@GaryDoug

Pleasee check the posts on testing the white wire from the PCM to the ICM. He is getting less than 1 VDC, below the typical target of 1-4 VAC. And his tach is moving but only “twitching”. He has spark, but it’s “hit or miss”….. not a continuous series of spark.

Background - this car was purchased from someone who was parting it out. From the photos there are loose harness connectors, missing components, coolant hose connected to the EVAP connection on the throttle body, probably what caused the engine to hydrolock. The starter still seems to be laboring, not cranking the engine as fast as it should. Maybe the slow cranking results in the tach.

Is it possible the PCM is receiving intermittent low res pulses from the Opti? That would explain the limited tach movement and the intermittent spark. What accounts for the <1 VDC on the white wire? I threw some WAG’s out. I'm stumped.

 

I believe, until you get a good tach reading, we won't know.
if the computer is getting garbage in, it's probably responding with garbage out, like twitchy tach, intermitten spark, 1-4 vdc etc.

 

Let me reset things with information from the top, and what I've seen since then and what I think.

Car went through a few auctions in the last couple of years, unknown as to why. The last "owner," a mechanic, bought it mostly to get the SS hood and Borla exhaust off of it. He also took the Opti harness, but broke the clip off of the harness doing it, so everyone lost there. I bought it knowing the motor was seized, and the story told to me that there was water in the SS intake hose. I was given a stock airbag and hose with my purchase, a different hood, and no muffler. The disconnected harnesses when I bought it turned out to be the Opti connector, and I believe the IAC connector, being in the way of getting the Opti harness off. The steam pipe is open, and instead of it being routed to the radiator, the EVAP connection on the TB is. Corresponding EVAP solenoid connection is open.

When I got it home, the first thing I did after verifying the status, was to replace the battery. It still would not turn over at all, so next step was to remove plugs, thinking there might be water in the cylinders. #6 was particularly hard to break loose, but none of the plugs showed rust on the electrode or tip (just a minor amount at the end of the threads on some), and no water came out of the cylinders. The next step was to turn the crank, and it was exceedingly hard to do so. Marvel Mystery Oil was put into the spark plug holes, and two days later with some leverage, the crank started to turn. Not easily, not smoothly, but it was turning. The ignition was used to turn the motor over, working the piston rings along the walls and to expel the excess oil.

New NGK copper plugs went in, gapped at .050. Existing wires put back on. I verified the other day that it appears I put them on in the correct order. I discovered the Opti harness was missing, and ordered one from Michigan Motorsports. Once on, I verified that the ICM harness was giving good readings on A, C, and D. B gave me around .7VAC on almost all attempts, with only once hitting 1VAC. I verified that the Opti harness I received gives the expected values.

I verified coil was providing spark, and the video shows it to be a pretty good spark. I took video of the #4 plug wire providing spark to a good ground, and it did not in either strength or consistency. Seemed like it might be even skipping a cycle. Fuel pump primes when ignition goes to run. Security light goes away after 3 seconds or so. Fuel pressure at the rails gives a consistent 40+psi. Noid light on the injector harnesses is abnormal, but all injector harnesses provide 12v hot. I got pulsing on 1,3,5,7,2, but got to 4 and haven't had consistent results since. 4 has never shown pulse, and will often get a "one injector harness shows spark, the next one does not" situation. Even on ones that previously tested ok.

I cleaned up the pass fender ground, and the ground next to the ICM. I cleaned up the ignition junction on the pass fender, and positive and neg terminals. Each time I try a starting attempt or testing attempt I make sure I'm getting at least 12.4V across the terminals. Low impedance from neg terminal to mentioned grounds, positive terminal to other wires on junction block and fuses. Every single fuse under the hood and in driver dash verified by audible connectivity. I'll post a pic later of both just so you can tell me if I am straight up missing any, but I don't think so. Car has a manual fan control, appears to be sourced from green connector near PCM using in-line splices. Fan functions via this switch as expected. I will take another look at the tach today, maybe even take a video, but as Injuneer stated, the bit of difficulty the starter has at turning the flywheel over at expected speeds may be playing some role.

