What am I missing?
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Joined: Sep 2013
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From: Port Saint Lucie Florida
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.
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.
I did the starter bump test, and interestingly the fuel pump does kick back on after the bump, so the ECM must be getting something from the distributor but it is still not sending spark or injector pulses.
I had the ICM tested at Autozone. They ran the test three times and it works even after heating up. My coil is up to spec, so I think you're right it is the opti or pcm..
Team Owner
Joined: Aug 2004
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From: Park City Utah
I wonder if you could borrow a scan tool from a friend, shop...someone. I'd lend you mine if we lived close enough.
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Joined: Sep 2013
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From: Port Saint Lucie Florida
Maybe. IIWM, At this point I'd want a scan tool to see what's up. The pump running tells me that the PCM is getting a reference signal...but maybe it's from cycling the key from run to crank and back? I'm not sure. I'd want proof.
I wonder if you could borrow a scan tool from a friend, shop...someone. I'd lend you mine if we lived close enough.
I wonder if you could borrow a scan tool from a friend, shop...someone. I'd lend you mine if we lived close enough.
Flatulent Dwarf
Joined: Nov 2009
Posts: 729
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From: Poverty Holler NC
Just a hunch but I suspect that MSD in order to make a unit that fits a bunch of GM applications has something wired to a different terminal than what it's supposed to be for this situation, without a wiring diagram from them we have no way to be certain.
later, tiny
Last edited by 1800Wing; May 5, 2017 at 08:11 PM.
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Joined: Sep 2013
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From: Port Saint Lucie Florida
I bought a couple from Harbor Freight....the more expensive one can track your fuel trim and let you see all the sensors, it's no Snap-On but for most of what Ive run into it does pretty good. Just wondering, what was the primary reason for this....did it just quit on you or spit and sputter...I missed that memo.
Just a hunch but I suspect that MSD in order to make a unit that fits a bunch of GM applications has something wired to a different terminal than what it's supposed to be for this situation, without a wiring diagram from them we have no way to be certain.
later, tiny
Just a hunch but I suspect that MSD in order to make a unit that fits a bunch of GM applications has something wired to a different terminal than what it's supposed to be for this situation, without a wiring diagram from them we have no way to be certain.
later, tiny
The car just died while driving back in September. I towed it home and have been trying to figure this out It hasn't run since....
Flatulent Dwarf
Joined: Nov 2009
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From: Poverty Holler NC
Ya know...a sudden catastrophic failure like that makes me think a fusible link...or a fuse somewhere rather than components. I remember you said you checked the fuses...were they the ones in the side of the dash in front of the passenger seat ?
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Joined: Sep 2013
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From: Port Saint Lucie Florida
Yes, all the fuses in the passenger compartment and the two blocks in the engine bay are good. I cleaned and double checked the injector and ignition fuses too.
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Joined: Sep 2013
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From: Port Saint Lucie Florida
A "sudden catastrophic failure" like that makes me think the MSD lived up to its reputation: May Suddenly Die.
Does the tach indicate cranking speed while cranking?
Interesting that there seem to be reference pulses (fuel pump comes back on), but there is no spark. Could the white wire be grounded? This is the primary side of the ign coil. It also goes to the tachometer. If the tach input is shorted to ground, there would be reference pulses, but no spark.
Also check the TPS blue wire voltage to ground with the key on, and the TPS connected to the harness. Slide a paper clip up through the cable seal to make a connection. The voltage should be ~.50 Volt at closed throttle, but must be < ~2.5 volts or the system will enter Clear Flood Mode and turn off injection.
Does the tach indicate cranking speed while cranking?
Interesting that there seem to be reference pulses (fuel pump comes back on), but there is no spark. Could the white wire be grounded? This is the primary side of the ign coil. It also goes to the tachometer. If the tach input is shorted to ground, there would be reference pulses, but no spark.
Also check the TPS blue wire voltage to ground with the key on, and the TPS connected to the harness. Slide a paper clip up through the cable seal to make a connection. The voltage should be ~.50 Volt at closed throttle, but must be < ~2.5 volts or the system will enter Clear Flood Mode and turn off injection.
I'll check the tps too. Haven't tried that yet. Thanks a lot for the help
Last edited by 94c4seminole; May 8, 2017 at 12:28 AM.
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Thread Starter
Advanced
Joined: Sep 2013
Posts: 85
Likes: 2
From: Port Saint Lucie Florida
A "sudden catastrophic failure" like that makes me think the MSD lived up to its reputation: May Suddenly Die.
Does the tach indicate cranking speed while cranking?
Interesting that there seem to be reference pulses (fuel pump comes back on), but there is no spark. Could the white wire be grounded? This is the primary side of the ign coil. It also goes to the tachometer. If the tach input is shorted to ground, there would be reference pulses, but no spark.
Also check the TPS blue wire voltage to ground with the key on, and the TPS connected to the harness. Slide a paper clip up through the cable seal to make a connection. The voltage should be ~.50 Volt at closed throttle, but must be < ~2.5 volts or the system will enter Clear Flood Mode and turn off injection.
