I have a 10 year old Trane XL 80 that leaks from the metal box above the heater while running the air conditioner. I haven't been able to find exploded diagrams or service or installation manuals for troubleshooting purposes. A friend of mine said he basically had the same problem and that it was a clogged filter in the condensation line and a contractor helped him over the phone. Any ideas? I'd like to fix it myself if possible.

Leaks water or cold air ?

It looks like the word "water" accidentally got deleted when it limited my number of characters. Sorry about that.

Leaking water form an A coil can be a drip collection drain pan is clogged, or at least its drain line. Do you see the drain line coming out of the plunum? Casn you get at the pan under the coil?

I see the drain line coming laterally out of the plenum (white pvc to a "T"). The bottom of the "T" goes down under the house and then out to an outside wall. The top of the "T" goes up a few inches and is open to air. I don't think I can get to anything under the evaporator coil since is inside a metal box and the heater unit is directly under and attached to it.

Try putting a sjop vac on that open end of the T. It should pull some of the junk out, it would be better if you could plug the other end that goes under the house. Do you have a humidifier attached to the plenum

No humidifier.

Thanks

If the vac doesn't pull the junk you may need to cut yourself an opening in the plenum to clean out the pan. The humidifier would have been a nice existing opening

OK