LT1 Based Engine Tech 1993-1997 LT1/LT4 Engine Related

Diagnosing a bad Opti

Old 04-29-2008, 12:55 PM
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Diagnosing a bad Opti

Maybe a moderator can sticky this...

Check to see if you have a signal from the PCM to the ICM on the white wire. While someone cranks it over, check the white wire to ground for 1-4 volts on the AC scale. If that's there, the problem is between the ICM and the coil.

2 of the wires go from the ICM to the coil. With key ON engine OFF, those 2 wires have 12 volts on them. Find them and be sure they have 12 volts with the key on. One of the other 2 wires left goes to ground. Find it and confirm it goes to ground. The last wire left is the one that carries the control signal. It will test out as I said on the AC scale when someone cranks it with 1 to 4 volts AC. Also measure the resistance of the wires on the opti harness.

If you are getting 1-4 VAC on the white wire it means your opti is sending pulses to the PCM which is sending signals to the ICM to fire. Then your problem is not the PCM or harness. It would be ahead of that which leaves the ICM, coil, and opti.

http://shbox.com/1/95_ign_system_schematic.jpg (Diagram courtesy of Shoebox)

Disconnect the ICM connector. Leave the coil connected. Turn key ON engine OFF. Check for dc voltage at harness terminal "A" to ground and and also "D" to ground. You should get 10v dc or more on both terminals.

If you get no voltage then its the coil or ignition fuse.

If you have good voltage, switch the meter to AC and connect the leads to terminal "B" and ground. Watch the metre while someone cranks the engine. You should see between 1 and 4 VAC. If you don't see the proper ac voltage the problem could be the optispark, the harness to the optispark, the PCM or any of the wiring in between.

Measure the resistance of your coil. If its around 5000 ohms or so its fine. Any higher than 7000 ohms means you should replace it. Measure the resistance of the opti harness and if it seems very high, replace it.

So then if your car passes all the above tests, its the opti.

BTW, I didn't create the above method. I had it on my PC as a text file for years and just found it this morning. I don't remember who sent it to me.

Last edited by Gripenfelter; 04-29-2008 at 03:24 PM.
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Old 05-01-2008, 06:49 AM
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Yeah, this same basic info has been on my website for about 4 years.
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Old 06-04-2008, 11:01 PM
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Originally Posted by marshall93z View Post
Just tested my 93 at the ICM harness and it seems to be different than the schematic has it labeled.

12DC/1-4AC/Ground/12DC is what I have versus Ground/12DC/12DC/1-4AC
That's exactly like the schematic. Make sure you pay attention to where the ABCD letters are shown. The schematic on my site shows it CADB for readability. The actual harness plugs are always ABCD...
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Old 09-23-2008, 08:08 AM
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can you guys do something about the ICM .. BCM ..PTR.. the three letters don't mean anything to some of us... without defining what they at least once so we can understand... lol
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Old 09-23-2008, 12:35 PM
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ICM = Ignition Control Module

The other two you typed aren't mentioned in this thread.
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Old 11-03-2008, 10:17 PM
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If I can add one piece of info that was driving me nuts for a couple days....

I recently replaced all the ignition parts recently (Coil/ICM/Delphi OPTI) and I could not get it to restart for anything.

Needless to say, was getting fuel and gas so my logical line of thinking was that my timing events were off. I spent a nice evening (thankfully not a day since I have the !trap door) ripping out the rear carpet and panels checking the fuel pump. Then I thought it had to be me putting it the Opti on incorrectly.

About half way between removing all the coolant and taking all the intake stuff off to check the opti, I swallowed my pride and asked myself to check the basics out one last time.

Even though I labled every wire and plug and thought I had put everything back correctly, I managed to somehow cross two spark plug wires on the drivers' side of the opti.

We all like to overthink things sometimes but it really paid off when I was able to check the firing order and realize I wasn't the mechanic I thought I was. That little discrepancy kept the car from firing at all and wouldn't even get the car going.

