94c4seminole

RESOLVED! See page two.----------------------------------------

1994 Corvette base with crank/no start/no spark/no injector pulse

It has a new MSD opti, new ACDelco coil and coil module, new pigtails, PCM tested okay by SIA.

All voltages are as expected per FSM as follows:

ICM:

A and D: 12 volts with variation cranking
B: 0-4 v AC cranking
C: continuity with ground

This makes me think the PCM is sending a signal to the ICM and the ICM is getting power from the coil and has a good ground to the engine.

So it appears the PCM and opti are working

Coil:
Black connector: both have variation when cranking
Gray connector: 12v power and ground
Resistance between terminals are 0.5 ohms or so
Resistance from the spark outlet to the terminals are 9000 ohms

This makes me think the ICM is sending a pulse to the coil to fire and the coil has power and is grounded properly. The resistances seem up to spec as well.

This makes me think the ICM and coil are also good.

So what am I missing here?

For codes I have:
C12 (normal)
C34 (MAP sensor)- I haven't hooked the MSD vent hose up yet so I believe this is just the vacuum leak. I don't want to drill into the intake until I'm sure the opti is good.
H34
H41 (Open Ignition control circuit)- This is new since replacing the opti with MSD, makes me think the ICM may be sending a bad signal?.
H64 (O2 sensor - this was there before the car died).

Any help appreciated!

geb@abq

Have the local part store check the ICM...they will do it for free, if they still have the machine. Autozone did mine. Sounds crazy but I had 2 bad ones, the OEM, and a new Delphi. Make sure they run it at least a half dozen time to make sure the ICM is hot.

94c4seminole

Thanks for the advice. I have been looking for someone to test my ICM rather than buying another one, but none of my local part stores do it.
Tomorrow I will call some stores farther away. It is good to know there are still places that will test them.

1800Wing

You have likely already done this but I'll ask anyway....does the fuel pump come on and pressurize the system before you crank the engine ?

later, tiny

94c4seminole

Oh yeah, forgot to say. Fuel pump primes and holds pressure at 45 psi. I also checked all the fuses and they are good. I cleaned all the grounds I could access without removing the engine as well.

geb@abq

ok, well, that is just one thing to test. Mine all checked good with voltmeter, cranking, etc. changed coil and ICM connectors, did all that, and still ended up with a bad ICM...makes no sense.

1800Wing

I'm wondering if that H41 code (open ignition circuit) isn't whats holding you out, the ICM has to get a reference pulse from the Opti before it will fire the ignition circuit as I recall.

Did your new Opti match right up to the original harness...moreover, did the wires in the connector match up ?

I got a Delphi unit and I had to move one of the wires in the connector before it would work. Might be worth a look.

later, tiny

94c4seminole

That's good to know. My voltmeter shows imprecise data when cranking. It could very well be a bad signal. I've considered getting a scanner or oscilloscope to see the actual square waves but am hesitant to spend the money. If a shop can do it that is ideal.

94c4seminole

I had an intermittent connection at the harness to Opti connector on the intake manifold, and I tried to replace it with a connector online. It broke everytime I tried to build it so I just used heat shrink butt connectors to make sure.

I used the FSM to match the wires from the PCM to the MSD opti wires. The MSD harness it built in so I couldn't use the connector I had on there before. Here's how it's hooked up:

PCM:
D. Pink/black. Distributor reference low signal
C. Red. Distributor ignition feed
B. Purple/white. High resolution signal
A. Red/black. Low resolution signal

to

MSD Opti:
4. Red/black
3. Red
2. Orange/black
1. Yellow/black

Tom400CFI

You have no reference signal from the distributor. If you have no ignition, and no injector pulse...that is a pretty sure bet that the problem is in the distributor or the harness from the distributor to the ECM.

1800Wing

Did the MSD come with a diagram or schematic so we know that the wires are carrying the right signal from the Opti to the original harness...all it would take is for one to be out of place. Not trying to be a wise *** but it has to be something simple as hell that's been overlooked....don't ask how I know.

