Problem:
The problem I have is that I wanted to have more read outs from the car's computer in my line of view when I'm driving and specifically when I am logging data to re-tune the computer. As a inexpensive solution to this want/need, I picked up a ScanGauge II.
One issue I have is the car only has one ODBII Diagnostic port. Options are:

1. Splice in a 2nd ODBII port (I don't want to cut any wiring in the car, this is out).
2. Use an ODBII Splitter (this ads a bunch of extra cabling down at the foot well, annoying and possible driving hazard, this is out).
3. Make use of the port to daisy chain other ScanGauges together. That's what I did.

Research:
During this process, I learned that the ODBII system has a few different standards and it dictates how the pin outs are configured in your ODBII diagnostic port. Below are a couple examples (these are not all the configurations out there). From what I understand, most GMs use VPW (the '00-'05 Monte at least does).


Below is the shop book information about the port in our Montes (and my Grand Am I used to test with).


Based on the above information, I pulled an ODBII connector from a '98 Regal. Using that connector, ethernet cable and a volt meter, I did some testing with the ScanGauge II. I exposed all the ends of the ODBII connector and one side of the ethernet cable (this way I could test the various leads with the volt meter). Even though the above information shows I only need power, ground and data, the Regal connector used an extra PIN14, I traced it out anyways, figured why not.

Note: During this test, I found some of the ODBII pins pass-thru the ScanGauge and land on multiple ethernet pins. When this happened, all but one ethernet pin gave a result on the volt meter indicating it was NOT a straight pass-thru. As a result, I was only interested in pins that were straight/uninterrupted pass-thrus.

Below is a chart that I made with the results. ODBII port pin #14 for VPW is irrelevant, but I documented it anyways. You can see, both grounds come out the same side of the ethernet cable. Also, Ethernet has two main standards, T568B and T568A. Most cables are crimped with the B color pattern (and that is what I was testing with).



I twisted the various leads together, protected with electrical tape as seen below. Ran a test in my Grand Am with this cable and my ELM ODBII BlueTooth adapter and laptop. As planned, the cable does pass the ODBII pins to the salvage ODBII connector and everything appears to work perfectly.

 

Building the ODBII Pass-Thru Cable:
This chart below is going to be the important part to follow. As mentioned in the first post, if you don't have Pin #14, it's not important. I included it since I had it (who knows, maybe I'll have a future need for it, doubtful, but maybe).


Based on my testing both ODBII ground leads end up in the same end spot. Since the wire in the ethernet cable is very thing to work with, I wanted to reduce the connected weight involved.
In this case, one of the ground wires was thicker then the other. I decided to carry the thicker one to the ethernet cable.
Using a pair of wire strippers, I pulled the sheathing on the thick wire forward. I poked between the standards on the exposed area and tied the other ground wire in (making a good mechanical connection) and then dabbed it with some solder. Finished it with a piece of heat shrink tubing. Doing this avoided a break in the wire and avoided making three connections (in this method, it was only one connection).



Using my chart above, I twisted the various pins together and dabbed them with solder and also covered them with heat shrink tubing (make sure the tubing is in position before joining the wires). The heat shrink tubing helps keep the connections from flexing apart.



I put a small wrap of electrical tap over everything to help support the wiring. I then pulled a piece of red heat shrink tubing I put in place over all the connections and finished it off. The final product can be seen below.
Since the Monte is parked for the winter, I tested again with my Grand Am. This time I used the DHP PowrTuner and ran UV Scan to see if it was able to log data, passing through the ScanGauge II. As planned, this worked perfectly.


Note #1: Knowing all of this, since I am using this with a DHP PowrTuner, I could go one step further. The ODBII cable on the PowrTuner connects with a DB15 connection to the actual tuner. I could fabricate a ScanGauge II ethernet-to-DB15 cable in the future.
Note #2: I chose to use an ODBII connector because it's cheap and available at any junk yard (plus I wanted a proof of concept before I even consider note #1). This also gives an additional benefit, this being I can connect any ODBII tool to the car via that Pass-Thru cable (I can use the ELM Bluetooth unit or the SnapOn Scan Tool I've used many times). If it connects to the car with ODBII to read data, this allows me to still have the ScanGauge II in place and not mess with the connector under the dash.
Note #3: I will not re-flash the PCM using this passthru. I have my concerns about the data stream the DHP has to push going through the ScanGauge II (could it mess up the ScanGauge, brick a PCM, I don't know, don't want to find out).

I know people who may benefit from this information is limited, but I hope it helps someone else out there

 

Here is a short video I posted on YouTube testing this cable. You can see everything working with my ScanGauge II, DHP PowrTuner and my laptop.