It sucks that I was overanxious to get my rig back on the road but if I woulda taken that extra time to just check it, I could have been on the road days earlier and slept that much better those few nights.

Keep It Simple Stupid as they say Men,
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Old 11-04-2008, 12:40 AM
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Weird. I've crossed two wires before and I could BARELY tell it.
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Old 12-15-2008, 01:14 AM
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Originally Posted by marshall93z View Post
Weird. I've crossed two wires before and I could BARELY tell it.
But if one of them is the Number 1 wire, then the reference signal sent to the PCM from the Opti is totally off.

I've done the same with other plug wires and, agreed, it is sometimes hard to tell.

-Ivan
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Old 12-19-2008, 05:34 AM
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The cam position reference is sent by the optical sensor. That is not going to change based on mislocating the plug wires. How severe or noticable the crossed wires are will depend on which wires you have crossed.
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Old 03-26-2009, 09:09 PM
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I feel I can say this...
"IT'S NEVER THE COMPUTER" unless you've been submarining your Fbody or using the PCM as a ground for your welder.

As far as yours non-start... do a search for my name (dirtydavew) and look at all the posts about non-starts. Just as much as finding a solution, read what I found it was NOT so you won't tear your stuff apart. It's almost always something simple.
Best of luck and keep us posted.

Originally Posted by chndlr91 View Post
I have a 95 Z28 LT1 and I recently changed the starter, fuel sending unit, distributor cap, coil, fuel pump, fuel filter, and spark plug wires. Needless to say, I am still having problems. I can't get the car to crank. It acts like it wants to crank but It won't. I don't know what else to try. I took it to a shop and they think it is the computer. Does anyone have any suggestions? I AM DESPERATE.


We finally tried changing the computer and we are still having problems with the car. I can't figure it out. Please help if you can.

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Old 10-31-2009, 06:59 PM
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Talking

I'm trying to followed your directions.

" Check if you have signal from PCM to ICM on the white wire."
I can only guess the ICM is located right above the coil, mounted on the drivers side head. Do you disconnect the connector first???? If not, how do you get to the wire? With it disconnected, I'm getting .49 volt AC.

Locate the ICM located right above the coil, mounted on the drivers side head.
The connector has 4 wires marked :
A - pink/black stripe
B - white
C - black
D - white/black stripe
Check the white wire to a good ground for 2 to 4v AC. If you have these voltages your Opti is sending signals. If not your ?????? is bad.


"2 of the wires goes to the coil"

Two wires go to the coil A - pink/black stripe/D - white/black stripe. Check for about 12v DC. Check the C - black wire to ground.

Disconnect the ICM connector, leave the coil connected....etc...
Good instructions from here to checking the coil.

The coil has 4 wires, two on each side. Where do you check the resistance of the coil????

I don't mean to be a pain, but I was taught if you write instructions, write them so anyone can follow them. And yes it's a pain to be detailed, but some of us who don't play with cars all day, might not know muffler ball bearings froma chain strecher.

THanks
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Old 10-31-2009, 07:34 PM
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Old 11-05-2009, 02:51 PM
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Where would one buy the pigtail that goes from the opti to the ignition system test connector. Mine is cracked clean through and still working but i'm sure some of my missing during acceleration is being caused by that.
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Old 11-05-2009, 07:18 PM
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I got mine on Ebay.
Example:
http://cgi.ebay.com/ebaymotors/95-97...item4a9bda5ae2
Originally Posted by dalynchmob View Post
Where would one buy the pigtail that goes from the opti to the ignition system test connector. Mine is cracked clean through and still working but i'm sure some of my missing during acceleration is being caused by that.

Last edited by DirtyDaveW; 11-05-2009 at 07:24 PM.
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Old 03-04-2010, 09:39 PM
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96 lt1 into '93 z28

hello,
does anyone know if i can use [leave] the intake and opti spark from a '96 lt1 that i'm istalling into my 93 z28? thanks for your time.
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