Cadillac Repair: No 4th gear, electrical schematics, torque converter

Question
My 99 Sedan De Ville with 85000 miles no longer will shift into 4th gear.  A local shop says no trouble codes have been set and that "no electronic problems exist"  He is reluctant to go further without any good ideas of what could possibly be the problem.  Any Ideas?

Answer
Could be a faulty:

Torque converter release switch located beside the brake switch.

Faulty torque converter solenoid within the transmission.

I can provide you with electrical schematics and picture locations and test procedures of the suspect components if you e-mail me at: onlineautohelp@msn.com