I spent the last two weeks working strictly on making final decisions about the wiring. After months of consideration, this is what I came up with. It's probably more complicated than it needs to be, but since this is being taken from a donor vehicle, the complexity made it simpler in the long run, if that makes any sense.
The Mark VIII powertrain electronics are broken up into three main harnesses:
1. Engine bay harness. Contains the cruise control module, variable load control module (VLCM, controls the AC compressor and cooling fan), AC pressure switch connectors, engine bay fuse box, starter solenoid connector, engine harness connection, IP harness connection, and PCM harness connection.
2. Instrument panel harness. Contains HVAC controls, transmission connections, instrument panel fuse box, OBD2 port, ignition switch, indicator lamps, cruise control switches, PCM harness connection, engine bay harness connection.
3. PCM harness. This harness contains a firewall grommet that allows it to connect to both the IP harness and engine bay harness. It also, of course, has a large connector for the PCM.
Starting with the engine bay harness, this one probably had the most stuff removed from it. It used to go through the firewall in two places on the Mark VIII in order to connect to the IP harness - both the driver and passenger sides. After trimming, the only thing going through the passenger side grommet was an AC pressure switch wire. This was easy to relocate to the driver side end of the harness. That end, however did not use a grommet. It used a very large connector that actually mated to the firewall by design. I wasn't about to cut a hole that big into the Volvo firewall, so I found two connectors from the Mark VIII that were no longer needed. I took the leftover grommet from the passenger side, fed all of the wires through it, and soldered each wire to a flying lead on the recycled connectors. Same thing was done on the IP harness. Here's a picture of the large connector that was removed next to the finished product.
I then made the hole where the heater core hoses used to go a little bigger to fit the grommet. The end result will look pretty good, but it won't matter because the engine will be blocking it! You can see the PCM harness in that picture, too. More on that later.
The instrument panel harness received the same connector mod that the engine bay harness did. A number of minor and major modifications happened with this one. The mods were done in a way that does not have this harness becoming one with the Volvo harness. Connectors were used in all places where the harnesses had to interact.
First, four ignition switch circuits had either to be spliced in with the Volvo wiring or replace the Volvo wiring all together in some cases. Those cases were the hot at all times power coming from the fuse box, and the starter solenoid relay wire. It made sense from a safety standpoint to get the Volvo ignition switch on fused power from the Mark VIII fuse box, so I cut that wire from the Volvo switch and soldered in the appropriate wire from the original Mark VIII fuse box. Looking back, I probably could have just put that Volvo wire on the Mark VIII fuse box and only cut three wires, but I didn't. Oh well. It'll still work. Starter solenoid relay wire was necessary, though. The two splices were for getting connections to the "hot in acc and on" and "hot in on and start" parts of the switch.
The instrument panel fuse box was removed and replaced with six inline mini fuse holders, since only six fuses remained after all of the trimming.
Next, I used a mini mate-n-lok connector for the brake switch. The brake switch on the Volvo was re-wired to get its power from the Mark VIII fuse box, and return power was spliced in to the Mark VIII harness. Turns out the PCM wants to know when the car's brakes are being applied. The cruise control module also needs to know about this.
Two more mini mate-n-lok connectors were used for the dash indicator lights. These include check engine, oil pressure, battery, and cruise control. The Volvo doesn't have a light for cruise control, so I wired it into work with the overdrive light. If I understand the Mark VIII wiring manual correctly, the cruise control module actually needs its indicator lamp to work. The ground for that indicator light is controlled within the instrument panel. It will only give the cruise indicator light ground if the traction control and ABS lights are not turned on. Maybe all I needed to do was ground that wire.
Next big mod was adding connections for the fuel pump driver module (FPDM) and inertia switch. This module and switch were not originally connected to the IP harness - they were located in the trunk of the car. However, all of those wires originated from the IP harness. I decided to mount the FPDM and inertia switch where the old Volvo ECU's were. They fit quite well in there with some new, longer wires on the IP harness. Also connecting in that general area is the fuel pump. I found the wires from the main fuel pump, removed them from the Volvo harness, soldered some longer wires to them and ran them to the FPDM under the carpet.
The strangest mod I did was in the interest of general safety. The Mark VIII ignition switch actually had two separate "hot at all times" wires going to it. It switched a lot more powertrain electronics than the Volvo switch does. One of those things is the coil driver. In order to reduce the amount of current going through the Volvo ignition switch, I added a relay that controls the coil driver using the same circuit that controls the PCM relay. This way, the coil driver gets power directly from the fuse box instead of having to travel through a switch and a bunch of connectors.
For the reverse lights, I actually cut one end of that connector right out of the Volvo harness and soldered it in to the Mark VIII harness at the reverse lights wire coming off of the transmission neutral safety switch.
The other wire that was changed off of the transmission electronics was the ground wire for the starter relay. It goes through the transmission so the car can only be started in part or neutral, but after that, it used to go to the SCIL module where it would control its ground. I simply grounded that wire since the SCIL is no longer in the car.
The most frustrating thing I found on the IP harness was the air conditioning controls. I want to run my AC compressor using the Mark VIII controls in order to get accurate fan control from the VLCM, but unfortunately, the PCM tells the VLCM to engage the compressor clutch when it gets a good signal from the HVAC unit, which is connected to the low side pressure switch. The HVAC unit reads this signal and sends a data signal to the PCM. Really? Why can't it just send on/off 12 volts from the pressure switch directly to the PCM? Ugh. So, basically, this module, with its now limited connections, will be tucked away somewhere in the passenger foot well. The idea is, if you want to run the AC compressor, reach under and push the "Max AC" button.
For the cruise control switches, I removed the switches from the steering wheel and simplified the connections down to just one since they used to go through the clockspring, etc. This is sloppy, but the idea is to have these switches live in the little storage cubby at the bottom of the cluster, and I can pull them out when I want to use cruise. Long-term idea would be to create a little push button panel that mimics the switches. That would look much cleaner.
The PCM harness is the only harness on here that will be completely unmodified. It's a very small and short, but it has a lot of wires going through it. I had to find a place in the car where it would fit as is. The driver foot well toward the outside of the car worked very well for this. I was able to cut a chunk out of the metal plate that goes in the corner, and it seemed to naturally slide right in. I cut a hole straight above for the firewall grommet and all is well.
I did a brief test to see if the car still got power after all of this, and it did. Hooking a battery up to the fuse box gave me full body controls and brake lights! I'll see if the PCM shows signs of life later on. I still need to triple check everything on the harnesses and tape them up so they look nicer.
After all this was done, I put the engine and trans back in to get my driveshaft length. It's at a driveshaft shop right now and I hope I measured correctly. It's getting closer to completion every day!