High Tension/Plug Lead & Plug Cap Testing

Disconnect the h.t. (plug) lead from the coil.
Examine the full length of the lead, checking for cuts/abrasions to the insulation.
If the outer insulation is cut through (especially if the inner wire is exposed), the lead should be replaced.

If you wish to measure just the lead resistance, unscrew the plug cap from the end of the lead, unless the plug cap is moulded onto the lead (non-detachable).

With the aid of a multimeter, set to Ω (ohms) and measure the plug lead resistance readings, as follows:

• The meter should be set to the low ohms setting, e.g. Ω x 1, 200 ohms, or similar range.
  Insert one meter probe onto the metal crimp at the coil end of the lead. Insert the other meter probe directly into the end of the plug wire, making contact with the inner wire.
  If the plug cap is left attached to the lead, insert the second meter probe into the end of the plug cap, making contact with the brass insert inside the cap.
  If using copper/steel cored plug wire, the meter reading without plug cap attached should be very close to zero ohms.
  If the meter reading is high resistance or open circuit, the lead should be replaced.
  With copper cored plug wire, this is very unlikely to be the case, unless there is an internal fracture that cannot be seen (hidden by the outer insulation).
  
  If the plug cap is attached, the total resistance reading will depend on the type of plug cap fitted.
  If it is a non-resistor (non-suppressor) type, then the reading will be very close to zero ohms.
  If the plug cap is a resistor type, then the total reading will include that of the plug cap.
  
  Most plug caps (e.g. NGK and similar) are 5k resistor (suppressor) type.
  This is usually indicated in the part number printed on the cap, e.g. for LB05F type, the '05' indicates 5k resistor type.
  Other types, e.g. LZFH, indicates a non-resistor cap. This will have a resistance reading of very close to zero ohms.
  Some plug caps are higher resistance, e.g. 10k ohms, usually indicated by a '10' in the part number.
  For a  resistor plug cap, you should allow some tolerance on the meter reading, typically ±10%.
  If the meter reading shows a higher/lower resistance than expected (or is open circuit), the plug cap should be replaced.
  
  If you are using resistive plug wire (e.g. spiral core), then the reading through the plug wire will depend on the specification of the wire used.
  This may be marked on the outer insulation. Otherwise, you will need to check with the supplier/manufacturer.
  The resistance is often specified as ohms/foot, e.g. 50 ohms/foot.
  So for a plug lead length of one foot, the total reading will be approx. 50 ohms, not including the plug cap resistance.
  If the meter reading shows a higher resistance than expected (or is open circuit), the lead should be replaced.
  
  Note: to get the most accurate reading, short the meter probes together and note the reading. This will show the internal resistance of the meter/probes, if any.
  Subtract this reading from the plug lead/cap reading, to give the true resistance reading.