I’ve built a switched cable to jump the ABS relay to power bleed the brakes, does the truck need to be on or running in order to activate the ABS modulator?

 

Ignition off.

 

Thanks for the reply and the confirmation of my initial thoughts.

 

Does anyone have a write up on how to do this?

 

Here is the way that I did it and the results were pretty good. Mind you, it takes a while but you don't have to do it all at once (I didn't) Let me know if/what is unclear as I don't know how comfortable you are with bleeding brakes, I'm assuming you kind of are:
Step 1: Build your power brake bleeder cable. This is well worth your time you will save plus you can do the brake job by yourself. Make the cord long, like 15 feet so that it can reach from the fuse box all the way to the driver side rear wheel which is the furthest away.
Step 2: Open the fuse box, remove the ABS relay, and connect the cable to the 2 terminals which close when the relay is activated, I can't remember off hand which two they are but I think its the one turned 90 degrees to the other three and the one directly across from it. My relay had a diagram on it.
Step 3: Suck out the old brake fluid from the master cylinder and replace with fresh DOT 4 fluid. All throughout this procedure watch and top off the reservoir as you ABSOLUTELY don't want it to empty otherwise you will pump a huge air bubble in to the line and have to start all over.
Step 4: In turn, remove each wheel loosen the bleeder screw and activate the abs pump with your cable until the fluid runs clear. Close the bleeder screw while the pump is running to ensure no air sneaks back through. I stopped every 10 seconds or so to check my fluid level in the reservoir. There is some debate as to in which order to bleed the system. Traditionally you start with the wheel furthest away and move closer (RR,RL,FR,FL) but the Land Rover workshop manual calls for a different order (FR,FL,RR,RL). For what it's worth I used the land rover order and it worked well enough for me.
Step 5: With all of the brakes bled once go for a short drive, put your truck in low range and turn on the HDC. Accelerate up to 15 mph or so and let off the gas, you should hear the ABS buzzing away as the HDC system activates to slow you down (side note: you don't need to be on a hill for this to work, I did it in a parking lot). Repeat this 5 or six times to really give the pump a good chance to purge all the air from the HDC system.
Step 6: Redo step 4 and bleed all of the brakes again. This second bleed will push any air purged out of the HDC system out through the brake lines. Then you're done!

Bleeding the brakes twice kind of sucks but you have to bleed them first to push fresh fluid through one circuit in the ABS modulator then again after the HDC procedure to flush any old fluid/air out of a second circuit in the ABS modulator. Get 2 quarts of DOT 4 fluid, you end up flushing a lot of fluid through the system but I think it's 100% worth it because if you don't do the second flush and there is air or old fluid in the ABS modulator you kind of hobble your 4WD/HDC system. Let me know anything I can clarify.