Wiring Help 411 PCM
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Teching In
Joined: Aug 2013
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From: Sonoma County, CA
Wiring Help 411 PCM 1971 C10. 2001 LQ4 4l80E
Finally... almost there. Long story short, my harness is ill put together. I bought it in 2014 and there is no recourse for support / repair from the source. I already had to relocate 7 pins on the PCM connector (4 injectors swapped, wires for TPS, MAF & ALT in the wrong place). but now it starts, no codes when last I checked.
Went to put in in gear for the first time the other day, with dreams of putting it on the trailer to the exhaust shop, and.... nothing.
Checked out the Connector @ the 4L80 and everything is where its supposed to be. Checked the corresponding pins in the PCM connector and everything is there except for pin 75 on the blue connector. Turns out this is part of circuit 1020, one of two key on hot / start hot circuits, this one specifically for powering the transmission solenoids on Pin E of the 4l80 connector.
So, this leads me to believe that the guy who did my harness must have spliced in it somewhere else? Maybe with the rest of the pinks that are only key on hot. I think the only thing to do is to pull the whole harness back through the firewall and see where the wire from Pin E ends up. I can harvest wires from the original harness if I need to make a jumper.
I'm having a hard time deciphering how to fix this when I find it. Is it a Pin 75 to Pin E straight line or do I need to tie it in somewhere along the way. Spent so much time trying to figure it out, I can't trust my own crossed eyes.
Finally... almost there. Long story short, my harness is ill put together. I bought it in 2014 and there is no recourse for support / repair from the source. I already had to relocate 7 pins on the PCM connector (4 injectors swapped, wires for TPS, MAF & ALT in the wrong place). but now it starts, no codes when last I checked.
Went to put in in gear for the first time the other day, with dreams of putting it on the trailer to the exhaust shop, and.... nothing.
Checked out the Connector @ the 4L80 and everything is where its supposed to be. Checked the corresponding pins in the PCM connector and everything is there except for pin 75 on the blue connector. Turns out this is part of circuit 1020, one of two key on hot / start hot circuits, this one specifically for powering the transmission solenoids on Pin E of the 4l80 connector.
So, this leads me to believe that the guy who did my harness must have spliced in it somewhere else? Maybe with the rest of the pinks that are only key on hot. I think the only thing to do is to pull the whole harness back through the firewall and see where the wire from Pin E ends up. I can harvest wires from the original harness if I need to make a jumper.
I'm having a hard time deciphering how to fix this when I find it. Is it a Pin 75 to Pin E straight line or do I need to tie it in somewhere along the way. Spent so much time trying to figure it out, I can't trust my own crossed eyes.
Not 100% sure I'm following you, but if there is no wire in the blue connector pin 75, then you will need to add one. Just hook it into an ignition hot source, generally one that is powered up by the ignition relay.
After rereading your post, I think I know what you are thinking now. Pin 75 doesn't feed power to the transmission, it's just powered up by the same fuse. It's a PCM power, so as long as it has an ignition powered source, it will be fine. The harness builder must have just forgotten it.
After rereading your post, I think I know what you are thinking now. Pin 75 doesn't feed power to the transmission, it's just powered up by the same fuse. It's a PCM power, so as long as it has an ignition powered source, it will be fine. The harness builder must have just forgotten it.
Last edited by ls1nova71; May 13, 2022 at 11:20 PM.
Thread Starter
Teching In
Joined: Aug 2013
Posts: 4
Likes: 0
From: Sonoma County, CA
Not 100% sure I'm following you, but if there is no wire in the blue connector pin 75, then you will need to add one. Just hook it into an ignition hot source, generally one that is powered up by the ignition relay.
After rereading your post, I think I know what you are thinking now. Pin 75 doesn't feed power to the transmission, it's just powered up by the same fuse. It's a PCM power, so as long as it has an ignition powered source, it will be fine. The harness builder must have just forgotten it.
After rereading your post, I think I know what you are thinking now. Pin 75 doesn't feed power to the transmission, it's just powered up by the same fuse. It's a PCM power, so as long as it has an ignition powered source, it will be fine. The harness builder must have just forgotten it.
Then, because it's also on circuit 1020, Pin E on the trans connector needs to be powered the same way? (key on + start hot). Or am I misreading the diagrams from lt1swap.com?
Thanks so much for your help.
So, I need to make sure that pin 75 gets power (key on + start hot). From what I can tell, The only other circuit that's powered this way is the ignition/injectors @ pin 3. So feed from that relay or run a new one.
Then, because it's also on circuit 1020, Pin E on the trans connector needs to be powered the same way? (key on + start hot). Or am I misreading the diagrams from lt1swap.com?
Thanks so much for your help.
Then, because it's also on circuit 1020, Pin E on the trans connector needs to be powered the same way? (key on + start hot). Or am I misreading the diagrams from lt1swap.com?
Thanks so much for your help.
Thread Starter
Teching In
Joined: Aug 2013
Posts: 4
Likes: 0
From: Sonoma County, CA
Supplied Pin 75 with power from the ign circuit. Still no joy, really had my hopes on that missing pin.
Never seperated the tq converter and the trans, so I don't think it's a pump issue. Still no codes, I have access to a tech 2. I'm stumped.
Never seperated the tq converter and the trans, so I don't think it's a pump issue. Still no codes, I have access to a tech 2. I'm stumped.
