TM 1-1500-204-23-1 4 Depress and hold timing plunger on magneto, while continuing rotation of engine, until timing plunger is fully engaged. 5 See    that    timing    indicator    shows correct degree of advanced timing. (2) Ignition harness. The ignition harness,  as  shown  in  figure  7-18,  contains  an  insulated wire  for  each  cylinder  that  the  magneto  serves  In  the engine.      One   end   of   each   wire   Is   connected   to   the distributor  block  and  the  other  end  is  connected  to  the proper  spark  plug.    The  Ignition  harness  serves  a  dual purpose.    It  supports  the  wires  and  protects  them  from damage  by  engine  heat,  vibration,  or  weather.    It  also serves   as   a   conductor   for   stray   magnetic   fields   that surround the wires as they momentarily carry high-voltage current.    By  conducting  these  magnetic  lines  of  force  to the   ground,   the   ignition   harness   cuts   down   electrical interference  with  the  aircraft  radio  and  other  electrically sensitive equipment.  When the radio and other electrical equipment   are   protected   in   this   manner,   the   ignition harness   wiring   is   said   to   be   a   shield.      Without   this shielding,   radio   communication   would   become   virtually impossible.      The   ignition   harness   check   consists   of   a visual    and    an    electrical    check    using    the    following procedures. (a) Examine all accessible parts of distributors,   harnesses,   and   leads   for   chafing,   broken insulators, corrosion, moisture, damaged or dirty terminals,  and  security  of  mounting  at  the  time  specified in the applicable maintenance manual. Figure 7-18.  Ignition Harness (b) Repair  or  replace  defective  parts  as necessary.      Keep   all   parts   clean   and   free   of   foreign matter. NOTE The  high-tension  system  manifold  Is never  taken  apart  in  the  field,  as  this will destroy the elastic gel seal. 1 Clean dirt and grease from harness by wiping with a clean, dry cloth. 2 Clean terminals with acetone, Federal Specification O-A-51. NOTE Replace  ignition  harness  leads  when more    than    three    wire    strands    are broken  in  one  inch  of  harness  length or if Insulation is damaged (c) Perform    a    high-voltage    breakdown test  when  an  ignition  malfunction  occurs  and  after  cable replacement   in   rewireable   harness   or   leads.      Use   an approved high-voltage tester, and test for serviceability as follows. 1 Remove all leads from spark plugs. 2 Remove spark plug elbow assembly from ignition lead. 3 Examine lead insulation for evidence  of  chafing,  breaks,  or  holes.    Replace  parts  as required. 4 Ground   all   leads,   except   one   for number 1 cylinder, to engine. 5 Connect   ground   lead   of   tester   to engine,    and    connect    high-tension    lead    to    number    1 cylinder lead. 6 Test and record any leakage detected. 7 Test remaining leads. 8 When two leads from same distributor show leakage, remove ground from both leads and  repeat  test.    When  leakage  is  stopped  by  removing ground from other lead, leakage is probably between. 7-19