ATX SMT ALT DEV OVERSHOT DURING CLIMB OUT FROM CXY.

Narrative

THE FLT ORIGINATED WITH 2 CAPTS; MYSELF WITH 115 HOURS IN TYPE; AND A NEW CAPT WITH 20 HOURS IN TYPE. I FLEW SECOND IN COMMAND ON THE FIRST LEG FROM ALLEGHENY CO TO HARRISBURG INTERNATIONAL ARPT. DURING THIS LEG MY FELLOW CREW MEMBER DISCONNECTED THE ALT ALERT SYSTEM BECAUSE IT 'IRRITATED' HIM. I DISAGREED WITH THIS PROCEDURE BUT FAILED TO SAY ANYTHING AT THE TIME. IT WAS MY INTENTION TO RECONNECT THE SYSTEM AFTER LNDG. THE RETURN TRIP HOME OCCURRED AT NIGHT AND UPON ENTERING THE ACFT I FORGOT THE ALT ALERT SYSTEM WAS DISCONNECTED. EVEN WITH THE COCKPIT LIGHTS ON THE 'BOX' SET ON TOP THE INSTRUMENT PANEL IN THE DARK AND OF SIGHT AND OUT OF MIND. WE RECEIVED OUR CLRNC; RAN OUR CHECKLIST; AND TOOK OFF INTO THE NIGHT OK. AS WE WERE CLIMBING OUT OUR ASSIGNED ALT WAS NOT VISIBLE TO ME DURING MY NORMAL INSTRUMENT SCAN. WE HAD BEEN ASSIGNED 4000'; AND WAS FLYING FROM THE LEFT SEAT. AS WE BROKE THROUGH 5000' MY ACTING CO-PLT CAUGHT THE ERROR. I PULLED THE POWER BACK; AND AFTER TURNING THE ALT REPORTING ON THE TRANSPONDER OFF HEADED FOR 4000'. AS WE PASSED 4700' THE CTLR ASSIGNED US 8000' AND HANDED US OVER TO CLEVELAND CENTER. WE REPORTED OUT OF 4000' AND CALLED CLEVELAND. THE DEVIATION WAS NEVER POINTED OUT TO US BY ATC. THE REST OF THE FLT WAS NORMAL. THE FOLLOWING FACTORS CONTRIBUTED TO THIS SITUATION. THE FIRST CAPT DISCONNECTED THE ALT ALERT SYSTEM. I DID NOT MAKE MY DISAPPROVAL AT THIS PROCEDURE CLEAR TO HIM. I DID NOT RECONNECT THE SYSTEM. THE ATC CTLR FAILED TO NOTICE OUR ALT DEVIATION. THE NIGHT WAS CLEAR. THE FLT SHORT; AND I WAS FLYING WITH ANOTHER CAPT. I LED MYSELF INTO A FALSE SENSE OF SECURITY. WHAT COULD POSSIBLY GO WRONG. I FOUND OUT.