If I'm not mistaken, the PCM generates the tach signal off one of the opti signals. This is also what is needed to get the engine to start. What you are describing sounds like a wiring harness problem(I would hope that two "new" opti's would not have the same problem....but I also hope you are aware of the consequences of not getting a GM or Delphi opti unit). Inspect the wiring harness that runs from the opti and along the passenger side of the intake manifold very carefully for problems.

Watch the Buick Know How video in the opti sticky very carefully on how to check the opti's signals. This also relies on a multimeter that has a decent display update rate. The cheaper ones will not allow you to see some signal changes. The absolute best way to view the signals is with an oscilloscope.

You may need to back probe the harness with a T-pin(very carefully I might add...) in order to measure the signals. T-pins are located at your local Wal-mart in the sewing section.

 

I was not able to find a ride to work today so I stayed home. I was re-checking my readings with a meter and all the connections at the ICM seemed within specs. I then checked the readings at the opti harness connector.
A 5VDC
B 5VDC
C12VDC

However this reading is high. The specs I have show 60ohms with key on

D shows 145-148 with key on.

D shows -.03 with key off.

Is this the gremlin? If so where do I run a new wire from?
Praying this is it!!

 

Ok so I went back out using a diagram here and figured out which wire it was coming from the connector labeled "black" on the computer. Using the continuity tester mode on the meter I borrowed, which is a calibrated electronic unit. (I work for an electronics company). I measured the resistance from the computer connector to the opti connector and it measured 0.06. So is this the issue? bad wire? Should I try splicing this wire and bypassing the opti connector? EDIT I called a buddy at work and he said the wire is fine with .5 ohms. so I am thinking to jump/bypass the connector? or.... is this a reading sent from the opti to the computer??? or from the computer to the opti????
**EDIT** I went ahead and ran a "jumper" wire from the opti pigtail (bypassing the connector) to the pnk/blk wire coming out of the "black" connector on the computer. I tested for continuity it was good. I then put the connector back together and tried to start it. No luck...
well it helps if you're not a dumbass. I went back out there and I had neglected to reconnect the ICM from when I was testing it I connected it back and it fired right up!

 

Here's a test procedeure that will help.

4th Gen LT1 F-body Tech Articles - OPTI and Spark Test

 

Yes it is a very helpful source. I have been using it thank you.

 

D, on the opti connector, appears to be a ground connection through the PCM. I just checked mine and with the key off, you should have 0-5 ohms with the key off from D to ground(battery negative). My meter read 0.1ohms.

You may can get the harness re-terminated or spliced to fix the bad connection. Glad you found it.

Check and double check all the engine harness ground terminal connections. One of them could be giving you grief as well.

 

UPDATE...... yesterday on my way to work the car died on the highway. I pulled over and tried to re start. Smoke began rolling out from under the car. The cat was glowing. So I waited about 15 min. It started and ran fine. I got to work, went to lunch, ran errands went to dinner, etc. car did fine. This morning it would not start again....so pissed