Transmission woes

I have a friend with a '93 Cadillac that has tranny problems. The car shifts fine from first to second but will not shift into third or OD. Anyone out there have any experience with this phenomenon? According to my friend, this is the only problem with the car and if it is something simple, he would like to fix it himself. My theory is that it is most likely an electrical component that has gone bad but I know little to nothing about GM trannies. Any help is greatly appreciated.

just replaced transmission in 1989 Cadillac Deville (440T4 same as 4T60, same trans as in the 93) I had the same problem. one way to check and possibly repair (about a $30 shot in the dark) is to change fluid and filter. If fluid is badly burned and smells burned (really black color) and the pan has a bunch of grease looking crap in the bottom, the transmission is burned up. If all is clean, the filter was just stopped up (should be changed every 12000 miles)