PILATUS PC12 FLT CREW HAS AN ALTDEV DURING TEB 5 DEP.

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WE WERE CLBING ON THE TEB 5 DEP OFF OF RWY 24 AT TEB. THE PROC CALLS (FOR DME EQUIPPED ACFT; WHICH IS APPLICABLE TO OUR ACFT) TO DEPART RWY 24 ON RWY HDG TO 1500 FT MSL; THEN ONCE AT 1500 FT MSL TURN R TO A HDG OF 280 DEGS. MAINTAIN 1500 FT MSL UNTIL PASSING 4.5 DME FROM THE TEB VOR (118.4); THEN CLB AND MAINTAIN 2000 FT MSL. AS WE WERE CLBING OUT; WE HIT 1500 FT MSL AND TURNED R TO A HDG OF 280 DEGS. THEN THE CAPT CALLED OUT 4.5 DME FROM THE TEB VOR (THIS WAS VERIFIED BY ME REFING THE DME READOUT OF 4.5 DME AND CLBING ON MY SCREEN) WE PROCEEDED TO CLB AND LEVEL OFF AT 2000 FT MSL. AS WE LEVELED OFF AT 2000 FT MSL; WE WERE TOLD TO CONTACT NEW YORK DEP. I THEN PROCEEDED TO CONTACT NEW YORK DEP WITH OUR ALT AND HDG; HOWEVER; WHILE I WAS CONTACTING NEW YORK I NOTICED OUR DME HAD DROPPED TO APPROX 2.5 DME. THIS ERROR IN DME AND ALT WAS QUICKLY REALIZED BY BOTH US AND NEW YORK DEP (WHO COMMENTED THAT GIVING OUR CURRENT DISTANCE FROM THE TEB VOR THAT WE SHOULD BE AT 1500 FT MSL). WE BEGAN DSNDING BACK TO 1500 FT MSL AND EXPLAINED WHAT HAD HAPPENED. THE CTLR SAID TO REMAIN AT 2000 FT MSL AND BEGAN GIVING US VECTORS AND ALTS TO OUR FIRST FIX. WE DO NOT KNOW WHAT CAUSED THIS ERROR IN DME. WE HAD BOTH FLOWN THIS PROC MANY TIMES BEFORE AND ARE FAMILIAR WITH IT. THE TEB VOR DME DID CHK OUT ON THE GND READING .7 DME PRIOR TO DEPARTING AND WE HAD FLOWN THE VOR DME-A APPROX 1.5 HRS EARLIER WITHOUT PROBS.