I have discovery and need to replace the ac evaporator.how do I start?

the evaporator is the ac cooling part that is in the dash.if it starts to leak the only solution is to replace the evaporator. this requires dash removal. adding any sealer will ultimately cause problems with compressor valves and expansion/orfice tubes. [ don't do it...]

first get a new ac specialist to work on your vehicle simply because a specialist will check for leaks first and not waste your time gas or money
next , ac evaporators are generic-that is made by specialist shops- to fit a wide range of vehicles and to be charged at ridiculous prices simply because it comes in a genuine parts box
so by talking with an ac specialist shop they will have evaporators there that will fit straight in for a lot less than "genuine "
they also know how to get the dash assembly out quickly to carry out the repair ( assuming that it is not just a loose connection)

there most likely is no need to replace the oil as oil travels with the gas
it is normally put in through the compressor but the amount is critical as excessive oil will block the TX valve
now that you have replaced the evaporator, take it to an auto ac specialist shop , have the system evacuated right down , any oil required to be added and the correct amount of gas to be installed

On what? Front or rear? You will have to evacuate the AC system using a recovery machine. Most evaporator cores are labor intensive involving the entire dash assembly of the vehicle to be removed for access/replacement. Not a job for anyone who does not have mechanical experience in Ac repairs

The Expansion valve is located in the evaporator box

On this year Nissan it's not to bad of a job. The dash doesn't need to be removed like the newer cars.

1. To replace the expansion valve, you will need to discharge the AC system. Remove the two AC lines from the evaporator.
2. Then you will need to remove the glove box
3. Then you will need to unbolt the evaporator box
4. Tip it back and remove.
5. Once you have the evaporator box removed, then you can replace the expansion valve. I would also clean the evaporator box as well since you will have it removed.

To check it you need to pressurize the system run the compressor and then you can spray soapy water around it and see if you have a leak If you decide to replace the evap core for the ac be sure to replace the dryer as soon as you open the lines it will start to draw moisture and then the new system will fail or not put out cool air correctly. Unless you absolutely have to change the evap for the ac I suggest you leave it alone unless you have done this before or have someone to work with that know ac.

I would think that it is the ac evaporator unit and condensate in the ac box that is unable to get out the drain pipe from a blockage . It could be ice melting off the ac unit fins . check that the ac is operating properly

The serpentine belt routing diagram for a 1998 Land Rover Discovery 4.0 liters with A/C


CS = Crankshaft
F = Fan pulley
G = Guide pulley
Gen = Alternator
HP = Hydraulic Pump
T = Tensioner
AC = Air Conditioning
PAS = Power Assisted Steering

Separate electric fans are used for the A/C Check the fans

That is a AC compressor problem.
It may be no compressor oil in that compressor