Zrout

1996 formula, T56.

Video of the no fire/occassional weird sound
I'm out of ideas but will google my *** off, 2 months ago car died in a parking lot, I towed it home and it needed a lot of other items so I ripped it apart. It had no spark at the time, I tried few spare parts I had (ICM, coil), assumed it was opti and went to work to replace it...


the problem from then to now has not improved AT ALL. I have not checked again, but I assume still no spark (will confirm tomorrow, too angry now to look )

- car cranks
- does not fire
- sometimes it makes a really weird metallic grinding-like sound, almost as if the starter skipped on the flywheel or something, it usually scares me enough to let go of the key so IDK if the sound would stop if I kept at it.
- but when it cranks it can crank for 10, 20, 30 seconds... never fires

- NEW: coil, plugs, wires + unrelated misc seals/gaskets everywhere
- used parts from another good car: optispark, ICM

after of 2 months of wrenching in my spare time I'm back at the beginning, same fing problem

the optispark I changed was used, but from an engine from another car that I drove home after it got rear-ended, so it should be fine.

any ideas?

I can't take the flywheel inspection cover off, it's hung up on the starter but from the little I saw peeking in flywheel looks fine, plus the engine (when the noise doesn't happen) spins fine for a long time

I hear fuel pump kick in, I did not check fuel pressure yet, I assumed the sound was some weird misfire/backfire and opti-related, clearly I was wrong, lol

what could cause that noise? It has to be somehow related to the issue..

I've changed oil, no metal shavings, no coolant in oil etc.. plus prior to just dying the car ran very well

Initially I checked the codes, I will check them again tomorrow but IDK if there will be any considering the battery was disconnected for months. I forget what code it was but it led me to believe it was optispark related, I didn't write it down then and forgot what it was.

Zrout

ok scanned the engine, P1371 , pretty sure it's the same as what I had 2 months ago.

I've uploaded a video of the weird sound, I"m starting to think it's unrelated, the car previous would crank for maybe 1 second before firing so maybe I just never encountered the "weak" starter or whatever is causing that sound.

it seems to stop the car from cranking so I'm thinking just started being weak/not engaging/maybe a bad spot on the flywheel, but then right after I try cranking again and you can see how it spins healthy just fine, MOST of the time it just cranks with no fire like that, actually once the sound happens it's difficult to replicate it and car would crank for a while, it seems to take having to wait to get that sound to happen again.

I'm no expert, but I've been around these cars and engines for few decades, I thought for sure it was the opti. I'm pretty bummed out about this, it's a good thing it's winter and I don't want to drive the car

speaking of the opti, I'm gonna scan it in a few minutes and post any codes

Zrout

hmm, so P1371 is either bad opti (the cam sensor inside?) OR the harness to the opti?

I bought a new harness, but it was too short so I re-used the old one...

I really have a hard time thinking it's another bad opti, it just seems so unlikely, so maybe I'll hack up the new harness and extend it by 6" to test that, easy enough and it was cheap I suppose.

Zrout

maybe my rantings will help someone else down the road..

- checked resistance in old opti harness - it's fine (on 200 ohm scale like .3, same as new harness)
- in the process I broke the clip for the opti connector
- extended cheapo ebay new opti harness 6" , checked resistance before/after, same thing at .3 on that 200ohm scale so I plugged that in

I found a thread on how to diagnose opti, while I couldn't get anyone to help me crank the car tonight I did check for voltage on A and D of the ICM connector with key ON, both had 12V, also checked I think C for ground, it was fine.

tomorrow I will check to see if the white wire on ICM gets the voltage on VAC scale while someone cranks

Injuneer

When you try to start the car, cranking the engine, does the tach needle move up a few hundred RPM? If it does, the PCM is receiving the low resolution pulse from the Opti cam position sensor. If the tach stays on zero, no pulse signal.

The fact that P1371 is there, after you disconnected the battery, indicates the code set while the engine was cranking. That would indicate PCM was not receiving the low resolution puls. When the code sets, the PCM shuts down the fuel system.

Injuneer

Managed to watch the video, using a high end stereo headset. The screech sounds like the drive gear is not fully engaging the ring gear, and slipping off the ring gear. But once it does starts cranking more normally, there seems to be a knocking sound like some is hitting the engine with a 3# hammer. Does not sound normal. But that shoulgn't prevent the engine from firing.

Zrout

honestly when cranking the engine sounds fine, maybe the video/phone captured it weird

in any case, here are some findings and I'm still lost, it just makes no sense, is this opti really dead too? WTF? I KNOW I drove that engine home with that opti 3 years ago, it sat in a dry garage ever since

ICM Harness
YES - A = ~10vdc-12vdc (or your system voltage)
NO - B = 1vac-4vac while cranking
YES - C = ~ 0 ohms to ground with key OFF
YES - D = ~10vdc-12vdc (or your system voltage-might be slightly less than reading at "A")

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

so if I understand it right the PCM is NOT sending signal to the ICM on the white wire, right? my meter in VAC only has 750VAC scale but it didn't even flinch when cranking. I confirmed there is no spark at the spark plug byt attaching a new one to one of the wires and cranking while grounding the plug, makes sense if the coil isn't told to fire.

