[njoythejourney]

id check wire for continuity rather than tearing harness apart .
you can run a new wire theres 2 white wires 1 to coil and 1 to icm
Fuse box under hood has c1 connector too Dont know if they are related
also a ignition fuse to check
even if fuses are good the tarnish on terminals of fuse might be blocking it
Again it has to be sending signal for plugs to fire
I think its 2 wire system both coil and icm put off signal
ecm reads and counts off coil
for injector signal the icm makes a tac signal
it has the resistor inside that protects the tac
so I would probe test light in white wire on icm to good ground
and fire it up and see if its flashing if its not its icm
on old Hei the icm was inside distributor and had tach pin on the distributor

The 3800 get the tach from the ICM but the 5.7 gets it from the PCM CONNA pin 13 or PCM 8A-81-22 in the service manual

 

[TealHornet]

I actually think the service Manuel doesnt have a box that says pcm is resonsible hints at it but what
I did was look up the aftermarket tachs for it
msd has port and makes its own signal
But the after market Tachs do not they rely on the icm for their signal
and explain the diode in icm that gives the signal
then the manuel gives the way to check pulse with it disconnected from pcm with harnes gauge thats adjustable to 57 megahertz
still disconnected from pcm
its like a noid hooking to icm
then if signal check ground no signal you check or replace icm
all test done unhooked from pcm i believe key on but not running
believe its a 3.8 volt signal if its higher or lower the diode is dying

 

[TealHornet]

just looked at Shoebox ignition Diagram
I dont understand why their chasing ICM stuff your right
Because you just did the opti I would think I would check the Crank sensor plug to see if its clean
and plugged in right and that they didnt pull that yellow wire terminal out of the clicked in position
in plug
CPK pink power pink black opti signal the yellow is signal for going to pcm
then to Tach on the white wire coming from pcm
it could be the crank sensor got antifreeze in it or something their always covered in oil unless you see a break in yellow coming off the crank sensor I bet its either pulling out of crank sensor plug or crank sensor is bad and pcm doesnt throw code cause it monitors the opti low res signal for codes
the traction control wire for abs you dont have the orange and black wire dead ends over radio and has a cover on it and was for 3.8 liter engine option you have posi rear end..
so ignore any codes for it..
but if ICM IS SHORTING TO GROUND IT could be the weakest point in system that happens the most
if it shares ground with either Tach /gauges pcm or icm or opti
but im Thinking yellow wire pulling out plug or crank sensor itself is original
hit its 130.000 mile death being a 96 lol

 

[TealHornet]

Let me know if you got it fixed

 

[njoythejourney]

No codes crank sensors fine. Today I didn't have the opportunity to verify there is an output from the PCM at c 203 D which is the white wire going to the tach. I have not been able to locate the wire on the left ip connector but since none of it has been disturbed I am ready to call it as a brass tach.

 

[TealHornet]

Got 2 white wires 1 from pcm to Tach other goes to icm
these Icm have super heated hotter than the heads they bolt to so much they put washers between to try to keep them cool
the icm sends a 3.8 volt through a transistor diode to pcm via white wire
pcm counts tics and sends to Tack
about 50 people fixed tachs on 3 different forums by replacing icm
the service manuel for tach fix has you trouble shoot the icm directly
there is a last box that says you may be running on battery power
and not enough to run tach
Assuming you got 14.5 at battery and terminals are clean
Id throw a Delco icm at it
actually just priced one over 100 bucks so looked around
parts geek has brock icm that worked for 20 bucks or hit pick and pull
snitch a couple

 

[TealHornet]

Actually thinking you changed the coil right before opti
cause you were having problems
might be worth putting old coil back on and see if tach works lol
just remember to move the goo between back and forth

 

[njoythejourney]

I had chased tach wire to the passenger side kick panel. I did not find the wire on the drivers side. However, with a DMM that would do frequency I was able to comfortably determine there was a signal coming from the PCM. With that determination made I ordered an instrument cluster from Hawks Motorsports. While waiting on it I replaced the clutch and started the car a couple days ago. The tach for whatever reason is now working. Oh well I have a spare instrument cluster. I never like issues that seem to fix themselves they always seem to come back.

 

[TealHornet]

If it were a peter built Id tell you to shake the wiring harness right where it goes though fire wall .
and have it running and somebody watching tach .
But youl eliminate it if thats it .
Its so hard tracing wires .
Where abouts you live?

 

[njoythejourney]

If it were a peter built Id tell you to shake the wiring harness right where it goes though fire wall .
and have it running and somebody watching tach .
But youl eliminate it if thats it .
Its so hard tracing wires .
Where abouts you live?

Ks

 

[TealHornet]

Your to far away im in reno.
Got a short chaser it will find a break in wire but a short would drive you insane cause it shorts to grounds and every thing lights up and signals .
Let me know if new tach fixes it

 

[njoythejourney]

I am going to leave it alone for now. It is working. I have too many other irons in the fire to chase this one any further at the moment. Thanks for all of your input.

 

[TealHornet]

Keep in mind the fuse box is the side panel and i fix breakdowns at gas stations
from dirty fuses not bad fuses just dirty connection fuses with sand paper from like 97 on
just did that in a 98 suburban this morning

 

[njoythejourney]

Will do, thanks.