A B737-500 WAS DISPATCHED IN NON COMPLIANCE WITH A KNOWN AUTOPLT MODE CTL FAILURE WITH NO MAINT ACTION OR DEFERRAL IN LOGBOOK.

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WHILE AT CRUISE ALT; JUST PRIOR TO DSCNT; WE HEARD A CLICKING NOISE COME FROM THE GLARE SHIELD; AS WE STARTED OUR DSCNT INTO ZZZ1 APPROX 80 NM W ZZZ1; FL200 DSNDING. WX CONDITIONS AT THE TIME WERE LEVEL 3; 4 AND 5 TSTMS; MODERATE TURB; HVY RAIN; AND SOME ICING AT THE HIGHER ALTS. THE MCP LOCKED UP. AUTOPLT KICKED OFF AND WOULD NOT ENGAGE. THE MCP SHOWED ALL '8'S' IN THE WINDOWS EXCEPT FOR THE ALT WINDOW WHICH READ 50000 FT AND THE ALT WARNING HORN GOING OFF EVERY FEW SECONDS. THE FO'S COURSE SELECTOR SEEMED TO BE OPERATING NORMALLY. WE CONTACTED DISPATCH AND TOLD THEM OF OUR SIT. THEY STATED THAT THEY WERE GOING TO TALK TO MAINT CTL ABOUT IT AND GET BACK TO US. WE CONTINUED TO TROUBLESHOOT. THE QRH STATES TO PULL THE AFDS MCP DC AND AFDS MCP CRS 2 CIRCUIT BREAKERS BUT WE WERE VERY BUSY FLYING THE AIRPLANE TO TRY AND LOCATE THE 10 OR 12 CIRCUIT BREAKERS ASSOCIATED WITH THE MCP. (IT WOULD NOT HAVE FIXED THE PROB ANYWAY; SEE BELOW.) ATC WAS ADVISED OF OUR SIT AS WELL. THEY GAVE US RADAR VECTORS AROUND MOST OF THE HVY WX FOR AN ILS TO RWY 23 ZZZ1. ONCE AT THE GATE; WE CONTACTED DISPATCH AND MAINT. MAINT WANTED US TO PULL THE GRAY COLORED CIRCUIT BREAKERS ASSOCIATED WITH THE AFDS AND RESET THEM; BUT THAT DID NOT WORK. I GOT BACK ON THE PHONE AND GAVE THEM MORE DETAILS OF WHAT WE HEARD AND SAW. WE TOLD THEM OF THE NOISE THE MCP WAS MAKING PRIOR TO IT FAILING. THEY TALKED ABOUT FLYING A PART IN FROM ZZZ2. AT THIS POINT; MORE PEOPLE SEEMED TO GET INVOLVED IN MAINT AND DISPATCH. AT ONE POINT WE HAD 2 DISPATCHERS AND 2 MAINT PERSONNEL ON THE PHONE AT ONCE. THROUGHOUT THE COURSE OF THE CONVERSATION; ONE OF THE MAINT PERSONNEL WANTED US TO PWR DOWN THE AIRPLANE. SO WE DID; AND THAT SEEMED TO FIX THE PROB. NOW DISPATCH AND MAINT WANTED US TO LOAD UP THE PAX AND CONTINUE ON TO ZZZ3. I FELT UNCOMFORTABLE WITH THIS FIX; SINCE IN FACT WE DID HEAR THE MCP MAKING AN UNUSUAL CLICKING NOISE WHILE INFLT. WE WERE ALSO ABOUT TO GO INTO THE WX AGAIN THAT WAS EXTENDING ALL THE WAY TO ZZZ3. THEY ASSURED ME THAT ALL WAS OKAY WITH THE MCP; BUT I STILL FELT SOMETHING WAS NEEDED TO BE WRITTEN UP IN THE LOGBOOK. MAINT SAID THAT IF I WROTE UP A LOGBOOK ENTRY; THAT THEY WOULD HAVE TO CALL OUT CONTRACT MAINT. I STILL FELT THAT SOMETHING NEEDED TO BE PUT INTO THE LOGBOOK. ATC KNEW ABOUT OUR SIT; ZZZ1 HEARD OUR CONVERSATION WITH DISPATCH AND I FELT IT WAS JUST THE RIGHT THING TO DO; SINCE AN ACFT COMPONENT DID IN FACT FAIL INFLT. (THIS IS THE POINT I THINK THIS ASAP RPT IS WARRANTED.) MAINT SUGGESTED THAT I PUT THE WRITE-UP IN THE LOGBOOK AS AN 'INFO ONLY' ENTRY. I QUESTIONED THIS AT LEAST 3 TIMES. THEY SAID IT WAS OKAY TO DO IT THIS WAY. I THEN ASKED DISPATCH IF THEY CONCURRED AND THEY SAID YES. IT WAS WHEN I WAS HEADING TO ZZZ3 I REALIZED THE ONLY 'INFO ONLY' WRITE-UP A PLT MAY WRITE UP IS A TKOF WARNING HORN MALFUNCTION (FOM A.10.A). IN ANY CASE; THE MCP MADE THE SAME CLICKING NOISE ON DSCNT INTO ZZZ2. IT FAILED IN THE VERT SPD MODE AND AIRSPD SELECT MODE. ONCE ON THE GND; I ADVISED MAINT OF THE MCP PROBS WE HAD GOING INTO ZZZ2 AND THEN AGAIN ON DSCNT INTO ZZZ2. I ASKED IF I SHOULD WRITE IT UP IN THE MAINT LOGBOOK; AND THEY SAID NOT TO; THAT THEY WERE GOING TO CHANGE THE MCP OUT ANYWAY. I'M NOT SURE IF MAINT IS AUTH TO HAVE A PLT WRITE AN 'INFO ONLY' AT THEIR REQUEST. IF SO; MAYBE WE NEED TO PUT THAT INFO IN THE FOM. IF NOT; THIS WILL NEVER HAPPEN TO ME AGAIN.