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I was looking to drop this 4l80 today and have ran into this harness connection that seems to be hardwired into the transmission harness. Customer already wants the pigtail replace due to the condition it's in. Only other 4l80 I have rebuilt had the regular harness connection and harness plug so I'm a little bit confused by my find. I'm guessing I'm going to have to tell him that he also needs an internal transmission harness? Only thing I can think is to label the wires row by row so that I can easily wire in the new pigtail when it gets here and cut them so I can get the transmission dropped.
Unless you are going through really LEAN times there..... I'd recommend firing that customer.
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edit. That sounded a little harsh... Maybe a better strategy would be to pack a couple hours for chassis wire harness repairs to the estimate, in addition to the connector pigtail, and of course the internal wire harness.
.........hopefully that's just the donor rig that your CORE trans is coming from, with missing pan bolts, and lots of debris. IF it's the customer's actual rig you are repairing, I'd remain wary, it does look poverty stricken.
Last edited by Full Power; Mar 5, 2025 at 12:54 PM.
It's too late and I already accepted the work lol. Seriously though what kind of connector/connection is this. I looked at the other 4l80 I have and the harness connection looks nothing like this when you look at how the wires are organized. Back tracing the wires is always an option but decoding it from this connection is pretty much impossible right or is there a chart to show what wire should be what color?
Some kinda prototype/pre-production transmission ? What is the YEAR-Make-Model of your parts wagon ?
Does NOT look anything like MY 4L80 nor 4L60 connectors.
I don't know what you have there, hope the experts chime in soon.
Last edited by Full Power; Mar 5, 2025 at 03:20 PM.
Reason: spelllchex
I found these diagrams earlier hopefully the wiring coincides. For the record I am always on the ground when I do my uninstalls so I remove the pan before dropping to prevent fluid coming out when I work the transmissions off my 4"x10"x12" wooden box I crafted to attach to my regular floor jack and he is definitely getting charged extra. I think this is a pre 91' type harness I may be wrong and not even sure what a ft550/600 could be. Just happy to have it in the shop after finding that torx bolt this afternoon, I'll figure out the wiring later.
Looks like some kind of heat shrink on the wires there, probably the way it's made. I bet there's an outer piece missing, you can see the tab on the Connector like where it would lock into.
Definitely different from what we all know as the common GM Transmission Connector.
The First 3 Years of the 4L80E Transmission had a different design Internal Electrical Wiring Harness/ Case Connector and a different design mating Electrical Connector from all Model Years that followed.
Obviously you have the Original design/ 1991 - 1993 version.
You just need to purchase a new Internal Electrical Wiring Harness/ Case Connector and mating Electrical Connector.
It is a direct swap.
I bought a internal harness and pigtail. Labeled all the wires according to the old style plug on the 4l80 harness. The vehicle side has 12 wires and the plug connection I purchased only has 11? I can see by the diagrams I posted above you cannot just go by the lettering. I'm trying to assess which wire is the extra and what I am missing. Anybody know why please don't have the same number of wires?
So from what I'm noticing is the newer style has the A and B and TCC solenoid on the same positive feed. The older style 91-93 has its own dedicated positive feed for the TCC solenoid. So can I just leave it disconnected and let the other positive feed the three solenoids?
I wrote a whole paragraph and tried to upload a photo and lost everything. Long story short I traced back the high and low of the pressure control solenoid all the way to the standalone TCM as it is still has no power and full line pressure at idle. Following this wiring guide had me wire the low to the high and the high to the low while AllData contradicts the guide and has the EPC wired oppositely. Can this electronic pressure control solenoid be reverse polarized or is it setting some kind of failsafe where it still shifts through all the gears no issue?
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4l80e harness connection wiring 