Here is the new information I have gleamed over the last couple of days:
Starter looks immaculate, like someone changed it and didn't drive the vehicle after. Connections appear tight. Put an eye on the PCM ground nearby, but cannot get to it and also cannot readily get to starter connections to remove. Decided to pass on this part for now.
Old Opti also appears that it hadn't been used since installing. No signs of any liquid even touching the outside.
Water pump had basically zero fluid in it, or the connecting hoses. Again, some work was done on the car without it being able to cycle antifreeze through it.
Spark plug wires have some minimal wear to them, but are going to be replaced for completion's sake.


So...the car stopped working, and stopped being able to turn over, in some order. This is only a theory, but I think maybe that the car started having a start issue before it started having a crank issue. Optispark was replaced, starter was replaced at some point by some previous owner without being subsequently driven. Whatever the order was, those are facts.

The plan I personally have thought out is this:
I have a ADLD cable on order, should be here Thursday. I just ordered a Hantek 6022BE, will be here tomorrow.
In a few minutes, I'm going to go down and check continuity and impedance on the 423 ignition control white wire from PCM pin 5 on the black harness to the ICM harness terminal B. I'm going to check that same ICM harness to see if it is a physical issue. I am going to trace the block ground at the ICM to the chassis and see if I need to do anything there. I am going to have the ICM tested, the coil I don't think is the issue at this point, but barring any other results to this point I will test that. I will install the new ACD spark plug wires that arrived last night. I will, over the course of the next week, take on the task of removing the spark plugs and doing compression testing. I also need to correct the EVAP/TB/steam routing, and I'll pick up a hose today to make that happen.

I'm trying to follow any troubleshooting steps I can to my best ability. I'm just a dude who hasn't worked on cars in a long time, but has decent capability, working on it in my MIL's driveway.

 

I’ll be updating this post as I get data.

1238 - .02 ohm and solid continuity on the white wire. I think I can rule a break out there. PCM pins and connector look pristine.


1243 - pics and video. Fuse boxes and crank attempt.





1340 - Oreilly doesn’t test ignition control modules, at least per the Sunday moron crew. I was told differently on Tuesday. Autozone’s sign says they do, but not as a standalone part. Have to have a think about what else I want to look at today.

1610 - cleaned up strap ground on chassis from ICM/coil mount. Checked starter ground, less than .5V. Swapped in the wire that came with the new Opti, just to check. Still .7-.8VAC.

New starting video, I doubt this is the RPM bounce everyone is looking for.


1654 - all driver plugs smelled of gas.

1840 - all plugs are out, all have evidence of getting gas, or at least starting fluid. Smells more like gas, though. Couldn't really get the compression tester in the plug holes, this car really isn't made for this. Or my hands aren't. Attempting again tomorrow. Hope to have the car back together enough to use the oscilloscope late Monday or Tuesday.

 

Asking a couple of Hail Mary/educational questions.

The 1-4VAC is just an acceptable range, right? The PMC should be sending a consistent signal in that range, no matter how fast the engine is turning, and it's up to the Opti to determine frequency that the injectors and spark fire. In other words, the fact that it is cranking slowly isn't producing weak or inconsistent spark. Nothing about any compression or other timing issue should result in a weak voltage. I don't know that I've read about anyone else in this position.

Is there any other 12v supply or ground (minus the one near the starter) that I should check to see if there's a reason behind not enough voltage on that ICM line? The other confusion I have is the roundabout manner in which the electrical signals go. If the ICM isn't receiving the proper voltage, it can still be the ICM's fault?

It kinda feels like if that line voltage had more juice, I'd be much closer to a resolution.

 

THIS HAS BEEN REVISED 09/25/23

The signal from the PCM to the ICM is not actually AC. It's a DC reference voltage, I'm assuming 5 volts reference, with the base voltage on the white wire being 0 VDC, and the PCM sending a pulse (5 volts, just like the Opti) telling the ICM to ground the coil and release the secondary coil voltage to fire the plug. It's just a way to measure the DC voltage with an AC volt meter. It's swinging back and forth too rapidly to measure with a DC scale. Best way to measure it would be with an oscilloscope. The acceptable range is copied out of the factory service manual. I had the 5 VDC reference and 0 VDC pulse reversed.

