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Thread: HELP!! Crank, No Start, Possible PCM issue? Reply to Thread
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  Topic Review (Newest First)
11-06-2018 07:25 PM
Conner90 Just wanted to post an update in case anyone runs into this issue. I did indeed have an ECM issue. After contacting my local GM parts guy, he informed me the ECM was on national back-order but I could send mine in for repair. 6 weeks later(today) I received my rebuilt computer. I couldn't get it installed fast enough. As soon as I hit the key she fired right up. I just wanted to say thanks for the help guys.
09-14-2018 09:05 PM
shoebox You should get ~5vdc standing on A and B. Those are reference voltages sent from the ECM (ECM, because it is a 1993, PCM in later models). When the optical sensor inside the opti is sensing movement of the shutter wheel, you will then get variations that an oscilloscope will see. Since it is a 1993, you can short pins A and B at the DLC to read trouble codes. http://shbox.com/1/dlc2.jpg I think you probably already know this. Seeing as get no SES at key ON and don't see any voltage at the DLC, you may have an ECM issue or at least a power or ground issue feeding it. I don't have a 1993 ECM pinout on my website, but I'm sure you can find one on the web. I'd try to verify power and grounds with a meter.
09-14-2018 06:43 PM
Conner90 Without immediate access to an oscilloscope, I'm trying to work with what I have. I know it's not a sure fire way to tell but here's my thoughts.

From shbox's website I'll quote...

At the end of the optispark harness (disconnected from the opti) with the key ON, you should see:

Optispark Harness

A = ~5vdc
B = ~5vdc
C = ~12vdc (or your system voltage)
D = ~0 - 0.2 ohms ground with key OFF

This is where I'm stuck on stupid. I don't want to point my finger at the Opti, bc this should be isolating the Opti from the rest of the circuit. Why am I not getting the 5V on A&B? I feel like if the 5V is absent it's never going to produce a pulse bc nothing is there to start with. Like I say, maybe I'm misinterpreting something but shbox is very thorough and his troubleshooting matches 2 other sources I've found.
09-14-2018 03:49 PM
cocobolo95 Try going to shoebox site and open # 28 in how to section. shbox.com


I missed this in your 1st post but you should be getting battery voltage at C of opti. As I said before, opti needs to be spinning to pick up voltage signal at A & B Plus meter needs to be reading AC voltage.


However, reading voltage of low & high res outputs is just a general test. A scope is needed to read the square waveform of the signals. Low res signal is 360 pulses per crankshaft revolution. Kind of fast for most multimeters to show very well.


Here's a long shot for you. Coil has 2 connectors that only fit on the correct places on coil. I found out the hard way that little pieces of the connectors can break off. This renders coil hookup non foolproof. If connectors put on wrong places, a no spark condition will result. So check coil connectors carefully.


When dealing with parts store optis, which are either new China made or offshore reman, having 2 or more bad out of the box is not a uncommon thing. So check the low & high res signals with engine cranking and meter set to AC volts. If voltage is there now, check with scope for proper square wave.
09-14-2018 12:25 PM
Conner90 cocobolo95, Thanks for your reply. OK, I've tried a second Opti with the same results. Both have been brand new lifetime warranty units from the local parts chain. I feel like I am misinterpreting something here. On another thread, I found the old school Buick video showing design and diagnosing the Opti. I have also have attached part of the troubleshooting tree from a GM service manual. Both refer to checking for a 5V from pins A&B. I haven't checked with an oscilloscope yet, bc the lack of 5V made me believe there was an issue there. Is it there a chance I have a ground issue even though I get B+ from the PCM on pin C? Do I need to disregard the fact the 5V is missing even though it says to check it in the service manual?

also, when I turn the key on, the SES lamp doesn't illuminate. I was going to read the codes with our Snap-On Verus, but it would not communicate. I took a test light to check for voltage at the DLC and its dead. I checked the horn/cigar fuse to see if that was the reason the DLC wasn't powering up, but it and all other fuses check good. I feel like my head is spinning 1000 different directions trying to diagnose this car. Any help is appreciated
09-14-2018 11:11 AM
cocobolo95 Welcome to the forum.


You need to check the opti low resolution signal on A and the high resolution signal on B while engine is cranking over. And the only 100% way of checking the square waves being output by opti is with a oscilloscope.


Your shop should have one. If not, you can buy a pc based scope for around $70 on Ebay, Amazon, etc.



There will be no spark if low res signal is missing or malformed. You should check for codes as 4 ignition biggies don't light the check engine light. And if both low & high res signals are missing neither dtc 16 or 36 will set. 93 can be checked for codes by shorting aldl pins.


Ecm is most likely not the issue. Ecm provides the 5 volts on C. It then receives low & high res signals from opti to control spark timing.


Did you by chance buy a cheap new Chinese opti, or a reman unit other than Genuine GM , Delco, or Delphi? If yes, there's a 50-50 chance of opti being bad right out of the box.
09-13-2018 12:59 PM
Conner90
HELP!! Crank, No Start, Possible PCM issue?

Hello all, I'm a new member, but have followed the forum off and on for many years. My experience with LT1s is moderate, but not expert. I have had 3 F-bodies w/ LT1 and even swapped one into an S10 years ago.

I have recently obtained a 93 Z28 M6 that has been sitting for several years...one of those ran when parked kind of deals. Upon initial cranking of the engine, I have learned that ignition spark is MIA. Being somewhat seasoned with LT1s, I immediately blamed the Opti. I figured, What the heck, its several years old, its probably due for one whether it fixes the issue or not. I wasn't surprised to see the absence of spark after the new Opti. I found an OG service manual for the ignition system from a program we use in our shop at work. I started with the coil and ICM and worked my way back to the Opti. Checking the plug at the opti, then back to the one by the intake, the results are as followed. D=good ground, C=battery voltage, A&B neither have anything. The service manual says to check for 5V and I'm getting nothing. I checked cont. between the pigtail and PCM with little to no resistance. I also checked for the 5V directly at the PCM with poor results as well. I hate to point my fingers at the PCM, since its not 100% common for failure there, but that's what I'm lead to believe. I would like to hope I have overlooked something, beings the 93 PCM is like a dinosaur. I've called GM dealerships, and all PCMs are on national backorder. If in fact the PCM is toast, my only option is paying a premium for a used one someone has laying around. If anyone sees that I have overlooked something, please point me in the right direction. Thanks

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