In the end it all worked. The blue cable across the middle of the picture is the heat shrink with the three wires coming from the switches to the relays.
There's very little room to work on the wires of the Transmission Control Unit (TCU). Unscrewing it off the side panel does give a bit of extra space, but not much. I used a test light to make sure I was cutting into the right wires. Bit tricky at 100km/h, but with some help from my son it all worked out in the end. I saw a tip since then, suggesting to stick sewing pins through the wires you want to test, and then using your test light on the pins. Wish I read that before I did this part of the project.
This is Nissan's numbering of the terminals on the GQ TCU. It was correct for our Trol.
The flashing LED was an annoying mistake I made. Buying it as an individual component wasn't a good idea, I then had to work out what size resistor to use so it wouldn't burn out.
Next comes the challenge of insulating it. Some insulation off other wires, and some heat shrink was the best I could come up with.
Opinions vary as the whether resistors should be covered in heat shrink. I figured it may overheat if covered, and then I'd have the frustration of replacing it. With leaving it uncovered, the biggest risk is that it might hit on something and blow a fuse. I attached two wires to the positive side of the LED to make it easy to connect it to the momentary foot switch and to the hold in relay.
This plug was already filling a hole in the dash. The LED fitted inside it just right, and I just drilled a hole for the LED to stick through it, then glued the LED into it with silicone.
When I went to take power from the brake light switch to run the cut out relay, I found a real mess. One side of the switch is permanently live, the other side is only live when you press the pedal. Our aftermarket cruise control takes power from both sides of the switch, and it's obviously had electric trailer brakes attached to it previously too. As I was trying to sort this mess out, I broke one of the wires off the factory connector. In the end the easiest solution was to remove the connector, pull the terminals out of it, and connect new wires onto them. While I was doing that I added some extra branches to the new wires to make it easy to connect the various accessories to them. My next electrical job on the Trol is to put an electric brake controller in it again, so having the wire there ready to go will make that a lot easier.
This is the connector rewired and ready to go back in.
The best momentary foot switch I could find was one designed for an electric guitar sound box. I hope it lasts, though it wont be too hard to replace if I have to, just resolder the wires and wrap it in heat shrink again.
The bracket for the foot switch is just screwed onto the strip which runs up the side of the foot rest. With the switch here I shouldn't accidentally hit it, and I just have to roll my foot forward over the foot rest a bit to click on it.
This shot shows what I see when the system is active.
I made a few mistakes along the way. Buying the wrong type of flashing LED was an annoying one, also I should've used more colours of wires. For example when I ran the three long wires across under the dash, I had to put power into each one on the drivers end to work out where they had to connect to on the relay box. If they were three different colours I would've known which was which straight away. Same with the wire running from the TCU to the lock up solenoid. It is grey with a red stripe, so after cutting it I used grey wire to extend both ends of it up to the relays. If these were different colours, it may have reminded me that only one of these now goes to the TCU, the other goes to the transmission's lock up solenoid.
When I ordered heat shrink I could've done better with the sizes. Most of it was okay, I had plenty of really thin stuff, and enough 6mm and 25mm, but didn't realise I'd need something in between.I was able to buy a short piece of 20mm in town, but a length of 15mm would've been good.
The biggest mistake I made may have been prevented if I'd used more wire colours. After getting all the other other wiring right, I connected the last two wires to the wrong place. They were both grey, and I forgot that one was coming from the TCU and the other was going to the transmission. I swapped them over and fed power from the relays into the TCU instead of into the transmission, and now it looks like I've wrecked that part of the TCU. It could've been a lot worse, if it'd damaged the torque converter clutch, the transmission would've had to come out to fix it, and I really don't need that. As is I just have to lock the converter manually all of the time, as the TCU can't keep it locked up, it keeps cutting in and out.
I've done a 1500km trip (Sydney and back, mostly on the Hume Fwy) towing a 6 x 8 trailer since I fitted my lock up system, and it worked really well.
Feel free to ask any questions. For general torque converter lock up stuff, there'll be plenty here who can help. For specifics about my system, I'm happy to explain in more detail if needed.
Time is never wasted when your wasted all the time 