I have a 98 ram 1500 with the 5.9l v8. AC hasn't worked since I bought the truck a couple years ago, I figure it's about time I get it fixed! Any ideas on where to start as far as diagnosing the issue?

 

Most likely no freon in the system due to a leak. If you have freon then it may be to low for the compressor won't come on.

 

+1
start the engine, turn on the a/c system. open the hood and see if the compressor clutch is engaging...if not...verify the charge level.
-if the compressor is turning on....let it run for 4 minutes and feel the lines at the accumulator... the small hose is the high pressure side and will be hot/warm...the large diameter hose is the low side line and should be cold. if they are both warm...check the charge level.

 

If your AC hasn't worked in a couple of years, I would hook a gauge set to it first and see if there is any freon in the system.
If not, I would pull a vacuum on it for at least an hour, close the gauges and see if it holds a vacuum for at least 30 minutes.
Then fill it with freon or at least enough to have 50 to 75 psi in the system, then try starting the truck and see if the compressor
clutch will kick in. if it does then I would continue to add freon until you have put in the amount listed on the under hood sticker
or follow this charging chart.



If the compressor clutch doesn't engage then I would start checking the AC fuses and relay.
ALso I would recommend that you wash out the AC condenser.

 

If it's out of refrigerant you can jump the low pressure switch just to make sure the compressor will kick on, don't let it run for more than a few seconds. You don't want to vaccum the system and add refrigerant just to find out the compressor isn't working.

 

A bad low pressure switch is also a possibility... when they get old the plastic insert cracks.