How would I test the computer itself or the signal from optispark to PCM? Is there a way to tell if optispark is talking to the PCM?


the tach moves SLIGHTLY when cranking, like maybe 100rpms? but it does noticeably moves a bit and then goes down when key turns to off. Hence my confusion.

I cleared the P1371 again and it came back again, takes a bit of cranking for it to come, but I assume that's normal.

Just for sake of testing should I pull the PCM, find which wires go to it from the opti and check for continuity at least to make sure some rodent didn't eat one? and same from PCM to that white wire on ICM?

Injuneer

You mention you bought a new Opti harness. Was it specifically for the 95-97 Opti? The connector length that pushes into the Opti on the 95-97 Opti is longer than the 93/94 Opti. Compare connector on new harness to old one. Did you measure the Opti harness volts/ohms at the Opti harness end (part that plugs into the Opti) or at the test connector on the bracket on the passenger side of the intake manifold? With the questionable quality new harness and the missing clip, I would focus on that first.

If the only scale on the meter is 750 volts AC, not sure you would see the voltage on the white wire to the ICM.

Yes, the engine needs extended cranking before P1371 can set. The PCM is counting high resolution pulses from the Opti, and sets the code after a significant number of high res pulses, without seeing a low res pulse.

The tests you did on the Opti connector would seem to prove continuity from the PCM. What you need to check now is the voltage on the Opti signal wires (A and B) while the engine is rotated, harness connected. While the PCM provides 5 volt reference on those wires, the optical cam position sensor in the Opti pulls the voltage down to “0” as the slots in the optical wheel pass through a light beam. This generate a 0 / 5 volt square wave.

If you manually rotate the engine, you should see the voltage switching between 0 and 5 volts. Pin A (red/black wire) will switch relatively slowly, since one rotation of the crank only generates four variable length pulses. On pin B (purple/white wire) the voltage will change more rapidly, since one rotation of the crank generates 180 equal length pulses.

For continuity on the white wire to the ICM, the wire connects to pin B5 (B is the black PCM connector). Like to 95 pinouts, but 96/97 additions are marked in red, and do not affect the white wire.

4th Gen LT1 F-body Tech Articles

Zrout

I broke the clip on the OLD original harness, so that's why I extended the new one to at least be sure it's clipped in, I will check the length of it for sure but I'm pretty sure they are identical, I've compared them earlier and to be honest the cheapo ebay harness (if it wasn't too short) seems better made than the stock one

I measured all voltages at the end of the opti harness, not at the test connector higher up.

I guess I will rotate the engine by hand and measure the voltage as you suggested, I'm almost hoping that there is nothing, that would prove the opti is bad right? it would just be easier to replace it again than deal with some weird harness/PCM issue. So if I rotate 360 and neither A or B ever drops to 0 I can be sure the opti isn't working right?

next time I will start the car or try without the water pump to avoid the hassle of pulling that off again

Zrout

alright progress or lack of

1) with key ON and VDC probe on terminal B of the opti harness (while it was plugged in) I spun the motor by hand and it's going from 5VDC to 0.2VDC every tiny rotational bit, up/down/up/down...

2) then I put the probe on terminal A, spun the crank pulley 3 times around through what felt like few compression cycles and it never dropped from 5VDC once, stayed solid at 5ish VDC

does that suggest it's a busted opti? seems like it to me, but injuneer if you could confirm my diagnosis I would greatly appreciate it, I don't mind the expense of new one, I just want to keep my sanity if 3rd opti also doesn't fire, lol

3) because PCM only sees one out of two signals (for lack of better term) it would then not send signals to the white/B wire of the ICM right? hence the P1371 and lack of spark?

your help is greatly appreciated by the way! I've been reading your posts on here for what seems like decades.

Injuneer

Pin A is the low resolution pulse. P1371 plus no 5/0 voltage change on A sure seems to confirm a problem with the Opti. If it was the other way around, no 5/0 voltage change on B instead of A, the engine would start.

One last check - make sure the Opti connector pin for wire A is not damaged.

When installing the Opti, you should be able to just plug it in, turn the engine, and look for the 5/0 volt switching.

Zrout

I will look it over, but considering I have tried 2 optis, 2 opti harnesses it all leads to my bad luck, I suppose this used 2nd opti somehow crapped out on its own while sitting on a spare LT1 on an engine stand, just my luck.

gotta follow diagnostics steps so I'll throw a 3rd one at it and see what happens, I think I'll get a new starter too while I'm at it, I don't like that noise it makes at all and at that time I can get the inspection shield off and look at the flywheel too, that will probably lead to a new clutch because why not, lol.

actually the reverse on the T56 keeps popping out so that trans is coming out sooner rather than later, sigh..

it's all good, I will report back when it's done in about a week, just in case someone else follows this thread later on.