Help

[nsb29]

It’s been a while since I built my engine harness. Now I wiring the rest of the truck and I have two wires marked brake switch one from the ECM and one from trans do both go to brake switch?

[prodjay10]

Isn’t one power constant and the other goes to ecm? I believe both go to the brake switch.

[ls1nova71]

What year is the engine/harness?

[nsb29]

Originally Posted by ls1nova71

What year is the engine/harness?

2009 6.0 and 6l80E out of a Pontiac G8

[ls1nova71]

Well, I feel like I shouldn't help you since you must have killed a G8 to get that engine...... Lol!

But anyway, they both tie together and go to the ECM X1 pin 9. That being said, I have never messed with that harness so not sure how you would go about hooking it to the brake switch since the G8 uses a brake pedal position sensor that is wired to the BCM then the info is sent to the ECM via the CAN Bus. I'm sure there's a work around, and maybe you already know what it is?

[nsb29]

Originally Posted by ls1nova71

Well, I feel like I shouldn't help you since you must have killed a G8 to get that engine...... Lol!

But anyway, they both tie together and go to the ECM X1 pin 9. That being said, I have never messed with that harness so not sure how you would go about hooking it to the brake switch since the G8 uses a brake pedal position sensor that is wired to the BCM then the info is sent to the ECM via the CAN Bus. I'm sure there's a work around, and maybe you already know what it is?

Well the G8 was dead when I got if that helps Ha Ha. so far almost everything I did to the harness was the same as the Camaro other than a couple of wires to the gas pedal. Have no knowledge About the brake pedal positioning sensor if you’re right I guess I’m going to have to learn just assumed it was the same as the Camaro

[kipps]

Originally Posted by ls1nova71

...But anyway, they both tie together and go to the ECM X1 pin 9. That being said, I have never messed with that harness so not sure how you would go about hooking it to the brake switch since the G8 uses a brake pedal position sensor that is wired to the BCM then the info is sent to the ECM via the CAN Bus....

Assuming the G8 is similar to the gen V camaro--

The BCM has a dedicated three-pin brake sensor. Based on info from that sensor, the BCM creates the necessary 12v brake signals for everything else. This includes the brake lights and x1-9 on the ECM. There is literally a wire(circuit #6311) from the BCM to x1-9 on the ECM that's energized to 12v when the BCM decides the brakes are applied.

There's also some evidence of a brake signal being passed between the BCM and the ECM on the CAN bus. The cruise control "description-of-operation" makes mention of this dual signal path. If you don't intend to use a BCM and cruise, it's possible you won't need that CAN bus brake signal, and instead can keep only the x1-9 12v signal for torque converter lockup, etc. I don't know on that one.

I'm using a camaro tune on a vortec engine, to give me a clutch pedal sensor and a manual transmission VSS. I'm also intending to use a Camaro BCM for cruise control. Hence my research into the muddy waters of brake circuits.

Note that this is quite different from the truck engines. Those don't use the three wire variable brake pedal sensor. Instead, they have a more typical brake switch. This brake switch feeds the BCM for light control, but it also directly feeds the ECM via x1-9.

[clay68c10]

The CAN bus is kind of like internet for the car. It sends coded and addressed signals to all the modules connected to the bus instead of just power and ground voltages to one single component.
For instance, you want to roll up your R rear window in your 2019 Oldsmobuick. You press the switch on the drivers door. The switch pack is actually a small computer. It sees the switch signal, makes a coded message that says “roll up R rear window.” The switch module puts that message out on the CAN bus, addressed to the R rear door module. The R rear door module sees the message addressed for it, receives the message, and then provides appropriate power and ground to the window motor. All the other modules on the bus see that the message isn’t addressed to them and they ignore it.
Haha simple, right?
The actual benefits are that it is less complex from a wiring standpoint since you don’t have to directly wire things across the car. Just wire outputs like motors, and inputs like switches to the module that is physically closest.
It also allows sharing of info between systems. Previously you had separate coolant temp sensors for the temp gauge, the engine computer, and the climate control. Now there’s just one and the engine module sends that temp info out to whichever modules need it.
All that is way slick and works great when the whole car is set up for it. It’s not so easy when we start swapping these engines into old vehicles and need something as simple as a brake switch input or AC compressor request.

[kipps]

The DLC is simply GM's name for the obd2 port at the driver's left knee. On the Gen IV's, the DLC has three data wires pinned to it in addition to power and grounds. Two of those wires are high-speed bus wires, and one is low-speed. If you're not using the BCM, you can skip the low-speed wire.

Think of the CAN bus(both high speed and low speed) as the equivalent of a USB cord. A USB cord or Ethernet cord allows multiple computers to talk together and exchange data. They can move a lot of info over four wires. The CAN buses are similar. Using only one or two wires, they allow computer modules to talk together.