Now it makes sense. If the engine is craning slowly, the 5 VDC pulses are going to be less frequent, and the AC Volts might be lower than 1 VDC.

That's why I was looking to Gary to make sure I have this right. He's an electrical engineer…. I'm a mechanical engineer, it's “beyond my pay grade” .

Logically if the circuit is at 5 VDC, with a cyclic pulse pulldown to 0 VDC, the faster the pulses, the lower the AC voltage. There is the contradiction in my analysis. If the starter is cranking the engine too slowly, the pulses would be slower, and the AC voltage would be higher.

If I still had my car and it wasn’t seriously modified, I could check that logic. Except I also used a completely different engine management system, and ignition system, running 8 LSx coil-near-plug configuration. The setup was similar to the Bailey Engineering LTCC system.

 

Well, maybe the oscilloscope will provide some answers, if the learning curve isn’t too steep. I watched a very educational video from scannerdanner on a 94 Vette where all those pulses were shown.

I found some more things I can check in the meantime, some of these may be red herrings but they don’t take much effort. I did seem to have a bit better cranking sound after I cleaned up the pass fender ground, so maybe hitting the one at the motor mount will help too.

 

The scope will absolutely tell you if the Opti is sending the signals. I don't see any real tach movement. I do not trust the voltage readings in some troubleshooting procedures. Those can depend on the meter being used. 1-4v is a wildly wide tolerance to be of much use.

Here are two shots off my scope of the hi res and low res signals. The first shot is done at a horizontal resolution (200us/division) to show the high res best and the second shot was at a resolution (10ms/division) that shows the low res best. Low res is the top trace and hi res is the bottom trace in both images. Note the varying pulse widths of the low res signal. That is normal, the pulse width identifies the cylinder number for the PCM.

You do not have to view both signals at the same time so a single channel scope is fine.
I had that Hantek scope model for a while and it worked fine. I gave it away after buying my Rigol DSO so I can;t help with the setup as much as best.

 

By the looks of both of your videos, i don't see a good tach reading. i would say you need to rule out the Opti for SURE!

Do you have any idea where they got the opti from, what brand?

 

I know compression tests can be done on this car, but I am not that dude. I will cross that bridge by paying a toll at some point in the future.

 

If the starter is cranking the engine slowly, the tach isn't going to move much if the Opti low res pulse is being received by the PCM. That would also explain why the DC volts are on the low side.

I revised post #66 because I think I had the ICM pulse - base voltage vs. pulse voltage - reversed. That eliminates the contradiction in my thought process. It is my understanding the PCM cannot time the spark correctly without the Opti low res pulse, so it can't send a pulse. But there is spark. Would seem to indicate the PCM is getting the low res pulse. That is the entire basis for the Shoebox procedure, which is a restatement of the factory service manual procedure,

Can you try this - with the Opti harness connectors plugged in, attach the meter black lead to the gray harness connector pin "D" (reference low) and the red lead to pin "A". Turn key to "RUN". Rotate engine slowly via the crank bolt, and watch to see if the DC volts switch back and forth between 0 VDC and 5 VDC. I realize you may have to pull some plugs to allow the engine to rotate slowly. That's what makes the 'scope approach desirable.

 

Would you like me to do that before I hook the scope up? I did get it a short time ago, and since the manual is two pages, I can work my way through that stuff when I go down next.

 

I haven’t figured out how to directly get the waveforms out of the software, so this is what I have in the meantime.

Low, High, and ICM in order.

 

You’ll never guess what started happening during these tests - VATS activated and the security light won’t go off now. This is new today.


This car is cursed.

Hang on, I see it did it in my own video when I first got it. I wonder if it's just not putting the key in far enough, because it hasn't always done that. Temporarily disregard this post, I may come back to it though.

 

Confirm:

TOP = low res

MIDDLE ?? = no high res signal

BOTTOM = white wire to ICM