APU FIRE WARNING ON ACR MLG MANDATES A RETURN LAND AFTER EMER DECLARED.

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PRIOR TO TKOF OUR APU WAS SHUTDOWN. AFTER TKOF AND CLBING TO 10000 FT THE APU FIRE DETECT INOP LIGHT ILLUMINATED. THIS OCCURRED ABOUT 8000 FT. ABOUT 30 SECONDS LATER THE APU FIRE LIGHT ILLUMINATED WITH AURAL WARNING. THE APU FIRE CHKLIST WAS ACCOMPLISHED. ATC WAS NEXT INFORMED THAT WE WANTED TO RETURN TO NEWARK AND DECLARED AN EMER. WE WERE ABOUT 40 MI N OF THE NEWARK ARPT. ATC GAVE US A L TURN BACK TOWARD THE ARPT AND ALSO GAVE US DSCNT INSTRUCTIONS. WE DID NOT DEV FROM ANY ATC INSTRUCTIONS. IN THE COCKPIT I EXPLAINED TO OUR #1 FLT ATTENDANT WHAT OUR PROBLEM WAS; ASKED HER TO CHK FOR ANY INDICATIONS OF FIRE IN THE CABIN AND TO PREPARE THE CABIN FOR AN EMER LNDG. I MYSELF GAVE A SHORT BRIEFING TO THE PAX VIA THE PA SYS. WE ACCOMPLISHED THE NECESSARY CHKLISTS PRIOR TO LNDG VFR AT EWR. OTHER THAN THE FIRE LIGHT AND INTERMITTENT ALARM NO OTHER ACTUAL FIRE INDICATIONS WERE OBSERVED. AFTER LNDG I EXITED A HIGH SPD TAXIWAY WHERE EMER UNITS WERE PARKED. THE ACFT ENGS WERE SHUT DOWN AND NO INDICATIONS OF FIRE WAS OBSERVED FROM ANY EMER UNIT OR THE TWR FACILITY. WE WERE TOWED TO A PARKING GATE WITHOUT FURTHER INCIDENT.