Does the tach indicate cranking speed while cranking?
Interesting that there seem to be reference pulses (fuel pump comes back on), but there is no spark. Could the white wire be grounded? This is the primary side of the ign coil. It also goes to the tachometer. If the tach input is shorted to ground, there would be reference pulses, but no spark.
Also check the TPS blue wire voltage to ground with the key on, and the TPS connected to the harness. Slide a paper clip up through the cable seal to make a connection. The voltage should be ~.50 Volt at closed throttle, but must be < ~2.5 volts or the system will enter Clear Flood Mode and turn off injection.
I checked the primary side of the ignition coil (gray connector, white wire). It is grounded with key off, but not grounded with key on. Sound right?
The TPS is interesting. I'd love to hear your opinion. I do not believe the PCM is sending an appropriate signal. I followed the FSM algorithm, which I'll post below.
https://goo.gl/photos/ryq6PnkFdvdbf9iz9
With the harness plugged in, the blue wire reads 0.05V at closed throttle and 0.1V wide open. (should be 0.5V and 4.5V).
With the harness disconnected, the blue wire gets 0.05V, and jumping blue to gray reads -0.04V (both should be >4V).
In other news, I got an OTC 4000E but don't know how to work it. It powers on but says no words. I used the universal leads with black to terminal 5 and red to terminal 9.
Last edited by 94c4seminole; May 17, 2017 at 01:33 PM. Reason: link to diagram
Thread Starter
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Joined: Sep 2013
Posts: 85
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From: Port Saint Lucie Florida
I checked again today 0.04v with key in "on" position. Maybe it is the PCM! Tomorrow it gets towed to the shop. I'll update you all as to what happens. Thanks for the help!
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Joined: Sep 2013
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From: Port Saint Lucie Florida
RESOLVED
I finally took the car to Eismann's Corvette Center in Jupiter, and they figured it out. There was a short somewhere in the harness causing the 5v reference signal to be absent at all sensors. There was also a bad connection at the ICM harness, unfortunately due to my probing that connector.
The MSD opti was working great. To repeat, the problem was not the optispark! Although I am certain the original failure was due to the optispark (rusty inside and bent with code 16), the shop had a scan tool to show the optispark high and low resolution signals were normal. This means the replacement opti's I got may have all been fine... Unfortunately I was never able to use a scanner to check this because the one I bought did not have the right chip..
If you replaced an opti and suspect it is DOA, I suggest borrowing or buying a scanner that will show these signals. Don't waste time and money buying optis! With the MSD, my car has noticeably improved power, especially on the high end.
My other lesson learned is to never press multimeter leads into a female connector. Just hold it so it is touching the metal pin, as pressing it into the connector can lead to a loose connection. If you are working alone and need to crank the car, use back probing leads (except on PCM). Be careful because all these connectors can be exposed to the elements if the seals are damaged by probing.
Thanks to everyone who helped along the way. I don't think I would have been able to learn so much on my own. The shop said it took them a long time to figure it out, as it was a tricky case. I have learned a lot about the ignition system for this car, so if I can help someone reading this just send me a pm.
I finally took the car to Eismann's Corvette Center in Jupiter, and they figured it out. There was a short somewhere in the harness causing the 5v reference signal to be absent at all sensors. There was also a bad connection at the ICM harness, unfortunately due to my probing that connector.
The MSD opti was working great. To repeat, the problem was not the optispark! Although I am certain the original failure was due to the optispark (rusty inside and bent with code 16), the shop had a scan tool to show the optispark high and low resolution signals were normal. This means the replacement opti's I got may have all been fine... Unfortunately I was never able to use a scanner to check this because the one I bought did not have the right chip..
If you replaced an opti and suspect it is DOA, I suggest borrowing or buying a scanner that will show these signals. Don't waste time and money buying optis! With the MSD, my car has noticeably improved power, especially on the high end.
My other lesson learned is to never press multimeter leads into a female connector. Just hold it so it is touching the metal pin, as pressing it into the connector can lead to a loose connection. If you are working alone and need to crank the car, use back probing leads (except on PCM). Be careful because all these connectors can be exposed to the elements if the seals are damaged by probing.
Thanks to everyone who helped along the way. I don't think I would have been able to learn so much on my own. The shop said it took them a long time to figure it out, as it was a tricky case. I have learned a lot about the ignition system for this car, so if I can help someone reading this just send me a pm.
2nd Gear
Joined: Sep 2017
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i fixed my icm issue
the icm needs a proper heat sink. the aluminum on mine was not flat as a heat sink should be. it must be totally flush to a heat sink. besides being bent, its small. i bolted a pentium heat sink to it and by all means....took it off of the head and relocated it. problem solved. ps. if anyone thinks the cylinder head sinks the heat away from the icm, good for you. stay with that. im back on the road. relocate, use thermal compound, not dielectric grease from auto parts store brainiaks, and throw out the stock thin heat sink in favor of a bigger one.