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Old 12-04-2018, 07:41 PM
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djmac djmac is offline
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How to troubleshoot a GM OBD1 ignition

I know this is the fuel injection forum but I have a GM OBD1 system controlling fuel and spark in my 82 J10 with a Ford 460. I had trouble with my IAC and found that the stock wiring pinout was wrong for my new Holley Tbody. I reversed the wiring and corrected that issue but ended up with no spark. I have an MSD 6A ignition box and jumped the test terminal and got a very strong spark out of the coil. With the key on I have no voltage at the coil + terminal but I understand that is normal with just the key on. My issue is I don't know the sequence of operation between the ECM and Ignition Control module and coil. I disconnected my ecm to troubleshoot the IAC so I could have disturbed a connection there. I replaced the Ignition Control Module no luck. So does anyone know with the key on what voltage I should be reading at the wiring harness connection to the ICM? My module has a 2 pin and 4 pin connection on one side and a two pin (input?) connection on the other. I believe if I knew what 8 terminal should read voltage wise with the key on I could track the issue down. This system came out of a 91 Camaro if that helps and I a standard billet dizzy with the computer controlling spark. This link has a ICM similar to mine. http://www.fieros.de/en/v6help/code42.html
Thanks in advance for any ideas
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82 J10 SOA 6" SPRINGS D60-D70 W/DETROIT LOCKERS 38x15.5 TSL'S on 12" M/T CLASSICS 460 V-8 C-6 CUSTOM EFIS EFI AND HYDROBOOST
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  #2  
Old 12-05-2018, 07:24 AM
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Maybe it's early and I need more coffee but the diagram you posted shows you the module power pin.
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'88 GW (aka Babywag) and '90 GW (aka JUNKbucket) both fuel injected
1994 Caprice wagon
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Old 12-05-2018, 05:34 PM
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Quote:
Originally Posted by babywag
Maybe it's early and I need more coffee but the diagram you posted shows you the module power pin.

As helpful as the pinout appears it does not tell me what to anticipate and test for with the key on. Given that I am not familiar enough with the sequence of operation I need to know where to expect the switched 12v current to show up when you turn the key on. Usually, it would be at the + terminal of the coil? Or in this case does the ignition key send 12v to the MSD? What comes from the ECM when the key is on? There are 6 pins on one side of the module as you can see in the picture. When the key is on what Voltage should these terminals be outputting? There are 2 pins on the other side of the module what voltage should they be outputting when the key is on or cranking? Maybe I am making this harder than it needs to be but I it appears to me that's the info I need to troubleshoot this one.
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Old 12-05-2018, 06:47 PM
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Module should get power key on/crank "+" on diagram you linked.
MSD would need same on ignition wire.
ECM would also need same on ignition wire.

There is a voltage check sheet for ecm probably find it on thirdgen or gearhead-efi.
Also full wiring diagrams.
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'88 GW (aka Babywag) and '90 GW (aka JUNKbucket) both fuel injected
1994 Caprice wagon
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Old 12-06-2018, 07:13 AM
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Thumbs up

Exactly what I needed thanks alot.
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  #6  
Old 12-15-2018, 11:55 AM
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Quote:
Originally Posted by babywag
Module should get power key on/crank "+" on diagram you linked.
MSD would need same on ignition wire.
ECM would also need same on ignition wire.

There is a voltage check sheet for ecm probably find it on thirdgen or gearhead-efi.
Also full wiring diagrams.

Thanks alot for the info. The guys at thirdgen were great and had the details I needed. My beast is starting and running great. It turns out I damaged the ICM while rewiring the IAC. I had suspected that and replaced the ICM but I did not actually mount it but rather just plugged it into the wiring harness when I tested the new one. I thought the wiring harness would ground it but it does not. Once I ran a test jumper to ground it she started right up and then I bolted it down and all was good.
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Old 12-15-2018, 12:15 PM
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It should be grounded via the harness, if you have to add a jumper something isn't correct.
Very unusual for an ecm to actually be bad, 95% time it's a wiring issue and frequently poor grounds.

This is a resource for a TBI wiring diagram I modified for folks.
http://www.ifsja.org/forums/vb/showthread.php?t=169223
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'88 GW (aka Babywag) and '90 GW (aka JUNKbucket) both fuel injected
1994 Caprice wagon
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  #8  
Old 12-15-2018, 12:25 PM
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Thank, I will check it my harness out
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  #9  
Old 12-19-2018, 02:10 AM
Bill USN-1 Bill USN-1 is offline
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ICM must be mounted on a heat sink and bolted to ground. Preferably, directly to the engine, not the firewall or inner fender.

The module grounds thru the mounting screws.
There is a low ref signal gnd wire but that does not provide the ground for the high power transistor used to fire the coil.

It's all been covered on binderplanet.
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  #10  
Old 12-19-2018, 07:45 AM
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djmac djmac is offline
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Quote:
Originally Posted by Bill USN-1
ICM must be mounted on a heat sink and bolted to ground. Preferably, directly to the engine, not the firewall or inner fender.

The module grounds thru the mounting screws.
There is a low ref signal gnd wire but that does not provide the ground for the high power transistor used to fire the coil.

It's all been covered on binderplanet.

Thanks Bill!
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  #11  
Old 12-19-2018, 08:39 AM
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Quote:
Originally Posted by babywag
It should be grounded via the harness, if you have to add a jumper something isn't correct.
Very unusual for an ecm to actually be bad, 95% time it's a wiring issue and frequently poor grounds.

This is a resource for a TBI wiring diagram I modified for folks.
http://www.ifsja.org/forums/vb/showthread.php?t=169223

Apologies...my brain saw/read ECM. Bill is correct, ICM must be mounted properly.
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'88 GW (aka Babywag) and '90 GW (aka JUNKbucket) both fuel injected
1994 Caprice wagon
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