...when you convert to a Series 2 drivetrain in your pre-1996 vehicle. Keep in mind this is *mostly* accurate, but still could be missing a few elements. I will add those elements with time.

First, let us start with what exactly will be needed to do this sort of thing. Hopefully, you already have the Series 2 engine/PCM/firewall harness that came with your engine or engine/transmission combo, as well as the PCM that is matched to that combo (or just the engine if that is all that is being swapped). Since swaps like these are more or less permanent, ripping apart the original vehicles firewall harness shouldn't be an issue either, as it too will be needed. Soldering iron and solder, heat shrink tubing, various gauge butt connectors, spade connectors, vampire taps (Scotch-loks), zip ties, and convoluted tubing should round out the small stuff needed. As far as tools go, you should have a DMM (digital multimeter), a good quality crimp tool, heat gun (for the heat shrink tubing, but you can use the soldering iron or even a lighter in a pinch), wire stripper, and perhaps the most important tool of all...factory service manuals for the vehicle being worked on AND pinouts/schematics relevant to the new powertrain.

For most, keeping the OBD2 PCM on the outside of the cabin is the preferred method, since in this regard, everything will transfer over with little lost in translation. However, the bulkhead connector for the OBD2 harness will NOT mate up to the older vehicles bulkhead. The solution is to either obtain a same-vehicle harness, or use the original harness, bulkhead connector. However, keep the wires and harness leads of UNAFFECTED systems intact. This includes the following circuits:
Coolant temperature
Oil pressure
Variable steering pressure
Oil level sender circuits
Boost/vac signal (this is done through a separate harness, not through the bulkhead, but still needs to be kept)

Label and cut the other wires (leave about 10 or so inches of wire for plenty of grafting area) After that is done, label and cut the OBD2 harness in the same manor (the head on this harness is of no use to us).
You will notice that some wires exist in the new harness that have no place in the old head. These following circuits can be left out of the wiring phase, and can either be cut out of the harness entirely, or be taped back.
Traction control
Oil level indicator (this is handled through a separate system on older vehicles)
Fuel pump pulse width
Coolant temp signal (See the list for kept circuits on the old bulkhead connector)
VATS (this can be disabled through the use of a tuner, and is not compatible with older vehicle VATS systems anyways)
Oil change reset
Charge indication circuit
Boost gage signal (See the list for kept circuits on the old bulkhead connector). Even if your vehicle has no boost gage, you are better off installing a mechanical one.
Anything cruise control (this should stay outside the harness, taped back for later upgrade)
Magnasteer
Fuel tank pressure
Starter solenoid (The starting systems are different between Series 2 and pre-Series 2...more on that later)

I'm sure there are a few others, but it's stuff that simply isn't needed in older vehicles. We can now see that there are just a few wires that DO need to go through that bulkhead connection, mainly power and grounds, but AC and fuel pump are among the others (and the fuel pump enable circuit must be carried by one of the defunct circuits that go from the bulkhead to the old PCM inside the cabin). Since these wires will be exposed to the elements, the most ideal method of joining them together is solder and heatshrink. Save the buttsplicing and/or vampire taps for interior use. Also, now would be a good time to look over the OBD2 harness to see if there are any bare or damaged wires. Knock sensor connections tend to get oil-soaked or heat damaged, so pay close attention to those.

PCM Preparation
Few people think about having to prepare the new PCM for use in their vehicle, but it's a very key step in getting it all to work right. ************** But don't panic...there are several options you can consider. The easiest one for most people would be to send out the OBD2 PCM to an outside vendor such as INTENSE. They have the necessary equipment to turn off the VATS for good, however this is an option that you may also want other options programmed in while they have it. The next option is to have a friend with a PCM tuner compatible with your PCM pay you a visit. The last option would be to obtain the BCM or VATS module that is matched to the OBD2 PCM. This, of course, is not the most ideal option, as it would entail much more wiring to be done...and who needs that? What all this boils down to though is that in some form or fashion, the VATS system MUST be dealt with before the PCM can be used to control the new transplant.

Wiring Up the Accessories
OBD2 vehicles use a different AC system than previous models. Prior to 1996, the AC compressor used was an HR6 design, which went relatively unchanged throughout its long life. 1996 and up models saw the use of a new compressor, the V5, which is a variable-displacement design. These compressors used only one electrical sensor...a pressure sensor located on the output hose behind the compressor. Earlier models used switches in various places...in other words, this one sensor can do the job of all the switches the old system used. This is where it will pay to study the AC diagrams of the vehicle AND the new powertrain...so that you can eliminate which wires will be left after this conversion is all said and done.

Remember that story about the starting system being different? This is the case where you WANT to keep the older starting system intact. OBD2 systems used a new style park/neutral switch that did away with the starter solenoid feed wire...instead, the body control module (BCM) handled that function. Again, study the wiring diagrams as needed....the new wiring harness will have to have the old connections to the park/neutral switch grafted onto it, and the starter solenoid deleted from the new harness.

For those of you that have a vehicle equipped with T2V, or variable-assist power steering, pre-Magnasteer era...you WILL lose this function. Older PCMs had two outputs for speed...a 4k signal that went to cruise control, speedometer, and HUD, and a 2k signal for the chime module, which dinged at you for various things AND handled when the T2V would kick on and off. Since OBD2 computers only have a 4k signal, T2V will become inactive, and will stay in the "firm" setting. For some, this is a non-issue, but for others it might be one. The simplest way around that is to wire in a switch to control the T2V solenoid manually...though, from my experience, the firm setting isn't too bad at low speeds.

Cruise Control - The older vaccuum-style setup will not work at all with OBD2, so the fully-electronic cruise module that resides where the vaccuum tank would be on the firewall will need to be procured. However, you will need *some* of the wires that run from the old cruise module...like the "coast", "set", and "resume" command wires. The new PCM will also have to tap into the brake switch for cruise interrupt.

Performance shift option - Even if your vehicle does not have this option installed, you can easily add it to the new powertrain. However, you will need a resistor and a switch to do so. (I will post more on this when I have time to research the wiring diagrams). Based on several posts on this forum, the function may or may not need to be turned on via a tuner as well.

_________________
2005 GXP - White Gold Pearl, no mods...yet.
2000 SSEi - Resurrection in progress. Built L67 w/L32 fuel rail, ported heads, and cam. Camaro front brake system, GXP cluster, and much more in planning.