If the diff lock is engaged while the engine is running, neither the TC or ABS are effected. If you turn off the engine with the diff lock engaged, the ABS and TC lights will light up when it is restarted, and they will work at reduced efficiency.

You may need to move the vehicle a few feet forwards or backwards after engaging the CDL for the light to come on, and the same holds true when the CDL is disengaged.

If you can find a 2004 CDL shifter, it is the easiest to install (not having to mess with bending linkages and such; just connect a few cables, but is more expensive than the D1 linkage.

 

Mully J,
From my research the switch light to your instrument cluster is not hooked up. Fish around above your transfer case. See if you can find a black plug zip tied to your other wires. Very near the Hi/Lo swicht wire on the transfer case. I have one like this on my 2003. I am about to install the CDL front output shaft. You may have a switch/wire plug installed in your transfer case that should display on your instrument cluster in the top left corner. The switch could also be not installed. But I bet there is a wire for it is present from the wiring harness. Check out Joshes writeup, the pictures will give you clearer picture of what is going on.

http://www.landroverclubvi.com/2003cdlinstall.html

Scroll down in this link till you find this heading.

TURNING ON DIFF LOCK INDICATOR LIGHT on 2003 Discovery:
1. If you were unfortunate to not have the diff-lock switch harness zip tied somewhere near your transmission or tcase
and you have to cut and splice to get your diff light on, then read on.
2. Remove your center console.
3. Once removed, find connector C0681 and disconnect it from C0668

 

Well, I did some digging and when I got to the warning lamp switch on top of the TC it was kind of coroded and when I pulled the lead off of one side it just pulled the entire flat connector off. The diff lock mechanism looks to be all mechanical? If so then the switch is the problem. I cleaned it up and put some test leads on it and was able to get the dash light to come on so the switch and corresponding system seem good. I am going to try to put the switch back in and see which way the shaft needs to be turned for diff lock with the test leads. I am hoping Josh is wrong about the counterclockwise from the top to lock because that is where the shaft was when I first began, yikes. We'll see?

 

I am now more confused. I tested the diff lock lamp switch and it is working out of the vehicle. When I put it back in the TC, no go even with the test leads. Took it back out and turned the diff lock shaft. I can see through the switch hole that the selector fork is actuating (clockwise as Josh said) and should be activating the switch but it is not. Took the spacer nut off to seat the switch deeper and still nothing. The switch is pretty sensative and you only need to barely bump it to light the lamp by hand. Any ideas?

 

After turning the diff lock nipple, are you moving the truck? My experience has been that after I engage the diff lock I usually have to move the truck a little before it (the dash light) comes on...

 

I'm not sure why this thing took so long to lock in, but that was the problem. I finally had to do the back and forth in a str8 line several times to get this thing to lock in. once it did I could see that the selector fork covered the entire switch hole. Put the switch back in and all was good. I ended up using a flux core lead solder to fix a new flat electrical plug onto the switch. Originally I thought that the diff had been locked but it was not, it was in the center position. It's a lot easier to see whats going on from the top. What is the full CCW position for? Diff lock is full CW and the truck was in the center position when I started, is that right? Josh had his diff lock shaft parallel with the driveline in his pic in the link and showed full CW as locked.