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1994 camaro z28 convertible lt1 90k original miles
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Discussion Starter · #1 ·
1994 camaro z28 almost stock lt1 with a4, it should read between 1 and 4 volts ac with a multi meter when tested at pin b of the icm harness as the engine turns over, but mine only went up to 2.1 at the highest. I have the gm opti with only about 5k miles on it and I'm wondering if the problem could be that the optical sensor lens is dirty and if that could cause the problem so maby I can clean the lens and fix the problem because it's a non-vented unit. I would replace the cap and rotor with the msd "vented" verson. the car doesn't have any codes and it starts runs and drives but has a stumble at idle and hesitates when accelerating so it robs most of the power. I'm hoping I don't have to replace the whole unit since the one that's in there has such low miles on it. if anyone has any suggestions that would be great thanks.
 

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Discussion Starter · #2 ·
The main thing I need to know for now is if pin b on the icm harness is supposed to read anything between 1 and 4 volts ac or if its supposed to cycle between 1 and 4 volts ac when the engine is turning over. Mine goes between 1 and the low to mid 2s volts ac. From the things in have read that part wasn't clear. My car has been down for almost 3 years because it had multiple problems, the fuel pump was weak so now its new and 2 of the plug wires had problems so they are new as well and fixing those things has helped, it still has the main problem so I want to rule out the optispark as a problem as much as I can or fix it if that's the problem but I don't want to just throw parts at it without testing and having an idea what's going on first.
 

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The main thing I need to know for now is if pin b on the icm harness is supposed to read anything between 1 and 4 volts ac or if its supposed to cycle between 1 and 4 volts ac when the engine is turning over. Mine goes between 1 and the low to mid 2s volts ac. From the things in have read that part wasn't clear. My car has been down for almost 3 years because it had multiple problems, the fuel pump was weak so now its new and 2 of the plug wires had problems so they are new as well and fixing those things has helped, it still has the main problem so I want to rule out the optispark as a problem as much as I can or fix it if that's the problem but I don't want to just throw parts at it without testing and having an idea what's going on first.
From: Diagnosing GM LT1 V8 Engines

"In cases such as this, the use of a scope is the only answer. By pinning the high- and low-resolution signals at the same time, you can easily determine if there is a fault in the sensor assembly. Since the sensors are of the optical type, you can expect a clean square wave signal measuring 5 volts peak-to-peak. If the engine keeps turning over, the high-resolution pulses should just keep coming in a steady square wave."

The signal is not AC, it's pulsating DC. You can read it with some meters on AC Volt scale, but not all. Use a scope. Something like this would do. New Assembled DSO150 Digital Oscilloscope 2.4 inch LCD Display with Clip + Power | eBay

Your only problem will be learning how to use it and what it's telling you. It's worth doing. You can use it for lots of other things. Here's a video you can watch to see if you want to try.


Al in Tucson
 

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Discussion Starter · #4 ·
Thanks for the info, I ordered that today so I will be learning how to use a new tool in about a week, but at least I should be able to get more accurate information as to what's going on. the tests I was doing were the ones on the shoebox site but even they say it's better to use a scope. it would be very frustrating to do the work of replacing the opti and the cost of a new one if it didn't fix the problem so I'm hoping I can get good diagnostic info from this. I already have the ability to scan the car and program the ecm with tunercats but that doesn't help with spark diagnosis. I just want to drive my car again with the power it had before this stuff happened. I use scan9495 by garydoug, is there a different program for diagnostics that is different or better? Since its a 1994 it is obd1 so that limits things some. One more question, my new fuel pump seems like it came with a bad check valve because it doesn't hold fuel pressure for more than a few minutes after the engine is off, but it is the correct psi when it is running and when I pull the vacuum off of the fuel pressure regulator to simulate wot, can this cause issues because it starts fine as long as I let the pump prime before it turns over.
 

