ACR MLG ALTDEV OVERSHOT DURING GAR CAUSED BY A FALSE GEAR DOOR WARNING.

Narrative

ON FINAL APCH TO 17R AT OKC THE FO WAS FLYING THE ACFT AND I WAS PERFORMING THE PNF DUTIES. AT 1500 FT AGL THE LNDG GEAR WAS SELECTED DOWN AND SUBSEQUENTLY SHOWED '3 GREEN' DOWN AND LOCKED. AT APPROX 1200 FT AGL THE FLT WARNING SYS ANNUNCIATED THAT A GEAR DOOR (L MAIN) WAS NOT UP AND LOCKED. THE ALERT DID NOT RESET SO A GAR WAS INITIATED BUT I TOLD THE FO THAT WE WOULD LEAVE THE GEAR DOWN WHILE WE INVESTIGATED THE PROBLEM. WE NOTIFIED THE TWR THAT WE WERE MAKING A GAR AND WERE ASSIGNED A TURN 90 DEGS TO THE R AND A CLB TO 3000 FT MSL. THE FO CONTINUED TO FLY THE ACFT AND AFTER FLAP RETRACTION AND CLB WAS ESTABLISHED; I REACHED DOWN TO GET OUT THE FLT MANUAL TO CONSULT THE PROPER PROC. WHEN I LOOKED BACK UP; THE ACFT WAS AT 3400 FT AND CLBING. I CALLED OUT THE ALT ERROR TO THE FO AND HE BEGAN TO LEVEL OFF AND DSND BACK TO 3000 FT MSL. THE ALT ALERT DID NOT ACTIVATE BECAUSE THE GEAR WAS DOWN AND OUR MLG TYPE SILENCE THE ALT ALERTS IN THIS CONFIGN SO AS TO PREVENT NUISANCE WARNINGS ON FINAL APCH. WHILE ON DOWNWIND RETURNING TO LAND THE GEAR DOOR FAULT CLRED ITSELF WITHOUT ANY PLT ACTION. IN THE FUTURE BECAUSE OF THE HIGH WORKLOAD SITUATION THAT EXISTED; I THINK IT WOULD BE WISE FOR THE PNF TO STAY IN THE 'LOOP' UNTIL THE AIRPLANE HAS REACHED A STEADY HDG AND ALT.