TM10-4610-232-12 Table  4-1.  Unit  Preventive  Maintenance  Checks  and  Services Item No. 13. 14. Interval Semi– Annual Semi- Annual Item To Be Checked  Or Serviced Metal  Oxide  Va- ristor  (MOV) Junction  Box Electric Heater Procedure e. Check air compressor valve efficiency. Vent air pressure to 0 psi and close all vent/drain air vaIves. Start compressor and time it until it stops. Make sure no air is being used in the ROW- PU. A run time of 1 hour indi- cates good operation. One to two hours  indicates  increasing  valve wear. Over two hours indicates excessive wear and direct sup- port should be informed to change air compressor valves. f. Check compressor cylinder fins for cracks, damage and cleanli- ness. g. Drain oil and check for metal particles on magnetic plug and milky condition. h. Replace oil and check that dipstick is tight and breather is not clogged. i.    Check  electrical  connector  box and cables for tightness and proper mounting. Inspect for cor- roded, burnt, loose or missing cable pins and inserts. j. Check for leaks in air lines and interstage  relief  valves. k. Check fan belt for tension and wear. a. Check electrical connector box and cables for damage, tightness and  proper  mounting. b. Check for burnt, damaged, loose or missing cable pins, in- serts and wires. c. Check for damage and conti- nuity of MOVs. a. Check for loose or missing mounting screws. Check for damage  inserts. Not Mission Capable  If: Compressor  runs  over two hours. Any cylinder fin crack ex- tends into cylinder hous- ing. Oil is milky or has metal particles. Breather  is  unservice- able. Any cable connection, pin or insert is damaged. Any line or valve leak or unserviceable  part  is found. Fan belt is worn or dam- aged. Any cable connection, pin or insert is damaged. Any connector, pin, in- sert, screw or wire dam- aged, burnt or missing. No continuity exists. Any screw missing or in- sert  unserviceable  (Cold weather  operations  only). 4-8    Change  2