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Thanks for the info, I ordered that today so I will be learning how to use a new tool in about a week, but at least I should be able to get more accurate information as to what's going on. the tests I was doing were the ones on the shoebox site but even they say it's better to use a scope. it would be very frustrating to do the work of replacing the opti and the cost of a new one if it didn't fix the problem so I'm hoping I can get good diagnostic info from this. I already have the ability to scan the car and program the ecm with tunercats but that doesn't help with spark diagnosis. I just want to drive my car again with the power it had before this stuff happened. I use scan9495 by garydoug, is there a different program for diagnostics that is different or better? Since its a 1994 it is obd1 so that limits things some. One more question, my new fuel pump seems like it came with a bad check valve because it doesn't hold fuel pressure for more than a few minutes after the engine is off, but it is the correct psi when it is running and when I pull the vacuum off of the fuel pressure regulator to simulate wot, can this cause issues because it starts fine as long as I let the pump prime before it turns over.
When you get into this, take your time. Message me if you run into a "brick wall." My 95 has the original Opti, it's been flooded by the water pump and still works. I'm fairly up on it and how it works.

Al in Tucson
 

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Discussion Starter · #6 ·
I should get the oscilloscope on Tuesday but the one thing I am having a difficult time finding is a write up or video on where to hook up the leads with an opti still in the car. I can find what the wavelengths are supposed to look like for high and low resolution but not where to hook up the oscilloscope which is odd. if you or someone else could send a link or a simple explanation that would be great, thanks. I'm assuming that for the low res signal it is the same place I hooked up my multimeter for the test I did with that at the icm pin b.
 

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I should get the oscilloscope on Tuesday but the one thing I am having a difficult time finding is a write up or video on where to hook up the leads with an opti still in the car. I can find what the wavelengths are supposed to look like for high and low resolution but not where to hook up the oscilloscope which is odd. if you or someone else could send a link or a simple explanation that would be great, thanks. I'm assuming that for the low res signal it is the same place I hooked up my multimeter for the test I did with that at the icm pin b.
I should get the oscilloscope on Tuesday but the one thing I am having a difficult time finding is a write up or video on where to hook up the leads with an opti still in the car. I can find what the wavelengths are supposed to look like for high and low resolution but not where to hook up the oscilloscope which is odd. if you or someone else could send a link or a simple explanation that would be great, thanks. I'm assuming that for the low res signal it is the same place I hooked up my multimeter for the test I did with that at the icm pin b.
Code 16- Low resolution pulse (distributor). ECM is not detecting the 4X pulse from distributor. Check wiring connections, especially the pigtail connector from the optispark to main wiring harness. This is the black/red wire that goes to terminal "A" on the pigtail connector. On the 1992-93 ecm, this is wire C5 (green connector). 1994-95 ecm, this is wire B2 (black connector). If wiring checks out, you will have to replace the optical sensor module in the distributor.

Al in Tucson
 

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Discussion Starter · #8 ·
I'm not getting any codes, that's another reason I want to test it to try to rule the ignition out as a problem or not. but what I can't find info about is where to hook the leads from the oscilloscope to read the different signals from the opti. I don't want to hook up the probe from the scope to the wrong thing and mess up the scope or the car. I have been looking for videos or explanations but the best I can find is the screen of an oscilloscope but not how or where to connect it especially with the optispark rather than other styles of ignition.
 

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I'm not getting any codes, that's another reason I want to test it to try to rule the ignition out as a problem or not. but what I can't find info about is where to hook the leads from the oscilloscope to read the different signals from the opti. I don't want to hook up the probe from the scope to the wrong thing and mess up the scope or the car. I have been looking for videos or explanations but the best I can find is the screen of an oscilloscope but not how or where to connect it especially with the optispark rather than other styles of ignition.
All the signals from the Opti are about the same voltage. If, we're talking about at the plug. You can't hurt the Opti or the scope. I sent you the wire colors you need for the low-res pulses.
On my car, I had no problem getting to the wires with a straight pin going into the connector from the back.

There is more information than you'll ever need here: 4th Gen LT1 F-body Tech Articles

Al in Tucson
 

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All the signals from the Opti are about the same voltage. If, we're talking about at the plug. You can't hurt the Opti or the scope. I sent you the wire colors you need for the low-res pulses.
On my car, I had no problem getting to the wires with a straight pin going into the connector from the back.

There is more information than you'll ever need here: 4th Gen LT1 F-body Tech Articles

Al in Tucson
How's it going? Making any headway? Figure out the scope?