Another analog data transfer which might seem similar but it's not, is involving "resistor ladder" switches. Many of the functions on the dash are directly wired to the body control module. If every switch had it's own dedicated wires going to the BCM, there'd be way too many wires. Instead, a group of switches will all have a common hot and a common signal wire. Depending on which switch you press, it connects a different value of resistor between the hot and signal wires. This way, five switches only have two wires connecting them to the BCM. The cruise switches(and many others) are this way. I'm pasting a 2010 Camaro cruise control diagram, to show how this kind of switch works. Note that there is no computer stuff involved in that switch. It's purely analog.

The use of a BCM with a GEN IV swap is not well documented. I've never found any tutorials or how-to's for doing such. Enough folks have done it that I'm sure it can be done, but it's not as easy as many other aspects of these swaps.

I strongly recommend you buy a subscription to alldata for your donor vehicle. If you're doing a well-documented swap, such as a stock gen III, it might not be needed, but for anything involving transmission swaps, gen IV or V, DIY harness mods, donor vehicle fuse boxes, Body control modules, etc, you've got to have access to those factory diagrams.

[nsb29]

My sun has access to all data the problem there is knowing what I need or what I’m looking at. I have a lot of stuff printed out on this swap and so far it’s been very helpful. I grew up around points and carbs and this is my first LS swop, so this has been a learning curve for me but I’m learning thanks

[nsb29]

I have another question, What c/c switches do I use to activate the cruise on off set, resume?

[kipps]

What truck are you working with? I found this diagram for the '84+ four-wire cruise switch. I'm soldering up a small padboard with a couple of resistors to adapt the '84+ switch to the BCM. Haven't done it yet, though.

[nsb29]

My truck is the 65 with the stock steering column

[mongocanfly]

The ecm doesn't have anything to do with lighting..at least not on mine..mine are 02-03 engines

[nsb29]

Originally Posted by mongocanfly

The ecm doesn't have anything to do with lighting..at least not on mine..mine are 02-03 engines

Kipp brought it up in post 16 I have also read this in other posts where people were having a problem with torque converter lock up using LED lighting, at the moment I don’t plan on using LEDs. He also brought up that the ECM may want to see grounding when the brake pedal wasn’t pressed. Just figured if I’m trying to get my head around all this I might as well ask all my questions at once lol

[ls1nova71]

It's the same thing as cruise control on GenIIIs, if you run LEDs the cruise wont set. The PCM needs to see the ground path through the brake lights, the 3rd brake light on factory applications. As long as you have at least one brake light or a relay to provide that, it will work. It "sees" the ground through the filament in the brake light as long as its hooked to the normally open side of the brake switch. How it actually works is really irrelevant as long as you know it needs to have it.

[mongocanfly]

I dont have torque converter or cruise control....guess that's why it's not a issue on mine...but I didnt know the ecm had anything to do with running leds...

[ls1nova71]

Originally Posted by mongocanfly

..but I didnt know the ecm had anything to do with running leds...

It doesnt really have anything to do with running LEDs, but just wont function correctly with LED taillights.

[nsb29]

Well I want to thank all of you for your help, next is to gather up the parts I need and see if I can stick it all together and of course have it work Hope y’all had a good Christmas

[nsb29]

Well I thought I was out of questions, my son brought up the fact that with some manufacturers you need the BCM that went to the donor car or have the replacement recalibrated. Hopefully that only pertain to the Rice grinders he works on lol

[kipps]

Originally Posted by nsb29

Well I thought I was out of questions, my son brought up the fact that with some manufacturers you need the BCM that went to the donor car or have the replacement recalibrated. Hopefully that only pertain to the Rice grinders he works on lol

That's been a bit of an unknown for me too. I'm starting out with a truck engine and ECM. I'm reflashing the truck ECM to a Camaro tune for more guaranteed manual transmission compatibility. To avoid any chances of trouble with the BCM/ECM mismatch, I purchased a manual trans Camaro BCM, and will reflash my ECM to the same VIN. This should give me a matched set.

I've heard of some mismatching of the the BCM being okay. Since you will be deleting VATS, the BCM doesn't need to be a perfect match with the ECM to allow the vehicle to start. Also, you're likely not even running the start circuit through the BCM anyway. I'm not.

Some folks who've used the BCM for cruise, charging, and other simple stuff, say there's no problem using a mismatched BCM for the purpose. I think they all grabbed one from the same type of vehicle, though. I figured it was hoping a bit much to get a camaro ECM and a truck BCM to work together, which is why I wanted that matched set.

When I get things running, I may try tossing the truck BCM into the mix, and see if it works. I won't know for another month or two, though.

Edit to add: all that is a long way to say, that it's definitely best if you can find the BCM from your engine donor. Failing that, pull the BCM from a similar year and similiarly equipped G8. If, once everything is assembled, and the BCM and ECM don't like each other, have a dealer do a ECM reflash to match the BCM VIN. It appears the ECM's are more cross-compatible when reflashing then the BCM's are.