What was found
DURING CLIMB AFTER TAKE OFF NOSE DOWN PRIMARY TRIM WAS SELECTED AS THE AIRCRAFT ACCELERATED TO NO EFFECT. THE TRIM REMAINED AT -2.9. EACH TIME NOSE DOWN TRIM WAS ATTEMPTED THE AMBER CAS PRI STAB TRIM FAIL MESSAGE APPEARED. SECONDARY TRIM WAS ACTIVATED AND MANUAL TRIM FUNCTIONED NORMALLY AND THE AMBER PRI STAB TRIM FAIL MESSAGE REMAINED. AFTER LANDING AIRCRAFT WAS POWERED DOWN WITH SEC TRIM STILL SELECTED ON. CORRECTIVE ACTION: REMOVED SUSPECTED DEFECTIVE ACTUATOR CONTROL UNIT P/N 103000B S/N 129B AND INSTALLED OVERHAULED CUSTOMER SUPPLIED ACTUATOR CONTROL UNIT P/N 103000B S/N 312B. PERFORMED OPS CHECKS WITH NO DEFECTS NOTED. REF CE 680A AMM 27-40-20.