Craftsman 1/2' impact gun

Have you tried oiling the unit. Sometimes putting a bit of 3 in 1 oil or wd 40 in the air intake will loosen the parts so they work correctly.

If you are trying to loosen or tighten a bolt or nut the the impact just stops more than likely there is a problem in the hammer (clutch) mechanism (items# 24 through 30), there could be broken part causing the mechanism to jam, the hammer (clutch) needs to hit and then slip to let the motor turn as they are directly coupled (no gears). On the flip side in the motor there are vanes (blades)
(item# 17) that need to have free movement and could also cause a condition were the motor could have low power or cause a locked condition.

Follow this link for the owner/parts manual:
http://www.searspartsdirect.com/partsdirect/part-model/Craftsman-Parts/Craftsman-Parts/user-manuals/manual-downloads/Model-875198641/0247/0755200?modelNumber=875198641&diagramPageId=&documentId=P0710197

Most I.R. impacts require grease in the hammer mechanism (clutch) and oil in the air motor.
To lubricate the air motor simply add oil through the air inlet about 3 to 6 drops at every use.
To lubricate the hammer (clutch) mechanism you will need a grease gun with a needle, the grease fitting (item# 17) is located on the hammer case assembly (item# 15) 2 or 3 small squirts will sufficiently lube the hammer (clutch) mechanism.
With heavy use grease the tool once a month, with light use 2 to 3 times a year.
Follow this link for the parts list, also indicates the grease needed:
http://manualarchive.ingersollrandproducts.com/ir_pdfs/Tools/Vehicle%20Service%20Tools/Air%20Impact%20Tools/80164726_ed4.pdf

This is were you add oil to the hammer mechanism, to lube the motor just simply add it through the air inlet.

If you are just oiling the gun,hold it upside down and squirt some oil ine the air chuck fitting.(I just use an empty bottle of my wife's hair dye filled with ATF-it's cheaper than a can of "air tool oil".If you are referring to the grease fitting for the impact mechanism,look for a small hole with a captured metal ball.The only tool I found to fill this is a specially designed miniature grease gun(got mine at Sears).

Just put 3-5 drops of air tool oil down the air inlet coupler and then run the gun for 5 seconds. Do this everyday that you run the gun for best results.

I just bought one of these also. Manual says oil goes into "the air inlet" which I'm thinking is where you attach the hose. That way the oil is sprayed throughout the interior of the gun once you run air through it.
Question for you: I can't get an impact socket onto this gun, almost seems like the square hole is too small. (yes, both are 1/2" drive). How do you get a socket on this stupid thing?
(man that sounds like I'm an idiot, but I've spent a half hour leaning all my body wight, wiggling, etc. and can't get a damn socket on)

It needs to be taken apart and cleaned then lube it with air tool oil and reassemble it should make a big difference

Make sure that you have at least 50 psig of air pressure before you attempt to troubleshoot your impact. 70 is better, 90 is perfect. Impact wrenches are powered by internal sliding vanes, which can sometimes stick. There are a couple of ways to get it going again. Try 1 before going to 2, etc,:
1. Spray W-D 40 or a similar oil into the impact, while you are holding the trigger down. Give it a good shot. Connect the hose and pull the trigger. Tap the sides of the gun with a plastic hammer to help free up the vanes. Repeat. Normally, this is enough to get you back running.
2. Place it in a bucket and cover the gun with W-D 40. Work the anvil end and trigger to work the oil deep into the gun. Ley it soak 1 hour, and repeat the process, working the oil in, and the air out. Remove it from the oil, and drain the oil back into the bucket by holding the trigger down. Hook up the air, and blow it out.
3. Remove the cover on the head that covers the vanes. Make sure that the vanes will slide freely, and that all gaskets and "O" rings are in good condition. If you note any dry areas in the inner parts of the impact, lubricate with W-D 40, and reassemble.
4. I'm all out of ideas.. If that didn't fix it, I'd go get a new one.
Best of luck, --W/D--

Although I do not have a Craftsman, I have taken severl othe brands apart. They all seem to be much the same. You do not need specialized tool. You just have to be mechanically savey. I think the grease has thickened up too much so taking it apart and cleaning it should do the trick. Start at the working end end and separate the front end. Work on a clean bench where little items (screws, ball bearings etc) won't roll off and get lost. When the two parts are separated, you should see the hammer mechanizm and a lot of grease. remove and clean everything that you can. Re-lube the mechanizm with a light lube grease and re-assemble. Turn the socket drive by hand to make sure everything is free, then apply a liberal application of air tool oil to the air port of the gun and attach an air line and give it a try . Put a load on the socket to check for impact functioning properly. I hope this helps