http://www.troublecodes.net/GM/92-93_33N_mfi.shtml
26 Quad-driver sense voltage shows a low voltage when battery voltage should be present or if the sense voltage shows battery voltage when a low voltage should be present.
27 3rd gear switch was closed or grounded for 10 seconds when vehicle was in 3rd gear, or drive gear was open when engine was first started.
have your alternator checked, check your battery with a load tester, my guess is you have something going on with your charging system and the car missed a shift sometime. Just guesses, but now you know what the codes mean.