A/C w201
 

Need help fixing an a/c problem. I can see the compressor turn on, but ac is blowing out hot air.

Sure your mono valve isn't stuck open? I've had a ton of W201s where the heat was stuck on.

First, check the water valve behind the battery and make sure it's in the closed position. With the compressor running, feel the suction line on the drivers side, is it cold?
You really need to check the hi/low pressures.

Suction line is cold. I did check the pressure with my charging unit (the little attachment to add the refrigerant) and it seemed low. The valve behind the battery is probably functioning, but i removed the aux water pump and installed a manual ball valve in place so i know that no coolant is getting to the heater core because the valve is closed.

Tested the ac last night and it was working. Blew out cold air, but maybe for only 5-8 mins then it stopped working again. This is the case actually, it's not that it doesn't work, it will only work for the first few minutes after turning it on then it stops.

do not simply rely on the little guage provided by the can, the only way to understand what the system is doing is looking at both liquid and vapour pressures. The liquid pressure is pretty important when working with a TXV system.

For starters - tap the heater valve with a hammer and then retest, set the heat all the way and then tap the valve, Do that once in cold setting too.

When it stops blowing out cold, see if the compressor cuts off. When mine did this, the aux fans weren't turning on. Turned to be a blown fuse.

I found the problem by putting my gauges on and watching the high pressure. When it climbed up, the aux fans failed to turn on to lower the pressure, so the high pressure switch at the drier/receiver cut the compressor off. Replaced the aux fan fuse and no problems since.

I have manual fan control for both high and low speeds, the compressor remains on when the ac blows out cold sore. I'm not sure what i will be looking for when tapping on any component so that test has not been done.

When the ac stops blowing out cold air, is the compressor still engaged?

Yes, compressor is still engaged when the ac stops blowing cold air

Drove home today from work and ac remained on for the entire drive home. I made an interesting observation, when i am driving home from work it's a lot cooler maybe 70°F but throughout the day it's about 85°F+.

w201 a/c: RESOLVED
 

Evacuated the system and recharged to ~2.2 pounds of r134a. System is now back up and running with cold air

Good work... 2.2lbs? I think you may be overcharging a bit...

w201 a/c: RESOLVED
 

I don't have the sticker on my radiator bracket for the capacity, but I do remember it at 2.2 pounds and I also confirmed it with a search. Here is the post by whunter: http://www.peachparts.com/shopforum/general-information/303136-refrigerant-capacity-w201-earlier-chassis.html