards the faulty ignition coil module connection or faulty ignition coil module. Are the Ignition Control Module and Ignition coil module are the same?
 

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Code 41 sets because ignition control circuit is malfunctioning. Test the white wire that runs from PCM connector B cavity 5 to icm connector B. This test will use your ohm meter to test for continuity of the white wire. Disconnect car battery and unplug connector B from PCM when testing.

After continuity test hook connectors back up and battery. You are now going to use your AC voltmeter part of multimeter to test ignition control circuit. THe circuit acts like old time points did. Voltage will be high when coil is charging. Voltage will drop to 0 or 1 volt when PCM commands coil to fire. Starter must be turning engine over for test.

You need to get the PC based scope to test high & low resolution signals from opti. The thing about opti codes is; if both high & low res signals are faulty or missing, no opti code will set. If low resolution signal isn't the correct square wave, or is missing, then PCM will not drop voltage to fire coil. ignition control circuit will not fire the coil.
 

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Discussion Starter · #3 ·
Code 41 sets because ignition control circuit is malfunctioning. Test the white wire that runs from PCM connector B cavity 5 to icm connector B. This test will use your ohm meter to test for continuity of the white wire. Disconnect car battery and unplug connector B from PCM when testing.

After continuity test hook connectors back up and battery. You are now going to use your AC voltmeter part of multimeter to test ignition control circuit. THe circuit acts like old time points did. Voltage will be high when coil is charging. Voltage will drop to 0 or 1 volt when PCM commands coil to fire. Starter must be turning engine over for test.

You need to get the PC based scope to test high & low resolution signals from opti. The thing about opti codes is; if both high & low res signals are faulty or missing, no opti code will set. If low resolution signal isn't the correct square wave, or is missing, then PCM will not drop voltage to fire coil. ignition control circuit will not fire the coil.
Thanks for the advise. I went ahead and ordered an oscilloscope and it will be here on Wednesday. My question about tracing the white wire (B on the ICM) is if I tested it while cranking (per shbox) and it showed the 1-4 volts it was supposed to show, would there still be a continuity issue with the wire? If the wire is bad, do I need to run a whole new wire from the PCM to the ICM? I'm planning on getting a new spark tester today, but if I am getting a spark on the coil, but no spark at engine then wouldn't it be reduced to the opti as the problem, or could it still be something else?

Also, I read your post in the 94 lt1 no spark post listed below my post in recommending reading and you mention the theft detterent module. Is that a possible culprit. I read that the starter would be killed with that as well though.
 

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If your getting the 1-4 volts AC on the ignition control wire, then it has continuity.

The scope will tell you if low & high resolution signals are present & in the correct square wave. It is possible to have the low & high res signal present, but not have the proper square wave. That is why scope is only 100% way to test opti.

Having security problem will cut ignition, fuel injectors, & starter won't work. At least from factory. Seeing that car is 28 years old, and has been owned by multiple people over the years, it's very common for the starter disable part of security system to be bypassed.

Again the scope will be the way to see if fuel enable signal is reaching PCM. Again it's a square wave that is 50 hertz. Fuel enable signal goes from theft deterrent module (TDM) to connector A cavity 25 of PCM. It's a dark blue wire.

If you get the correct 50 hertz square wave with key on, then security system is ok.

1 last thing; the coil on your car has 2 connectors going to it. They are a foolproof pair that can only connect if they are in proper position. But years of hot & cold heat cycles will make plastic connectors brittle. The pieces of the connectors that make them foolproof, can break off, allowing the connectors to be put on in the wrong place.

If that happens, you will get no spark.
 

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Discussion Starter · #5 ·
If your getting the 1-4 volts AC on the ignition control wire, then it has continuity.

The scope will tell you if low & high resolution signals are present & in the correct square wave. It is possible to have the low & high res signal present, but not have the proper square wave. That is why scope is only 100% way to test opti.

Having security problem will cut ignition, fuel injectors, & starter won't work. At least from factory. Seeing that car is 28 years old, and has been owned by multiple people over the years, it's very common for the starter disable part of security system to be bypassed.

Again the scope will be the way to see if fuel enable signal is reaching PCM. Again it's a square wave that is 50 hertz. Fuel enable signal goes from theft deterrent module (TDM) to connector A cavity 25 of PCM. It's a dark blue wire.

If you get the correct 50 hertz square wave with key on, then security system is ok.

1 last thing; the coil on your car has 2 connectors going to it. They are a foolproof pair that can only connect if they are in proper position. But years of hot & cold heat cycles will make plastic connectors brittle. The pieces of the connectors that make them foolproof, can break off, allowing the connectors to be put on in the wrong place.

If that happens, you will get no spark.
Thanks for the feedback. The connectors are connected properly to the coil as they are fairly new and will only go one way. I tested two coils and the coil to opti wire tonight and they pass. I have the oscilloscope and want to check the opti now. Where do I connect each lead? Do I disconnect the connector to the opti by the fuel injectors and then put the probes in the opti side of the cable? Which ones A-D or wire colors do I connect the probes to? Also, I tested my spark tester on another car and it worked with the car running. However, when I connect it to the 94 coil it will not light. With both coils bench testing okay, I am for sure not getting spark at the coil. Although, I did test it by taping the positive probe of my multimeter to the coil wire connector part of the coil and grounded the other probe. It showed 12 volts with key on and 18 while cranking. Not sure if that helps. I think I may have to check my theft deterrent to make sure the ground wire didn't disconnect.
 

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When you test opti, leave connector connected. Low resolution signal is on the red/black wire @ cavity A. The high res signal is @ either a purple/white or a light blue/black wire @ cavity B.

When you test the opti, red positive probe will be connected to either cavity A or cavity B, depending on which signal you want to test. The black negative probe will go to a good ground. Ignition switch must be on during test.

Have you ever used a scope? If yes, get at it. If no, here are some things you will need to adjust. The voltage knob to 5 volts or higher. The time knob, you will have to play with it to get a viewable square wave signal. Some scopes also have a knob to center the signal.

Youtube has videos on how to use a scope.
 

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Discussion Starter · #7 ·
When you test opti, leave connector connected. Low resolution signal is on the red/black wire @ cavity A. The high res signal is @ either a purple/white or a light blue/black wire @ cavity B.

When you test the opti, red positive probe will be connected to either cavity A or cavity B, depending on which signal you want to test. The black negative probe will go to a good ground. Ignition switch must be on during test.

Have you ever used a scope? If yes, get at it. If no, here are some things you will need to adjust. The voltage knob to 5 volts or higher. The time knob, you will have to play with it to get a viewable square wave signal. Some scopes also have a knob to center the signal.

Youtube has videos on how to use a scope.
Thanks Cocobolo95. It looks like I will have to use sewing pins or needles to probe the wires as the oscilloscope has alligator clips. I did watch some videos on youtube and will look for some more to help me through the process. I noticed one where the guy used a multi-meter for a basic test. In his next video he goes over the oscilloscope. I'm doing it on my own, so I will probably use my phone or try to set it up so that I can see it while cranking it. Hopefully, I can get the right results and get to the heart of the problem! It's going on 4 weeks, and I have to make time for it. I'm planning on researching the theft deterrent on this site and ruling that out.
 

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While a multimeter does many things well, only a scope can check for a proper squre wave. Your security system uses a 50 hertz squarewave to open fuel injectors.
 
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