94c4seminole

That's what I was afraid of. I bought a brand new MSD unit, so maybe I wired it wrong. I was very careful but perhaps I messed it up. I think the wiring from the opti is in this order:

From driver to passenger side,

D. Dist ref signal (grounded)
C. Dist ignition feed (12v)
B. High res signal (5v)
A. Low res signal (5v)

Is that what you have? This diagram suggests the opposite but I was careful to keep it as it was: http://shbox.com/1/95_ign_system_schematic.jpg

The DTC 41 schematic in the FSM leads me to believe it is "faulty ignition coil module connection or faulty icm", and it says "If a DTC 41 is detected, the PCM will disable the fuel injectors to prevent flooding." So maybe that could be a cause of no spark/ no inj pulse too.

94c4seminole

Unfortunately no diagram from MSD. Here are their instructions: http://documents.msdperformance.com/8381.pdf

I used this diagram to identify the original harness wires: http://shbox.com/1/95_ign_system_schematic.jpg

However, I have it wired from driver to passenger: D, C, B, A, which I believe is how it was originally. Can you confirm this? I am looking at images trying to find out.

I totally agree with you, and I hope it is just something that simple! Thanks for helping out

1800Wing

Looking at my schematic in the FSM for my 96 it appears identical to what you used, but without knowing how MSD wired theirs...well.

Sure would make it easier if MSD had included a basic wiring chart if nothing else, otherwise we're just guessing that it's correct for this application

That was one of the reasons I bought a Delphi replacement for mine...and even it did'nt bolt up without the alteration to the harness.

94c4seminole

I found the original harness connector and lined it up with the MSD connector. It looks like I have it wired up correctly. I think my next step is to get the ICM tested at AutoZone if I can find someone who knows how to do it.

TorchTarga94

This.

You may have a DOA MSD opti.

Tom400CFI

The MSD should be plug and play...just like any other replacement opti. No way you can really mess up wiring, other than spark plug wire order -and even that is pretty straight forward.



If you have no injector pulse...it ain't your ICM. ICM is AFTER the PCM in the flow of things...No injector pulse means that:

Figure our why the ECM isn't "seeing" a reference pulse from the distributor. My BET, is that it's the MSD distributor.


EDIT: Do you have access to a scan tool? If so, you can "data stream" and watch your tech, hi res and low res signals in real time. With that feature enabled, crank the engine and you'll know instantly if the new distributor is working; if you see RPM, hi and low res signals...it's working. If not....it's not.



.

94c4seminole

I tried Wwp, Cardone, AIP X2 before this. The only one that would test it was AIP who confirmed it was bad. Is it likely I have had 5 bad opti's? The PCM is sending 0-4v AC to the ICM during cranking.

94c4seminole

I'll order one. You're right. It is better to see the exact signal than trying new parts. Would an OTC show if the ICM is working too?

Tom400CFI

No it won't.

An ICM is functionally, nothing more than a "relay". The ECM grounds the circuit on the "primary" side of the ICM and electronically (solid state), the ICM "pulls the magnet to close the contacts" on the secondary/high current side. This high current side is what is grounding the ignition coil to charge it; similar in function to points that are closed. The ICM is used b/c the solid state components in the ECM itself aren't robust enough to handle the current that the coil needs. Later ECM's are more robust and have the "ICM" internal -one for each coil on plug, and there is no external "ICM" unit. I say all this so you can see why the ECM/scan tool won't tell you if the ICM is working; there is no feed back; it's just a dumb "relay"/switching, on/off, actuator. If you have a dwell meter, you could test the ground side of the coil circuit and "see" the ICM grounding and ungrounding the coil...just like you could test on a points triggered ignition system.

Again, if you for sure have no injector pulse, and no spark, it's strong evidence that there is no signal from the distributor. The Scan tool suggestion is just to be able to PROVE to yourself what is going on. It gives you "eyes" to see the distributor functioning...or not.

Here is another simple test; turn the key on, fuel pump will run for a second, right? Then shut off. Try bumping the start...just enough to turn the crank a 1/4 or 1/2 turn or so. A 1/4 turn should sent two low res signals and 90 high res signals to the ECM...fuel pump should kick on for a second. Does it? If not, Distributor is not sending any signals out. (or they're not getting to the ECM). One reference pulse from the distributor should fire up the fuel pump.