2004 John Deer Buck Utility ATV 500, 500EX and 500EXT Service Manual, Page 165Get this manual
BLACKGREEN wires or between the RED YELLOW and GREEN wires while starting the engineIf there is no voltage, either the MPEM or the wiring harness is defectiveDisconnect then remove the ignition coil or unplug the spark plug cableUnscrew the spark plug one turnClean the spark plug and cylinder head with pressurized airUnscrew the spark plug completely Due to the integrated diode, it is not possible to take any resistance measurementThe output voltage should not be less than 12 kV (12 000 V)The ignition coil outlet caps and the spark plug caps include resistanceThey should be checked prior to replacing suspected ignition coilMeasure resistance between ignition coil outlet caps and spark plug capsThe obtained value should be between 5.4 and 5.7 kWIf resistance is good, continue checkIf not, change spark plug wiresDYNAMIC TEST: An ignition coil tester available from after-market tool equipment suppliers can be used
The proper heat range of the spark plugs is determined by the spark plugs ability to dissipate the heat generated by combustionThe longer the heat path between the electrode tip to the plug shell, the hotter the spark plug operating temperature will be and inversely, the shorter the heat path, the colder the operating temperature will beA "cold" type plug has relatively short insulator nose and transfers heat very rapidly into the cylinder headSuch plug is used in heavy duty or continuous high speed operation to avoid overheatingThe "hot" type plug has longer insulator nose and transfers heat more slowly away from its firing endIt runs hotter and burns off combustion deposits which might tend to foul the plug during prolonged idle or low speed operation