Al in Tucson
 

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Discussion Starter · #11 ·
I have been messing around with the scope and it seems to work and I think I understand it enough to give it a try now. it took a while for one because I was trying to figure out the scope and I had to assemble the intake so I could plug all the sensors in so everything is working as it should when I start the car to test it, and I had taken the plugs out to look at them so I got those put back in and the plug wires hooked up now. So you took a pin and pushed it into the connector at the right wire? I think that sounds better than stripping a section of the insulation off of the correct wire since it seems that those wires would be sensitive enough that I don't want to accidentally strip away or cut any of the strands of wire, or am I over thinking this? I have read of people stripping a small section and hooking something up so that it can be tested with ease at any point in the future if problems come up and I was considering that but wanted to see what you thought.
 

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I have been messing around with the scope and it seems to work and I think I understand it enough to give it a try now. it took a while for one because I was trying to figure out the scope and I had to assemble the intake so I could plug all the sensors in so everything is working as it should when I start the car to test it, and I had taken the plugs out to look at them so I got those put back in and the plug wires hooked up now. So you took a pin and pushed it into the connector at the right wire? I think that sounds better than stripping a section of the insulation off of the correct wire since it seems that those wires would be sensitive enough that I don't want to accidentally strip away or cut any of the strands of wire, or am I over thinking this? I have read of people stripping a small section and hooking something up so that it can be tested with ease at any point in the future if problems come up and I was considering that but wanted to see what you thought.
If you strip insulation, seal it with: 4 Oz. Liquid Tape - Electrical You can peel it off later to get to the wire. Try not to nick the wire. You can just skin the top off the insulation, you don't need to actually remove a section of it.

Home Depot and Lowe's sell it too.

Keep in mind there are two grounds, sensor ground and chassis ground. Hook the scope up and take picture of the display. I can help decipher them. If you get weird signals you may have a ground problem. It's all easy to see with the scope.

Al in Tucson
 

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Discussion Starter · #13 ·
I took a lot of pictures of the screen so let me know if you want to see more, this is a test to see if I can get them uploaded on here. I can see there are extra lines and things that should tell you something at least. thank you very much for the help.
Hood Computer hardware Gadget Electrical wiring Gas
Communication Device Gadget Audio equipment Gas Display device
 

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In the first picture, you're seeing differences in the signal as the distributor turns. I don't see a problem there. Second picture is of what? Are those both the low res signals?

Al in Tucson
 

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One more thing, the scope should be set o DC coupling and where the zero line is, is important. In the first picture you're seeing differences in the signal as the distributor turns. You're looking at a small section of the complete waveform. One revolution of the distributor would be one complete string of pulses. You're seeing little sections piled on top of each other.

You're doing good. I would like to see zero, and the waveform.

Al in Tucson
 

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Discussion Starter · #16 ·
all the pictures I took of the screen is from the same test, the scope was connected to the pin b wire that comes from where it hooks up to the ICM. I have more pics from that test that show different things I just don't know what they mean.
 

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Discussion Starter · #17 ·
It has a switch on the top of the scope that says ac, dc and ground I think but that was set to dc for the test
 

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Discussion Starter · #20 ·
no codes on the computer at all, it sounds like a slight misfire from the moment I start the car. there is no change between open and closed loop. It is most noticeable when accelerating, it smells a little rich and has no power when accelerating. I can drive the speed limit it just gets there slower than my old 4 cylinder ford ranger. when I hit the gas hard it bogs down then will accelerate but not at all like it always has. It had a few things go wrong at the same time, the fuel pump was weak so I replaced it and there was a plug wire that was bad and arcing so I put new plug wires in and plugs but that just made things better by a little bit. I have been trying to figure this out for almost 3 years so it has been very frustrating. I have the obd1 cord for pc so I have data logged many times and things show up good except the fuel trims are a little off like it thinks its a little lean so its adding extra fuel that it doesn't need. The o2 sensors are cycling good and as far as I can tell the sensors are all reading like they should, maf, map, iat, tps all reading good. I have replaced the maf a while ago because it fit the symptom's but my old one worked fine so I have an extra, same with the coil. The only thing I can think of it being is the opti but that is a gm unit that only has a few thousand miles on it so I don't want to put all that time and money in until I know more so its not just a guess. It also starts right up weather its a cold start or its warmed up but I can hear the "misfire" right away. If I wasn't as familiar with the car I probably would only notice the problem when accelerating but I have owned the car over 10 years so I know how it should sound and feel, I am probably missing something because I have tried a lot to fix it with no luck. If you have any suggestions please let me know because I miss my car, thanks for the help.
 
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