1998 Discovery Series I, 4.0L with Auto Trans and A/C
The belt turns out to be easier than it seemed at first. I didn't really understand any of the postings I found here and I kept thinking I needed to unbolt the idler somehow to release the tension. For anyone also confused or not sure, you DO NOT release the tension by unbolting the idler. Rather you pull the idler pulley to the right which releases the tension on the belt. The idler pulley is preftty stiff so you need to be reasonably fit and have good leverage. Here's how I did it and when I finally did it the right way it was pretty easy. Just hard to figure out at first.
1) I removed the top fan shroud over the main fan by releasing the clips on the left and right side.
2) I removed the old belt by cutting it loose since I hadn't yet figured out how to release the idler tension.
3) I prepositioned the new belt around all the pulleys except the altenator. I used a spring clamp on the belt over the idler pulley to keep belt tight so it wouldn't fall out of place from the main pulley
4) I used my 3/8" drive ratchet and the correct socket to fit the bolt on the idler pulley. I extended the leverage on the ratchet (you need as much leverage as will fit between the idler and the A/C compressor) by using my short extension (about 3 inches long) and my small sparkplug socket. The sparkplug socket fit over the end of the ratchet handle perfectly and the 3 inch extension was all that would fit. It gave me just enough leverage to pull and hold the idler to the right hand limit.
5) While holding the idler pulley as described above I then slipped the last loop of the belt over the alternator pulley from bottom right up over the top of the alternator pulley to the left. At this point I released the idler and the job was done!