Cutting my own Harness... Questions for the experts

[knomadd]

Okay all of you LS swap wiring guru's... I need your help. I think I've got my game plan for cutting my harness sorted with a few questions still remaining.

First off, this is a 2016 Chevy Express 3500 6.0L (L96) and 6L90E trans combo. It uses the E78 ECM and a mechanical fan (no fan trigger wires in harness). It's going in a 71 C10 with Vintage Air, Dakota Digital Gauges with OBD-II BIM controller, and the van's fuel pump in an aluminum tank. I have a new PRC aluminum radiator for an LS swap and dual Spal fans. I plan on running the fans off a trinary switch and an Entropy LS Swap fan controller since there isn't a fan trigger in my harness.

Easier Questions:
*Can you confirm that I do not need the A/C pressure sensor, or the A/C Relay Control from the ECM, or the A/C Request from the van Harness?
*I also have 6 wires that have "brake apply sensor" in the description that goes to a "brake position sensor" in the van. Can I delete these? I plan for the TCC Brake Signal to go to a relay for the brake switch, or I'll replace the 2 pin switch with a 4 pin switch.

More complicated Stuff:
*There is a dark blue wire described as "accessory wakeup serial data" that goes from the ECM to the trans, and the diagrams show it going to the BCM and Instrument panel. I'm planning on leaving the ECM to trans connection and deleting the rest of it. Is this okay, or does this need to go anywhere other than the trans, like the OBD-II port?
*There are other wires that go inside the van, but not to the ECM that I think I can delete. They are: "Serial Data Communication Enable", "CAN Bus high terminated serial data", and "CAN Bus low terminated serial data". The CAN Bus stuff I'm not sure about, but I do have the "High Speed GMLAN Serial Data" wires, both + and - going to the DLC (OBD-II port) pin 6 & 14. See below for DLC pin-out info.


My DLC (OBD-II) is shown to be like this for the van:
1 - Low Speed GMLAN Serial Data (I assume this is not needed, it goes to the BCM in the diagrams)
4&5 - Ground
6 - High Speed GMLAN Serial Data (+)
14 - High Speed GMLAN Serial Data (-)
16 - Battery Positive - 20A Fused from truck harness.

Any help is greatly appreciated. Let me know if I've left anything out that you might need to know to answer these questions.

[boostedc10]

Duane,
Here's my input.

If you run the AC system as a standalone and wire it to a trinary switch like below you can get rid of the ECM wires.


IMAGE BORROWED FROM HARTS POST

The accessory wakeup serial data wire is not needed. This wire is nothing more than an aux power to the ECM that wakes it up for the low speed (Interior) CAN bus. The termination wires for the high and low speed CAN can also be eliminated.

Your DLC looks good and usually you only need one of the two grounds for it 4 or 5, but not sure with the E78. You are also correct on the low speed wire not needed, that is the diagnostic wire for the interior low speed CAN bus which you don't have.

Hope this helps.

[knomadd]

Awesome, thanks Brandon! Do you think I can delete the "brake apply sensor" wires?

The "Serial Data Communication Enable" doesn't even go to the ECM, so I'm pretty sure I can just delete it. Would you say that's a safe thing to do?

[boostedc10]

Duane,
You should be ok to get rid of the brake apply sensor wires. You said 6 wires though and I only see 3 on the x3 connector (pins 22, 38, 58).

Where is the serial data enable wire going in the diagram?

[knomadd]

You are correct on the 3 wires in the X3 connector. Those wires are #4, 5 & 6 of the Brake Pedal Position Sensor. It appears the other 3 wires go to the BCM directly so they don't exist in my harness.

I've pulled all of this info from GMupfitter.com which does custom vehicles like ambulances, police vehicles and box vans.

It appears that the serial data enable wire is coming from the BCM to the X100 connector (instrument panel harness to engine harness), then to the X101 connector (chassis harness to engine harness) and then to the electronic brake control module. Based on that, and the fact it doesn't connect to the ECM or the engine, I'd say I can delete it. I'm just a bit nervous about the fact it says "serial data" which I have read is important.

[boostedc10]

Duane,
The brake apply sensor stuff can go. As for the serial wire I will have to look at the diagram when I get home. I am almost sure you can eliminate it also based on what you said. It sounds like it is just a communication line from the BCM to the EBCM. Ill double check when I get home tonight.

[knomadd]

I got another one for you...

I was planning on running a starter relay, like it shows in the wiring diagrams for the van, but I'm curious how the signal would actually work?



Based on this wiring diagram, the ECM triggers the relay which starts the engine. That isn't possible without the BCM taking the weird a$$ ignition switch's signal and telling the ECM to start the engine.

So, do I need that wire (yellow/black from ECM X1-27 to relay pin 85)? Does it need to be connected with a wire from the existing ignition switch in the truck and tied together to the relay?

OR should I forget the starter relay all together, delete the yellow/black wire and just run the original starter wire to the starter wire in the engine harness?

[boostedc10]

Not sure on this one. Let me talk to my buddy at GM and what is going across that communication wire from the ignition to the ECM.

[solidaxel]

OR should I forget the starter relay all together, delete the yellow/black wire and just run the original starter wire to the starter wire in the engine harness?

B I N G O !

[knomadd]

OK, so I'm deleting the starter relay and the ECM wire. But now I've thought about another one... There's a relay in the van schematics that control the coils, injectors, MAF and O2 sensors. Looking at the wiring for it pin 85 & 30 are direct to battery + while pin 87 sends power to the fuses that control each function. That's all clear.

The concern/question I have is with pin 86, which comes from the ECM and is a triggered ground titled "Powertrain Relay Coil Control". How is this controlled? The schematics aren't clear on that.

Shouldn't the ignition related stuff be controlled from the ignition switch "ignition" wire? Maybe I need to run the pink ignition wire from the ignition switch to the relay pin 85 and a constant ground to pin 86? If I do this, what do I do with the ECM wire? I'm afraid if I wire it like the van, it won't power the coils, injectors, etc. But, if I don't connect the ECM wire, will it cause issues?

Here's a screen shot of the schematic. The wire in question is 3rd from the left.

[clinebarger]

The Ignition Run/Crank relay applies power to Circuit 439/Pink on the left of your diagram & "wakes" the ECM up, Then the ECM grounds Circuit 5991/Yellow to activate the Engine Controls Ignition Relay.

Along with waking up the ECM, The Ignition Run/Crank Relay also wakes the TCM up via circuit 2139/Pink.

You don't absolutely need the Run/Crank relay, Circuits 439 & 2139 just need power when the ignition switch is at RUN & Start positions (Thru a 15 amp fuse for each circuit), Wire the Engine Controls Ignition Relay just like your diagram shows.

[knomadd]

Awesome, thanks clinebarger, that's some great info. I really appreciate it!

[Hcb3200]

How did you wire your brake switch / TCC. What pin on ecm? Seems there is conflict on is it hot 12 while brake is of or hot 12 when brake on and does open circuit need resistor ground.

[knomadd]

It appears the TCC wire is light-blue/white from ECM pin 33 on connector X1. I'm not far enough with this to tell you how I've hooked it up, but I know the ECM and the Trans are connected. I'll have to add a 4-pin brake switch or do a relay to use the 2-pin switch. I haven't researched it enough yet though.